Scan generators. Purpose. Scheme. Synchronization of sweep generators

An oscilloscope is an electronic instrument for measuring and observing electrical signals in a circuit.
Determining the shape and parameters of vibrations is necessary to monitor the correct operation of the equipment. The first attempts to create a device for determining electrical vibrations date back to 1880. They were made by French and Russian physicists. The first oscilloscopes were analog. Since the 1980s, signals have been recorded using digital equipment.

Design and principle of operation of the device

Let us explain the structure of an analog oscilloscope simply, “for dummies.” The device consists of the following elements:

• ray tube;
• power unit;
• vertical/horizontal deflection channel;
• beam modulation channel;
• synchronization and sweep trigger device.

There are controls to control the signal parameters and display it on the screen. Older models did not have a screen. The image was recorded on photographic tape.

Principle of operation

When the device starts, the signal is sent to the input of the vertical deflection channel. It has high input impedance. A voltmeter that measures voltage works on the same principle. However, the voltmeter does not show a time graph of voltage fluctuations.

The signal is amplified to the required level after being applied to the input. It is displayed on the screen along the vertical axis. Gain is required to operate the beam tube deflection system or the analog to digital signal converter. It allows you to change the scale of display of vibrations on the screen from large to small.

Device

The ray tube is sensitive to electrical impulses. The lower their frequency, the higher the sensitivity. In current tubes, the number of beams can range from one to 16. Their number corresponds to the number of signal inputs and graphs displayed simultaneously.

The peculiarity of a digital oscilloscope is that it has a screen and an analog signal converter. It has a memory to store data about the received oscillation graph. Some of the information is analyzed automatically and displayed in processed form. An analog oscilloscope does not store data, but only displays it in real time.

A sweep is the trajectory of a beam that picks up vibrations and displays an image on the screen. It comes in different shapes - elliptical, circular. The sweep value is adjusted depending on the signal being studied along the horizontal (time) axis.

The power supply supplies voltage from the 220 V network to the electronic circuits. There are also battery-powered models that can operate autonomously.

Types of oscillographic scans

In general, the sweep voltage is the voltage that determines the trajectory and speed of movement of the CRT beam in the absence of the signal being studied. The trajectory described by the beam, or the trace it creates on the screen under the action of the scanning voltage, is called scanning. If the development is obtained as a result of applying a development voltage to one pair of deflection plates (usually horizontal deflectors), then it is called according to the shape of the development voltage - sawtooth, exponential, sinusoidal. When the scan is created by applying voltage to both pairs of plates simultaneously (and to the radially deflecting electrode - in special tubes), its name is determined by the shape of the trajectory drawn by the beam: circular, elliptical, spiral, radial.

Linear periodic sweep

is created by a sawtooth, i.e. linearly varying voltage (Fig. 10.2).

Rice. 10.2. Linear periodic sweep:

A

– form of continuous periodic voltage;
b
– unfolding shape

voltage in the form of periodically repeating pulses; V

– scan line

on the screen

At a minimum voltage value (point 0 in Fig. 10.2, a

) the beam is in the extreme left position on the horizontal line of the screen. As the sawtooth voltage increases, the beam moves from left to right at a constant speed. This movement, called the forward path of the beam, occurs over a period of time until the development voltage reaches its maximum value (point A). With proper selection of the amplitude of the sawtooth voltage, the beam will move to the extreme right position of the screen during the forward stroke. When the voltage drops from A to B, the beam makes a reverse stroke - within a time it quickly returns to its original position in order to repeat the cycle consisting of a forward and reverse stroke in the next period.

The main characteristics of continuous periodic scanning (Fig. 10.2, a

): the sweep period or frequency and the maximum deflection of the beam during the period, determined by the amplitude of the sweep voltage.
Pulse periodic scanning (Fig. 10.2, c
) is characterized by duration (instead of period), pulse repetition rate
F
and maximum beam deflection. To obtain a high-quality image of the process under study, the condition must be met. In modern oscilloscopes this requirement is always met. In addition, the beam is extinguished during the reverse stroke or illuminated during the forward stroke. In practice, we can assume that or .

To ensure that the scan line or signal image does not flicker during observation, the beam must trace the same trajectory at least 25 - 30 times per second. This takes advantage of the inertial ability of the human eye to retain a visual impression for approximately 1/15 s.

The image appears stationary to the observer if the beam, with each forward stroke, traces the same curve, starting at the same phase. To obtain a stationary oscillogram, it is necessary that the period of the sweeping voltage (or period T

) was equal to or a multiple of the period of the signal under study, i.e.

or (10.1)

This is achieved by synchronizing the sweep voltage with the signal under study or external voltage with a period corresponding to condition (10.1).

Sawtooth voltage is not strictly linear. It often changes exponentially, close to a straight line, and the degree of linearization depends on the sweep generator circuit.

A quantitative measure of nonlinearity is the nonlinearity coefficient γ

, characterizing the degree of variability in the rate of voltage rise at the beginning and end of the forward stroke of the beam

(10.2)

In oscilloscopes used to observe the voltage waveform, the nonlinearity coefficient, depending on the accuracy class of the device, ranges from 3% (class I) to 20% (class IV), and in oscillographic time interval meters it is much less and amounts to tenths and hundredths percent.

To study various pulse processes and single pulses, a waiting scan

. Its essence lies in the fact that the sweeping voltage is applied to the horizontal deflection plates only when the pulse under study arrives at the input of the oscilloscope. After the beam completes one cycle of forward and reverse motion under the influence of the sweeping voltage, the sweep stops and “awaits” the arrival of a new impulse that triggers it (Fig. 10.3).

expressed in mm/ms, mm/μs or mm/ns ( – sensitivity of the tube to horizontal deflection, mm/V). Often the scan rate is also expressed in cm/µs, etc.

A sinusoidal sweep is obtained by applying a sinusoidal voltage to the horizontal deflection plates. In this case, the scan line appears straight to the observer, but the speed of the beam is uneven in different parts of the screen.

channel transmission coefficients X

and
Y
of the oscilloscope must be chosen so that the horizontal and vertical beam deflections are equal.
The trajectory of the beam is a circle (Fig. 10.4, a
), and the beam makes one revolution in a time equal to the period of the sinusoidal unfolding voltage.
This development is a special case of an elliptical development (Fig. 10.4, b
). If two harmonic voltages are applied to both pairs of deflecting plates, which are necessary to obtain a circular scan, but differ in that their amplitudes change linearly in time, then the beam will describe an Archimedean spiral. This type of scan is called a spiral scan.

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Types of oscilloscopes

Based on their operating principle, oscilloscopes are either digital or analog. There are mixed analog-digital devices. Virtual ones are increasingly being released. There, another device is used as a screen - a computer monitor, a TV monitor.

The operation of some models is based on the electromechanical principle:

• electrodynamic;
• electrostatic;
• rectifying;
• electromagnetic;
• magnetoelectric;
• thermoelectric.

The device can work independently or be an attachment to other equipment (for example, a computer). In the second case, the price is lower, but the device itself is dependent on an external device.

Types of scans

In different operating modes of the oscilloscope, linear (created by a sawtooth voltage) sweeps may differ:

• One-time. The generator starts once, then locks out. This sweep is needed to capture non-repeating signals.
• Waiting. The launch occurs immediately after the signal. Needed to monitor rare fluctuations.
• Self-oscillatory. The generator turns on periodically when there is no signal. Convenient for displaying frequent periodic pulses.

Processes measured

According to the principle of operation, devices are divided into:

• Special. They have blocks for intended use (for example, television oscilloscopes).
• Stroboscopic. Sensitive instruments for studying short-term repetitive processes.
• Express. Used to record processes at high speed (with an accuracy of nano- and picoseconds).
• Memorable. Save the resulting image. Typically used to study rare, one-time actions.
• Universal. Explore different processes.

Where are oscilloscopes used?

Information provided by the oscilloscope:

• voltage values, time parameters of oscillations;
• phase shift, pulse distortion in different parts of the circuit;
• frequency (determined by recording its temporal characteristics);
• variable and constant components of oscillations;
• processes in the chain.

Oscilloscopes are used for both practical and research purposes. For simple measurements, you can use a multimeter, but in most cases an oscilloscope is indispensable.

Instruments for measuring vibrations are used when setting up electronic equipment. For example, to adjust a television signal, it is necessary to obtain its oscillographic image. The devices are also used in repairing power supplies and diagnosing printed circuit boards.

When repairing cars, the device will help to obtain data on the position of the crankshaft and camshaft, position sensors. These oscillograms will tell you about the presence of a pulse on the coil, indicate a malfunction of the spark plugs and wires, and the diode bridge of the generator.

Medical equipment (cardiographs, encephalographs) also works on the principle of oscillography. Only the electrical vibrations measured by them occur in living organisms.

Small, simple oscilloscope

Some time ago I came across a kit that allows me to assemble a simple oscilloscope called DSO138.
Everything would be fine, but it had a very limited frequency of the measured signal. I won’t say that the device under review is much cooler, but it has a stated bandwidth of up to 4 MHz. Let's see what it is and how it works. This oscilloscope is initially positioned even on the seller's page as an option for training, i.e. designed for an untrained, novice user who is very far from managing more “sophisticated” models and may well get confused.

To begin with, the technical specifications, I immediately apologize for the crooked translation in places. Sampling rate: 20MSa/s Analog bandwidth: 4MHz Sampling precision: 8 bits Cache size: 650 bytes Vertical sensitivity: 10mV/div ~ 5V/div (1-2-5 progressive) Horizontal scan rate: 1.5us/s div ~ 6ms/div (1-2-5 progressive) Display: 2.4 inch TFT 320X240 (ILI9325 driver chip) Input impedance: 1MΩ Maximum input voltage: 40Vpp (1:1 probe), 400Vpp (10:1 probe) 1) Input signal: AC Waveform hold (HOLD function)

The oscilloscope is sold in several configuration options (approximate prices from the product page): 1. Oscilloscope + USB power cable - $17.40 2. Oscilloscope + power cable + probe - $20.09

You can also buy separately: 3. 40 MHz probe - $2.69 4. BNC-BNC + BNC+ alligator cable - $1.74

I ordered option number two. As a result, I received two such packages. By the way, the oscilloscope was ordered from the same seller as the LCR meter, so the delivery cost was slightly reduced.

Since the cost of delivery depends on weight, I first weighed the set according to point 1, and then the complete version according to point 2.

Let's move on to the inspection, first the equipment. The large bag contained a probe, power cable and all sorts of small things. The package is tight with a “valve”, it is convenient to store all additional “equipment” in the future.

The power cable has a familiar USB plug at one end, and a round plug with a diameter of 3.5mm at the other. The probe is the most common, the cable is soft.

The kit included instructions, after reading which I realized that the kit was still not entirely complete; it lacked a special grounding contact in the form of a spring and four colored rings. Well, these are rings, you can survive, but it’s a pity for additional contact, it would be very useful to me

The probe has a built-in 1:10 divider, with a corresponding switch. The ground contact is covered in insulation, although the crocodile is quite “oaky”. Above I showed the instructions, according to it my probe is designed for a frequency of up to 40 MHz and a voltage of up to 600 Volts. The oscilloscope itself has more modest boundaries, so everything here has a decent margin.

The probe has the ability to be adjusted. There was also a screwdriver for adjustment, but I found it very useful for working with an oscilloscope rather than a probe. but keep in mind that the screwdriver comes with the probe, not the oscilloscope. By the way, the price of the probe is very low, in my opinion, in our offline they cost much more.

And here is the subject of the review. Outwardly, it’s a typical “skillful hands” club at school, a simple body, although there is laser engraving, and not banal stickers, but this is not relevant.

On top of the case there is a color display with a diagonal of 2.4 inches and a resolution of 320x240. My DSO203 has a larger display both in size and resolution (400x240), although not by much. On the right are control buttons, and the controls are extremely simple, there are no menus, settings, etc. Just five buttons - 1, 2. Input voltage from 0.01 to 5 V per cell. 9 steps. 3, 4. Sweep, from 1.5 μS to 6 mS per cell, 12 steps. 5. Hold button, simply fixes the readings on the display. As it turns out, the most used button in some situations.

At the top end of the case there is a BNC input connector, as well as a switch and power connector. By the way, the device consumption is only about 150 mA, which makes it possible to organize its autonomous power supply, but since the device is quite sensitive, it is recommended to use a pair of lithium cells and a low-dropout linear voltage regulator. On the Internet, search by prefix - Low Drop.

At the bottom there is a hole for access to the zero adjustment trimmer resistor.

This design is very easy to disassemble; first, unscrew the four screws from the bottom.

Then four screws on top and remove the board. There is a hole drilled in the case for the connector, so you need to remove the board from the hole.

Inside you can see the oscilloscope board and a display quite familiar to many radio amateurs. If I'm not confused, the same display is used in the DSO138.

The display is held only by fixing it in the connector, it is pressed against the top by the body, and two plastic risers are melted at the bottom.

The printed circuit board is made quite well; next to each element there is not only a position number, but also a nominal value, which is extremely rare. Just a “repairman’s dream” I once filmed a video on how to determine the value of a burnt out resistor; this would not be necessary here.

Power supply and input operational amplifier assembly. The soldering is quite good, but there is a feeling that some components were changed after assembly; traces of flux are visible.

Input circuits and signal dividers. Unfortunately, the oscilloscope can only work with alternating current, but for most tasks this is more than enough. At the input there is a 330nF 250 Volt capacitor.

Input divider. There are 5 reed relays on the board, the divider has 9 input voltage options. The first three relays operate in the circuit of the first op-amp, then another pair in the circuit of the second op-amp, resulting in 3x3=9 options.

Zero setting resistor. Initially, the oscilloscope came with a “floating” zero and installed it, but practice has shown that the zero still likes to “float” sometimes, so a screwdriver is needed quite often.

Oscilloscope elements: 1. Input dual op-amp LM6172 with a maximum frequency of 100 MHz. 2. ADC - ADS830E, maximum frequency 60 MHz 3. Asynchronous FIFO buffer with access time no more than 12 nS. 4. Atmega16A microcontroller, 20 MHz quartz resonator on the left. 5. Just a logic chip 6. Voltage converter 7660, forms the negative pole of 5 Volts. The board also has a 3.3 Volt linear voltage regulator, it can be seen above in the photo.

The soldering on the bottom is relatively high quality, but there is flux, there is a lot of it.

In addition to the fact that the component ratings are indicated on the board, there is also a circuit diagram. The truth is in the version with a different diet. Here it is not the 7660 that is responsible for power supply, but simply a network power supply unit with bipolar power supply. Unfortunately, the quality of the circuit has let us down a bit, but that's what it is. The input attenuator, ADC, buffer and microcontroller with display are visible. The circuitry is as simple as three kopecks, but quite good for what is essentially a toy.

Let's take a closer look at what the oscilloscope is built on. Immediately after the first attenuator, the signal goes to the amplifier. A fairly good op-amp with a frequency of up to 100 MHz was used, which is more than a reserve at the stated 4 MHz.

Next is a good 8-bit ADC manufactured by Burr-Brown with a top frequency of 60 MHz, which is also a huge margin. It’s interesting that the DS203, which I use, has a dual ADC, but it only has 40 Megasamples.

FIFO buffer, as far as I understand, the maximum operating frequency is about 80 MHz. IDT7205 is used. It seems that this series is produced in military version.

But then the Atmega16A is engaged in displaying the signal on the screen, as well as the scale grid and measuring the frequency.

First of all, I first decided to evaluate the noise level. The input was not shorted, if shorted, then there is just a straight line on the screen. On the left, the oscilloscope is simply lying on the table, on the right, I put my hand on the body near the input attenuator.

Don’t be alarmed, in fact the oscilloscope screen looks much more beautiful, everything is clear and contrasty.

It’s just that since the oscilloscope can’t take screenshots, I had to resort to the “old-fashioned method.”

To begin with, I used the built-in one in my usual DS203 as a generator. Saw and triangle 20 kHz, respectively, as the monitor and mine see it, quite well.

Sine and square 20 kHz. The sine coincides, but the leading edge of the rectangle is heavily blocked.

I will assume that the above generator worked in DDS mode, so I increased the frequency above 20 kHz, since it is precisely the rectangular pulse generator that works in this mode. 200 and 500 kHz. Perhaps I would say that it’s even not bad, if not for the fact that one oscillogram is littered with one thing, and another with something else. It seems like the image is a mirror image. In both cases, a cable from the DS203 was used, connecting alternately to the input of one and the other oscilloscope.

And then I accidentally saw one interesting feature, perhaps this was an error in the program, perhaps this was intended, but the oscilloscope allows you to significantly reduce the sweep time than the stated 1.5 μs per cell. I started switching scan modes (they go in a circle) and by increasing the time I was able to stretch the signal. The frequency meter, of course, began to show the value “from the bullshit”.

Okay, now I’m curious, let’s apply 1 MHz.

On the left is 1.5 µs, on the right is the “wrong” stretched mode.

Let's apply 2 MHz.

Well, I keep thinking, “Bobby is dead,” it’s nonsense on the screen, you can’t see it in the first mode, in the second it’s almost a triangle.

But I don’t give up and will give it 4 MHz. Already on the screen of my oscilloscope there is something faintly resembling a rectangle.

And on the screen of the monitored animal there is generally darkness, but... 1. The original signal is at the shortest sweep, the frequency meter works normally, displays the applied 4 MHz. But you can’t look at the signal without tears. 2. We increase the sweep time, as I did above, well, a triangle. 3. Let's change the input attenuator from 1 V per cell to 0.5 V. Oh, that's noticeably better. 4. Well, now let’s stretch the scan some more. It even looks like a rectangle

On the other hand, no one actually promised anything above 4 MHz.

The next experiment was carried out in this mode; by the way, the built-in frequency counter at a frequency of 6 MHz is already starting to show nonsense. But as it turned out, the frequency displayed on the screen is still a multiple of the real frequency of the input signal. 1. 6 MHz, displays on the screen as 2 kHz, i.e. 3000 times less. 2. 8 MHz, on the screen 2.8 kHz, which is also about 3000 times less than 8 MHz.

But it works. I got the impression that somehow not the entire real signal is output, and it is very separated, i.e. Most of it was “taken out” and it acquired a sane appearance. Unfortunately, I don’t have much to test at high frequencies.

In general, to be fair, I first tried to conduct tests with a different signal generator.

And I wouldn't have added them to the review if it weren't for some little things that I noticed along the way. To begin with, the 8 MHz signal is in its normal form and stretched, as I did above.

But if you stretch it even more, it takes on this form; perhaps this information will give someone food for thought.

And this is what a triangle and a saw look like, fed from this generator to the oscilloscope under review and my main one. Frequency 65 kHz.

Since I’m already testing, I’ll check how this oscilloscope works with more real signals. For example, an oscillogram from one of my reviews of the power supply. True, a capacitor was used here in parallel with the probe, as I did in recent reviews of power supplies.

The same power supply, approximately the same load, but with different signal output parameters. It seems? In my opinion yes.

Perhaps for some, the oscillogram that I showed above will not seem very clear, so I selected one of the power supplies, where the ripples have a more familiar appearance. The same power supply, on the left the load is 50%, on the right 100%. In both cases, the oscillograms coincide, and in the monitored one you can still stretch the picture 2 or 4 times. But at the same time, my oscilloscope operates at the minimum possible 50 mV per cell, and the sensitivity of the reviewer can be increased by another 5 times, bringing it to 10 mV per cell. True, a small “feature” was also discovered: one oscilloscope had a larger ripple range than the other. By the way, the monitor’s full span value is displayed quite correctly.

Group photo, DSO138, reviewer and DS203.

As conclusions, I can say that the oscilloscope pleasantly surprised me, first of all, with its very good element base and simplicity of the circuit design. In terms of functionality, it will of course lose even to the DSO138, not to mention the DS203, but in terms of characteristics it stands head and shoulders above the DSO138 and I would say that in some ways it is not much worse than mine. Do not forget that the DS203 uses an ADC with a maximum frequency of 40 MHz, and the one under review is 60 MHz. The input attenuator is built without tricky switches, only on the basis of the simplest relays, but this solution works. Of the minuses, I note that the input mode is only AC, and not AC/DC, like the DSO138 and DS203. But one of the advantages is the simplest control, which unfortunately still added a fly in the ointment in the form of some difficulties in the operation of the built-in trigger, which is responsible for holding the signal on the screen. This is exactly what I wrote about above when talking about the Hold button. In some situations, the oscilloscope cannot keep the signal on the screen stable and it begins to “twitch”; when you press the Hold button, the result is most often normal, you just need to get used to it. The biggest oddity is the rectangle at 20 kHz.

Otherwise, a very interesting option for the most novice radio amateurs, which is easy to operate and allows you to use it in practice, for example, when working with power supplies. In addition, this oscilloscope is sold in a housing (this is both an advantage and a disadvantage at the same time), and also has a 5 Volt power supply. I tried powering it from a power bank, it works great.

Video review - https://www.youtube.com/watch?v=pNcID30bFwo

I bought it through an intermediary yoybuy.com, the cost of the set is about 22 dollars, the cost of delivery depends on the country, the weight of the components is indicated in the review. Referral link for registration, as far as I remember, you can get a bonus of 10 dollars from 50. The link is not mine, my bonuses are not there

Link to the product on the Taobao website.
That's all for me, as always, I'm waiting for questions, I hope that the review was useful.

Measurement technique

The oscilloscope measures electrical voltage and generates an amplitude graph of electrical fluctuations. Digital devices can remember the resulting schedule and return to it.

Oscillations are displayed on the screen in a two-dimensional coordinate system (voltage is the vertical axis, time is the horizontal axis), forming a graph - an oscillogram. There is a third component of research - signal intensity (or brightness).

If there are no input pulses on the screen, the horizontal line is “zero”, indicating the absence of voltage. As soon as voltage is applied to the input (or inputs) of the device, one or several graphs become visible on the screen simultaneously (depending on the number of measured signals).

The shape of a graph of electrical vibrations can be:

• sinusoid;
• damped sinusoid;
• rectangle;
• meander;
• triangles;
• sawtooth vibrations;
• pulse;
• difference;
• complex signal.

To obtain a stable graph of oscillations, the device contains a synchronization unit. It is possible to obtain a cyclic display of oscillations only after setting the synchronization value. It is taken as the “starting” one and serves as the starting point of the schedule. All jumps are displayed relative to this point.

Scan generators. Purpose. Scheme. Synchronization of sweep generators

To obtain a still image of the signal under study on the oscilloscope screen, a voltage linearly varying with time (LIN) is applied to the vertical deflection plates. This voltage is generated in an electronic oscilloscope by a sweep generator.

Rice. 2.12. LIN chart

To ensure image stillness, the generator is synchronized by the process under study. A typical LIN graph is shown in Fig. 2.12. This voltage U(t) is characterized by the following parameters:

The duration of the forward stroke Tpr, the repetition period Tr, the duration of the reverse stroke Tobr, the recovery time. TU, amplitude Um and linearity of forward stroke bР:

, (2.16)

In general, oscilloscope sweep generators must generate voltage with high linearity of the forward stroke, short reverse stroke time, have a short recovery time, allow the possibility of synchronizing their operation, and have high efficiency. using power supply voltage.

Oscilloscope scan generators are divided: according to the speed of change of LIN - into slow sweep generators (Tr = 10s - 20ms); medium sweep speed (Tr = 0.1s –1 μs) and fast sweep (Tr < 1 μs); according to the permissible nonlinearity of the scan - into precise bР < 5%) and conventional (bР = 10% - 20%);. according to the method of synchronization by the process under study to continuous standby sweep generators; according to the principle of construction on generators with parallel (Fig. 2.13, a) and serial (Fig. 2.13, b) connection of the switching element.

In the first circuit (Fig. 2.13a), the element that stores the energy of the electric current during the forward stroke is charged from a source E , and discharged during the reverse stroke. In the second circuit (Fig. 2.13,b), during the forward stroke, this element is discharged through the discharge circuit, and during the reverse stroke it is quickly charged through the switching element. For synchronization, it is possible to control the moment of the beginning of a new period of operation of the generator by applying a synchronizing pulse, for example, to a switching element.

Currently, a large number of LIN generator circuits have been proposed. Characteristic of most of them is the presence of a capacitive integrating circuit. To obtain the most linear voltage possible at the output of such a generator, they try in one way or another to obtain, perhaps, a more constant capacitor charge current. In this case

, (2.17)

if i = const, then ULIN(t) = U0 ± Kt.

The constancy of the charging current can be achieved by using a high voltage compared to Um to charge the capacitance, i.e. using only the initial section of the exponential voltage to form the LIN (the main disadvantage of such a circuit is the low efficiency of using the power source voltage); the use of current-stabilizing two-terminal networks and compensation methods using positive and negative feedback. In modern oscilloscopes, LIN generators, built according to one of the listed methods, are controlled by rectangular pulses, the duration of which is equal to TP. For this purpose, each scan generator has a control device.

Enter exit

Rice. 2.14. Block diagram of a sweep generator with a control multivibrator

In the control unit circuit (Fig. 2.14) with a multivibrator in standby or self-oscillating mode, it produces rectangular pulses that are used to control clays. In standby mode, the multivibrator is triggered by short pulses coming from the synchronization and trigger device. In continuous mode (periodic sweep), the multivibrator is synchronized by a synchronization circuit with the signal under study. To ensure that the range of um LIN does not change when switching the sweep duration in the generator, the time setting the elements of the multivibrator and the clays is simultaneously switched.

In the control unit circuit with a trigger (Fig. 2.15), the sweep generator operates in standby mode. By changing the operating mode of the control unit using the “stability” resistor, the trigger can be turned into a control device with one stable state, which corresponds to the forward sweep, which in this case operates in continuous mode.

enter exit

Rice. 2.15. Block diagram of a sweep generator with a trigger

The standby sweep generator allows you to set the duration of the forward stroke by switching only the timing elements of the clays. The duration of the pulses of the control unit is set automatically thanks to the trigger. The pulse coming from the synchronization and triggering device transfers the trigger from the initial state to the operating state. A linearly varying clay voltage is supplied to a comparing device, the output signal of which, when the LIN reaches a certain level, transfers the trigger to its initial state, after which the forward sweep stops. At the output of the trigger, rectangular control pulses are generated, the duration of which is determined by the sweep speed. At a constant comparison level, the range of um does not change when switching the timing elements of the clays. In this case, the sweep generator is “blocked,” i.e., it becomes insensitive to trigger pulses.

To obtain a still image of the signal under study on the oscilloscope screen, a voltage linearly varying with time (LIN) is applied to the vertical deflection plates. This voltage is generated in an electronic oscilloscope by a sweep generator.

Rice. 2.12. LIN chart

To ensure image stillness, the generator is synchronized by the process under study. A typical LIN graph is shown in Fig. 2.12. This voltage U(t) is characterized by the following parameters:

The duration of the forward stroke Tpr, the repetition period Tr, the duration of the reverse stroke Tobr, the recovery time. TU, amplitude Um and linearity of forward stroke bР:

, (2.16)

In general, oscilloscope sweep generators must generate voltage with high linearity of the forward stroke, short reverse stroke time, have a short recovery time, allow the possibility of synchronizing their operation, and have high efficiency. using power supply voltage.

Oscilloscope scan generators are divided: according to the speed of change of LIN - into slow sweep generators (Tr = 10s - 20ms); medium sweep speed (Tr = 0.1s –1 μs) and fast sweep (Tr < 1 μs); according to the permissible nonlinearity of the scan - into precise bР < 5%) and conventional (bР = 10% - 20%);. according to the method of synchronization by the process under study to continuous standby sweep generators; according to the principle of construction on generators with parallel (Fig. 2.13, a) and serial (Fig. 2.13, b) connection of the switching element.

In the first circuit (Fig. 2.13a), the element that stores the energy of the electric current during the forward stroke is charged from a source E , and discharged during the reverse stroke. In the second circuit (Fig. 2.13,b), during the forward stroke, this element is discharged through the discharge circuit, and during the reverse stroke it is quickly charged through the switching element. For synchronization, it is possible to control the moment of the beginning of a new period of operation of the generator by applying a synchronizing pulse, for example, to a switching element.

Currently, a large number of LIN generator circuits have been proposed. Characteristic of most of them is the presence of a capacitive integrating circuit. To obtain the most linear voltage possible at the output of such a generator, they try in one way or another to obtain, perhaps, a more constant capacitor charge current. In this case

, (2.17)

if i = const, then ULIN(t) = U0 ± Kt.

The constancy of the charging current can be achieved by using a high voltage compared to Um to charge the capacitance, i.e. using only the initial section of the exponential voltage to form the LIN (the main disadvantage of such a circuit is the low efficiency of using the power source voltage); the use of current-stabilizing two-terminal networks and compensation methods using positive and negative feedback. In modern oscilloscopes, LIN generators, built according to one of the listed methods, are controlled by rectangular pulses, the duration of which is equal to TP. For this purpose, each scan generator has a control device.

Enter exit

Rice. 2.14. Block diagram of a sweep generator with a control multivibrator

In the control unit circuit (Fig. 2.14) with a multivibrator in standby or self-oscillating mode, it produces rectangular pulses that are used to control clays. In standby mode, the multivibrator is triggered by short pulses coming from the synchronization and trigger device. In continuous mode (periodic sweep), the multivibrator is synchronized by a synchronization circuit with the signal under study. To ensure that the range of um LIN does not change when switching the sweep duration in the generator, the time setting the elements of the multivibrator and the clays is simultaneously switched.

In the control unit circuit with a trigger (Fig. 2.15), the sweep generator operates in standby mode. By changing the operating mode of the control unit using the “stability” resistor, the trigger can be turned into a control device with one stable state, which corresponds to the forward sweep, which in this case operates in continuous mode.

enter exit

Rice. 2.15. Block diagram of a sweep generator with a trigger

The standby sweep generator allows you to set the duration of the forward stroke by switching only the timing elements of the clays. The duration of the pulses of the control unit is set automatically thanks to the trigger. The pulse coming from the synchronization and triggering device transfers the trigger from the initial state to the operating state. A linearly varying clay voltage is supplied to a comparing device, the output signal of which, when the LIN reaches a certain level, transfers the trigger to its initial state, after which the forward sweep stops. At the output of the trigger, rectangular control pulses are generated, the duration of which is determined by the sweep speed. At a constant comparison level, the range of um does not change when switching the timing elements of the clays. In this case, the sweep generator is “blocked,” i.e., it becomes insensitive to trigger pulses.

How to choose

You need to imagine for what purposes and how often the device will be used, to study what signals it is intended for. Consider the number of points for simultaneous measurement, singleness or periodicity of oscillations. Sometimes Soviet-made devices are used. But getting precise tuning with them is difficult.

Number of channels

By the number of channels, oscilloscopes can be single-channel, simple (2-4 channels), advanced (up to 16 channels). Multiple channels allow you to simultaneously analyze incoming signals.

Power type

You can take the device with the battery with you when traveling. This is convenient for technicians who check equipment at its location. If you do not make field trips, it is better to take a network-powered oscilloscope, since it is more stable and reliable.

Sampling frequency

Sampling frequency is important for single-shot and transient measurements. The higher this parameter, the more accurate the signal image on the screen can be obtained.

Bandwidth

For simple studies of digital circuits and amplifiers, the optimal audio frequency is 25 MHz. For professional measurements, you need a device with this parameter - up to 200 or even up to 500 MHz. Modern communication lines operate at very high frequencies. The frequency of the signals under study should be 3-5 times less than the bandwidth.

Types of scans in a universal oscilloscope

A cathode ray tube is a vacuum glass flask containing an electron gun, deflection plates and a fluorescent screen. The electron gun consists of a heated cathode K,

modulator (grid) of the brightness of the light spot
M,
electrodes for focusing and accelerating the electron beam - focusing anode
A1
, accelerating anode
A2
and main anode
A3.

Block diagram of a universal oscilloscope.
The brightness of the CRT phosphor is adjusted by changing the negative voltage on the modulator M.

The voltage at the first anode
A1
focuses the electron flow into a narrow beam.
To give the electrons the speed necessary for the phosphor to glow,
a fairly high (up to 2000 V) positive voltage is applied
A2 For additional acceleration of electrons, anode A3 is used,
to which a high positive voltage is applied (up to 10 ... 15 kV).

The main purpose of an electron gun is to form a narrow electron beam, which, when it hits a luminescent screen, creates a luminous spot on the screen.

Operation of CRT deflection systems: an electron beam (beam) passes between two pairs of mutually perpendicular metal deflection plates: vertical deflection Y

and horizontally deflecting
X.
If voltage is applied to the deflecting plates, then an electric field will exist between them, which will cause the electron beam to deflect in one direction or another.
When voltage is applied to the vertical deflection plates, the spot will move along the Y
;
if voltage is applied to the horizontally deflecting plates, then the light spot on the tube screen will deflect along the X

Y
plates , and to the
X
plates sawtooth voltage, then under the combined influence of two voltages the beam will draw an oscillogram on the tube screen, reflecting the dependence of the input voltage on time.

Beam vertical deflection channel

serves to transmit

the test signal
uc(t)

the Y
input to Y . The vertical beam deflection channel contains an attenuator, a delay line and an amplifier
Y.

uc(t)
to be attenuated by a certain number of times, and the adjustable delay line provides a small time shift of the signal on the
Y
of the CRT relative to the beginning of the sweep voltage
Ux,
which is important for the standby mode.
Amplifier Y
provides

sufficient
(t).
This amplifier contains an input amplifier with a variable gain and a paraphase (with antiphase output signals of the same amplitude) amplifier, which ensures the position of the light spot in the center of the screen in the absence of the signals being studied. The signal from the calibrator is fed to the input of the first amplifier to set the specified gain. Main characteristics of the vertical deflection channel:

• upper limit frequency (about 100 MHz or more);

• sensitivity;

• input resistance (1... 3 MOhm) and capacitance (1... 5 pF);

• measurement error of voltage amplitude and time intervals is about 5-7%.

A switched isolation capacitor is also included in the input circuit of the vertical deflection channel, which allows, if necessary, to exclude the supply of a constant component of the signal under study to the oscilloscope input (the so-called “closed” input).

Horizontal beam deflection channel

serves to create a horizontally deflecting - sweeping
-
voltage
Ux
using the voltage of the scan generator or to transmit (via an attenuator and amplifier) ​​to the plates
X
the signal under study, supplied to the input
X.
The synchronization circuit (and start of the scan) controls the scan generator and ensures the multiplicity of the periods of the test signal and sweep. To obtain a still image, the start of the scan must be associated with the same characteristic point of the signal (front, maximum amplitude, etc.). This is achieved by synchronizing the sweep voltage with the signal voltage, so the sweep period must be equal to or a multiple of the period of the signal being studied:

Trazv = pTs,

where
n =
1, 2, 3, 4, ….

Scan

- the line that the beam draws on the screen in the absence of the signal under study as a result of the action of only one
sweeping voltage.
The process of linking the sweep to characteristic points of the signal is called
synchronization in
the self-oscillating mode of the generator and launching

in the standby mode. Synchronization and start of the scan are carried out by a special synchronization pulse supplied to the generator from a synchronization device.

The oscilloscope has two trigger modes: internal (Internal)

and
external.
With internal synchronization (switches
P1 and P2
are in position
1),
clock pulses are generated from the amplified input signal before it is delayed.
With external synchronization (switches P1
and
P2
are in position 2

, the synchronization signal

of the oscilloscope

The horizontal deflection channel is characterized by sensitivity and bandwidth, the indicators of which are almost two times less than in the vertical deflection channel. The main block in the horizontal deflection channel is a scan generator operating in continuous or standby mode. There are a number of specific requirements for the shape of the generator’s sawtooth voltage:

• the time of the reverse stroke of the beam should be much less than the time of the forward stroke, i.e.

Tobr << Tpr; otherwise part of the signal image will be missing;

• the scanning voltage during the forward stroke of the beam must be linear, otherwise the electron beam will move along the tube screen at different speeds and the uniformity of the time scale along the X

This may lead to distortion of the signal being studied.

Brightness control channel

(beam modulation by brightness) is designed to illuminate the forward path of the beam. Illumination is carried out by transmitting from input Z to the control electrode (modulator M) a CRT signal that modulates the beam flow and, consequently, the brightness of the phosphor. The circuit of this channel includes: an attenuator, a polarity changing circuit and amplifier Z. To form the required voltage level of the modulator, amplifier Z is used. The amplifier may have an additional input, which makes it possible to modulate the image in brightness with an external signal. The Z channel is also used to create luminance marks for measuring frequency and phase.

Calibrator

- a voltage generator that generates a periodic pulse signal with a known amplitude, duration and frequency for calibrating the oscilloscope, i.e. to ensure correct measurements of the parameters of the signal under study.

Cathode-ray tube

The method of obtaining a focused beam and the principle of beam control in an oscilloscope can be explained using the diagram shown in Fig. 5.6. As noted above, in a CRT a set of electrodes K, M, A1, A2, A3

called an electron gun, which emits a narrow beam of electrons. For this

Voltages are applied to the electrodes, the approximate values ​​of which are given in Fig. 5.6.

Main characteristics of CRT

— sensitivity, bandwidth, afterglow duration, screen area.
Tube sensitivity ST
=
LT /UT,
where
LT
is the deflection of the beam on the tube screen under the influence of voltage
UT
applied to a pair of deflection plates.
Typically S
t is 1 mm/V.

As the frequency of the signal being studied increases, the sensitivity of the tube decreases. The upper limit of the CRT bandwidth is set at a level where the sensitivity is approximately 0.7 of the nominal value. For universal oscilloscopes widely used, this frequency reaches 200 MHz. Modern oscilloscopes often use multi-beam tubes; To do this, increase the number of electrodes. It is more economical to use a single-beam oscilloscope in the mode of alternately supplying two signals to deflection plates (dual-channel oscilloscopes). Due to the afterglow effect of the tube and the properties of the human eye, the simultaneous image of two signals is observed on the screen, although they are presented alternately.

CRT light parameters include:

• the diameter of the light spot, which at optimal brightness determines the resolution of the CRT;

• maximum screen brightness; depends on the density of the electron beam and is regulated by changing the negative voltage on the modulator;

• screen glow color; the most commonly used colors are green and yellow, which ensure the least eye fatigue;

• afterglow time; To improve the visual perception of the oscillogram, the time the screen is illuminated must exceed the time it is exposed to electrons.

• If it is necessary to observe processes with a frequency of less than 10 Hz, use screens with an average duration of afterglow of up to 100 ms.

For photographic recording, a phosphor with a low (0.01 s) afterglow is more preferable. When studying slowly changing processes, screens with an afterglow of more than 0.1 s are used.

The scanning voltage during the forward stroke of the tube beam must be linear, otherwise distortions of the signal under study will appear (Fig. 5.7, a).

The nonlinearity of the working section of the forward sweep of the beam is characterized by
the nonlinearity coefficient:
the physical meaning of which is explained in Fig. 5.7, b.

Rice. 5.7. Signal waveform distortion:

A

— nonlinearity of the scan;
b —
— illustrations for the concept

nonlinearity coefficient; n - start of sweep; k - end of sweep

The nonlinearity coefficient expresses the relative change in the rate of voltage rise at the beginning and end of the sweep stroke; should be less than 1%.

Almost linear scanning on a CRT screen at a limited supply voltage level E

can be created in op-amp integrator circuits (Fig. 5.8).
An operational amplifier is an “ideal” device - therefore, in the circuit, the current i
0 = 0. Taking this equality into account, the currents
iR = uBX/R
and
ic = - CduBХ/dt.
Equating these currents and assuming
RC
= tа, after simple transformations, we obtain:

those. The device performs linear integration of the sweep voltage.

Storage oscilloscopes

When studying single pulses and periodic signals with high duty cycle, storage oscilloscopes are used, the basis of which are storage tubes.

Memorable

CRTs contain the same elements as the CRTs of a universal oscilloscope, and are also additionally equipped with a memory unit and an image reproduction system. The memory unit consists of two flat grid electrodes located parallel to the screen (Fig. 5.9). Directly next to the screen there is a target covered with a layer of dielectric. Another electrode in the form of a grid with a larger structure—a collector—is placed on top of the target.

The image is recorded by a high energy electron beam (recording beam). The electrons of the beam are deposited on the target, and the amount of charge is proportional to the beam current. When the beam moves on the target, a potential relief is created that repeats the shape of the oscillogram. After the signal ceases, the potential relief

the target remains for a long time. The recorded image is observed by a reproducing system consisting of a heated cathode K',

anode
A'2
and modulator
M'
(see Fig. 5.9).
The cathode of the tube creates a flow of low-energy electrons, the density of which is controlled by the modulator M'.
As a result, a wide defocused beam of electrons is formed, uniformly irradiating the target. The target potential is selected in such a way that, in the absence of a recorded image, slow electrons of the reproducing beam cannot pass through it. If there is a potential relief at these points of the target, some electrons pass to the screen, causing it to glow. An oscillogram appears on the screen, repeating the shape of the potential relief of the target. The recording is erased by applying a negative pulse to the collector, equalizing the potential of the target.

The storage CRT has three characteristic operating modes:

• observation of a signal without recording an image; there is a small positive voltage on the collector Ucol = +

50 V, the target has zero potential
Umish
= 0 and it is transparent to fast-moving electrons;

• recording mode; Ukol

= + 50 V, a positive potential
Umish
= 30 V is applied to the target, and the target becomes less transparent, so fast-flying electrons knock out secondary electrons and create a positive potential relief on the target, which remains for a long time;

• playback mode; the target potential again becomes zero U

mish
=
0, except for those places where the relief is recorded; the target is irradiated by a wide stream of slowly flying electrons from the reproducing system; for this stream, the target is transparent only in the places of the relief where the signal is recorded.

Memory CRTs are characterized by the following parameters:

• the brightness of the screen in playback mode is controlled by the voltage of the playback system modulator and can be high, since playback is performed continuously;

• image reproduction time is mainly limited by the resistance of the potential relief to ion bombardment; in modern CRTs, playback time can reach tens of minutes;

• the recording storage time is determined with the voltage removed from the CRT;

• recording speed characterizes the performance of a CRT in storage mode and is determined by the time required to create a potential relief of sufficient magnitude.

The latest models of storage CRTs have signal recording speeds from 2.5 to 4000 km/s.

Matrix indicator panel.

A new display device used in modern oscilloscopes with analog-to-digital and fully digital conversion of the signal under study is a matrix indicator panel. It is a collection of individual discrete emitters (liquid crystal, gas discharge, solid state, plasma, etc.) located in a certain way. In Fig. Figure 5.10 shows the design of a matrix gas-discharge panel.

The matrix panel contains two glass plates 1, on the outer surfaces of which thin conductive strips - anodes - are deposited. 2

and cathodes
3.
Anodes are placed on the front plate through which light radiation passes, so they are made transparent.

A dielectric matrix is ​​placed between the plates 4

with holes forming gas-discharge (or other) cells at the crossing points of the electrodes.
The panel is filled with a helium-neon mixture and sealed. The image of the signal under study is reproduced by alternately glowing gas-discharge cells. To do this, positive and negative ignition voltage pulses are supplied, respectively, from the panel control circuit to the anodes and cathodes of the plates. The number of the anode to which the ignition voltage pulse is applied determines the scan row, and the cathode number determines the column; at their crosshairs there is a luminous panel cell. This principle of scanning beam control is called matrix
in practice, it is implemented by digital methods and devices.

Advantages of matrix display panels: small dimensions and weight, low supply voltage; there are no geometric distortions in them, the luminous point is stable. Panels with internal memory have been developed that are capable of not only reproducing, but also storing the signal image. The digital control principle allows you to quite simply combine the signal image with the alphanumeric indication of its parameters on one screen. The disadvantages of matrix display panels include the complexity of the control circuit, relatively low resolution and low performance.

Storage digital oscilloscopes.

In recent years, digital storage oscilloscopes (DSOs) have found widespread use in measurement technology.
The block diagram of the CC is shown in Fig. 5.11. The oscilloscope can operate in two modes. If the dual switch P is in position 1,
then the circuit represents a regular universal oscilloscope, and if it is in position
2
, then the circuit operates as a digital signal center.

Rice. 5.11. Block diagram of a digital storage oscilloscope

Operating principle of the CC: test signal uc(t)

from input
Y
is fed through an attenuator to the information input of an analog-to-digital converter (ADC).
Ut
with a repetition period T
are also supplied from the controller (control device) to the ADC.
one

arrives
c
ti
)
into binary code U
ti), i.e.
. a set of code numbers 0 and 1. At the end of such a conversion, the ADC outputs a corresponding signal to the controller. In this case, the digital code is transferred to a specific cell of the storage device (memory).

During the study of the signal U(t)

the codes of its amplitudes
U(ti), U
(
ti
+
T
),
U
(
ti
+ 2
T
), etc. are accumulated in the memory;
there they can be stored for any time. To reproduce stored information at the command of the controller from the memory, the codes are read in the required sequence and at a given pace and fed to a digital-to-analog converter (DAC), which converts each code into its corresponding voltage. These voltages are transmitted through an amplifier to the Y
. The oscillogram is a set of luminous dots.

Advantages of the storage center: almost unlimited storage time of information; wide limits of its reading speed; possibility of slow playback of individual sections of the stored signal; bright and clear waveforms; the ability to process information digitally on a computer or using a built-in microprocessor. The disadvantage of the digital digital converter is that due to the relatively low speed of the ADC, most oscilloscopes can store signals with a frequency of no higher than 100 MHz. The CRT cathode ray tube also has a number of disadvantages: large dimensions (length), high supply voltages, and relatively low durability. Therefore, in recent years, matrix gas-discharge and liquid crystal display panels have been used in the control center.

Digital oscilloscopes

A digital oscilloscope allows you to simultaneously observe a signal on the screen and obtain numerical values ​​for a number of its parameters with greater accuracy than is possible by reading quantitative values ​​directly from the screen of a conventional oscilloscope. This is possible because the signal parameters are measured directly at the input of the digital oscilloscope, while the signal passed through the vertical deflection channel can be measured with significant errors (up to 10%).

On the screen of a modern digital oscilloscope, in addition to the oscillograms themselves, the state of the controls (sensitivity, sweep duration, etc.) is displayed. Provision is made for outputting information from the oscilloscope to printing and other functionality. However, this does not limit the capabilities of digital oscilloscopes. Interfacing digital oscilloscopes with microprocessors allows you to determine the effective value of the signal voltage and even calculate and display Fourier transforms for any type of signal. Digital oscilloscope devices perform full digital signal processing, so they typically use the latest panel displays.

In digital oscilloscopes, the measurement result is displayed in three ways:

• in parallel with observing the signal image on the screen, its numerical parameters are displayed on the display;

• the operator places light marks on the signal image on the screen in order to mark the parameter being measured, and by the number on the corresponding adjustment determines the value of the parameter of interest;

• use special indicators and a raster method for forming an image of the signals and digital information under study.

Modern digital oscilloscopes automatically set the optimal image size on the tube screen. Below are the parameters of a modern digital automated oscilloscope, which is a typical representative of this class of devices.

The block diagram of a digital oscilloscope contains: an input signal attenuator; vertical and horizontal deflection amplifiers; amplitude and time interval meters; signal and meter interfaces; microprocessor controller; sweep generator; synchronization circuit and cathode ray tube.

Specifications

typical modern digital oscilloscope:

bandwidth 0... 100 MHz; screen size 80 x 100 mm; digital measurement error 2… 3%.

Functionality:

automatic setting of image sizes; automatic synchronization; difference measurements between two marks; automatic measurement of peak-to-peak, maximum and minimum signal amplitude, period, duration, pause, pulse rise and fall; entrance to a public channel.

From the block diagram, it is clear that the amplitude and time parameters of the signal under study are determined using meters built into the device. Based on the measurement data, the microprocessor controller calculates the required deflection and sweep coefficients and, through the interface, sets these coefficients in the hardware of the vertical and horizontal deflection channels. This ensures constant image dimensions vertically and horizontally, as well as automatic signal synchronization. The microprocessor controller also polls the position of the front panel controls, and the polling data, after encoding, is again sent to the controller, which, through the interface, turns on the appropriate automatic measurement mode. The measurement results are displayed on a separate light display (it can be built into the tube screen), and the amplitude and time parameters of the signal are displayed simultaneously.

Control questions

1. For what purposes are oscilloscopes used?

2. What blocks are included in the block diagram of a universal oscilloscope? Their purpose?

3. Why is oscilloscope scan synchronization used?

4. List the main types of synchronization.

5. For what purposes are amplitude calibrators used in oscilloscopes?

General information

Visual or visual reproduction of the vibration shape is an important task in radio engineering measurements, since the shape allows one to immediately evaluate many vibration parameters. One of the main instruments used for visual observation and study of the shape of electrical signals is the oscilloscope (from the Latin “oscillum” - oscillation and the Greek “grapho” - write). Most modern oscilloscopes in use are equipped with a cathode ray tube (CRT) and are called cathode ray oscilloscopes. At the same time, in the latest developments of oscilloscopes, matrix display panels are used as display devices - gas-discharge, plasma, liquid crystal, solid-state, etc.

Cathode ray oscilloscope

- a measuring device for visual observation in a rectangular coordinate system of electrical signals and measurement of their parameters. Using an oscilloscope, periodic continuous and pulsed signals, non-periodic and random signals, single pulses are observed and their parameters are assessed. Most often, an oscilloscope is used to observe the dependence of voltage on time, and, as a rule, the time axis is the abscissa axis, and the ordinate axis reflects the signal level. From the images obtained on the oscilloscope screen, you can measure amplitude, frequency and phase shift, parameters of modulated signals and a number of other indicators. Instruments have been created on the basis of an oscilloscope to study the transient, frequency and amplitude characteristics of various radio devices.

For many purposes, various types of cathode ray oscilloscopes have been developed and used: universal, high-speed, stroboscopic, storage, special, etc. Differing in technical characteristics, circuit and design solutions, these oscilloscopes use the general principle of obtaining oscillograms. The ability to observe the shape of the signal under study and simultaneous measurement of its parameters and characteristics puts the electron beam oscilloscope in the category of universal devices.

The most widely used universal oscilloscopes are

making it possible to study electrical signals with a duration from a few nanoseconds to several seconds in the amplitude range from fractions of millivolts to hundreds of volts, as well as to measure the parameters of such signals with an error acceptable for practice (5...7%). The bandwidth of the best universal oscilloscopes is 300...500 MHz or more.

Repeated short-term processes are examined using stroboscopic oscilloscopes.

Based on their operating principle, stroboscopic oscilloscopes are classified as time-scale conversion devices and are characterized by high sensitivity and a wide (up to 10 GHz) operating band.

Storage oscilloscopes,

having special CRTs, they have the ability to save and reproduce the signal image for a long time after it disappears at the input. The main purpose of storage oscilloscopes is to study single and rarely repeated processes. Storage oscilloscopes have almost the same characteristics as general-purpose oscilloscopes, but have expanded functionality.

Special oscilloscopes

equipped with additional blocks for specific purposes. These include television oscilloscopes, which allow you to observe a video signal of a given image line, and digital ones, which make it possible not only to observe the signal, but also to transfer it in digital form to a computer for further processing. Special oscilloscopes are equipped with multimeters that allow you to measure voltages, currents and resistances, as well as devices for studying the current-voltage characteristics of semiconductor devices.

Based on the number of signals simultaneously observed on the CRT screen, single-channel

and
multichannel oscilloscopes.
Combining images of several input signals on the screen is realized either using a special multi-beam tube, or by periodically switching signals to different inputs using an electronic switch.

Universal oscilloscopes

Let's consider a simplified block diagram of a universal oscilloscope shown in Fig. 5.1. In the circuit of this oscilloscope, in addition to the CRT, the following functional blocks can be distinguished: channels of vertical and horizontal deviations, a synchronization and sweep trigger device, a beam modulation channel, auxiliary devices, and a power supply. In an oscilloscope, the electrical signal under study is supplied through the vertical deflection channel to a vertically deflecting CRT system, and the horizontal deflection of the electron beam of the tube is carried out using the horizontal scan voltage.

A cathode ray tube is a vacuum glass flask containing an electron gun, deflection plates and a fluorescent screen. The electron gun consists of a heated cathode K,

modulator (grid) of the brightness of the light spot
M,
electrodes for focusing and accelerating the electron beam - focusing anode
A1
, accelerating anode
A2
and main anode
A3.
Block diagram of a universal oscilloscope

The brightness of the CRT phosphor is adjusted by changing the negative voltage on the modulator M.

The voltage at the first anode
A1
focuses the electron flow into a narrow beam.
To give the electrons the speed necessary for the phosphor to glow,
a fairly high (up to 2000 V) positive voltage is applied
A2 For additional acceleration of electrons, anode A3 is used,
to which a high positive voltage is applied (up to 10 ... 15 kV).

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Oscilloscope setup

Before using a new device, it is calibrated using the rectangular pulse generator located on the housing. The signal probe is connected to the calibration output, and a “saw” appears on the screen - a zigzag line. It is necessary to check the operation of all functions and controls.

Oscilloscopes are now regularly used in the electronics industry. There is a large selection of devices that allow you to monitor the parameters of electrical vibrations. Neither a professional engineer nor an ordinary radio electronics enthusiast can do without an oscilloscope.

Building an input attenuator

The second, and perhaps the most important thing that I would like to explain is the construction of the input attenuator (divider). Not everyone understands (observation from practice) that the input attenuator does not create pre-emphasis for the subsequent amplifier, IT ITSELF IS A SOURCE OF DISTORTION .

The blockage and splash are largely his “merit.” If you look at what is happening in the input circuits of the oscilloscope with an external device, you will find out that most of the distortion appears precisely due to the input divider. But the capacitors in it are precisely used for tuning. Remove all these fillets and splashes.

Amplifiers also have their own distortion, which can be noticeable on the oscillogram, but it is much less if the amplifier is properly built.

Hence the conclusion - the input attenuator cannot be full-fledged if EACH ITS STAGE does not have its own frequency correction circuits.

Those. input attenuator built according to the circuit

(I gave it earlier) Can be set up normally (example in the picture)

(the angles of the rectangular pulses are quite even) only on one of the ranges (switch positions S1)

On the other two we will observe this

Or that

The problem with this circuit is that capacitor C1, placed in parallel with resistor R1, will be set to one specific switch position.

No one will rebuild it every time we switch bands, and it’s inconvenient. Not to mention the fact that its value can change by two orders of magnitude, i.e. You can't do without re-soldering.

I’m not ready to completely rule out that in some cases normal configuration is possible, but I couldn’t do it. Neither theoretically (on simulators), nor practically. In addition, from experience, input attenuators built according to the same circuit, using parts from the same batch, operating on the same type of op-amp, will not necessarily have the same capacitance of the frequency correction capacitors. Those. You shouldn’t count on being lucky right now.

Different mounting capacitance, slightly different op-amp parameters (there is always technological variation), different probes for the oscilloscope (This is also important to mention. The oscilloscope must be configured with “native” probes ).

Therefore, the only normal way to construct input circuits is to follow the principles laid down in diagram 2

The important difference is that not only the lower resistor of the divider, the lower arm, as I usually say, switches, but also the upper one.

Those. A switch with TWO GROUPS OF CONTACTS is required. And in fact, these are several different and independent divisors. And we switch them completely, thus eliminating the influence on each other.

And only this method can guarantee the possibility of adjustment in any position of the switch.

Those who have read the previous parts of my reviews are probably surprised why I repeat some of the pictures and some of the information.

But the fact is that I have already encountered the use of defective input divider circuits in homemade products by people who have definitely read the previous parts specifically dedicated to this.

A faulty input attenuator has already ruined several serious and interesting developments that I have come across. It also destroyed some of the entry-level oscilloscopes produced by the industry for radio amateurs. It shouldn't be this way.

It is important to understand that a normal input attenuator MUST have switching with two groups of contacts.