Oscilloscope... no tube. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Electrical signal. Can you see him?

"Of course," you say. For this, there are special devices - oscilloscopes, the main part of which is a cathode ray tube. Of course, you know how the image on its screen is obtained. Therefore, we will not dwell on this issue.

Is it possible to do without a tube?

It turns out you can. If instead of a tube, an electro-optical indicator (EOI) is used. It is found in almost all tube radios and tape recorders.

A block diagram of such an oscilloscope is shown in Figure 1.

Rice. 1. Block diagram of the oscilloscope: 1 - EOI, 2 - disk, 3 - electric motor.

Let us place an opaque disk with narrow radial slots in front of the EOI screen and rotate it uniformly at such a speed that the slot has time to pass from one edge of the luminous strip to the other in a time equal to one period of the input signal. The screen will show the envelope of one oscillation of the input signal (Fig. 2). And since the input signal is periodic, the next slit, located at a distance of the width of the EOI screen from the first one, will give exactly the same image of the envelope (due to the inertia of human visual perception, we will see a fixed picture of the input signal shape).

Fig.2. This is how mechanical scanning works: 1 - EOI, 2 - screen, 3 - disk with slots.

To obtain a stable image, high stability of the disk rotation speed is required. Therefore, the electric motor, on the shaft of which the disk is located, is powered by a stabilized power source.

Brief technical data

Input impedance - 300 kOhm
Sensitivity - 200 mm/V
Bandwidth at unevenness 6dV - 50-13000 Hz
Sweep frequency - 0-1500 Hz
Image size at 15% non-linearity - 6X6 mm
Maximum image size - 15 X 6 mm
Range of measured voltages - 0,01-10 V
Range of measured frequencies - 100-10000 Hz
Power consumed from the network - 6 W
Dimensions - 164x148X84 mm

Schematic diagram oscilloscope - in Figure 3. Block 1 has two amplification stages on transistors T2, T3 and an emitter follower (T1) to increase the input resistance. T1 and T2 are connected to each other by direct current. The output stage must provide an undistorted signal with an amplitude of 7-8 V, so it uses a transistor with a high value of the collector-emitter junction voltage. All cascades are thermally stabilized, have auto-tuning of the operating mode, which allows you to install transistors in them without preliminary selection. To prevent interference from the electric motor, power is supplied to the amplifier through the filtering circuit Dr1, C6 and is stabilized by diode D1.

Fig.3. Schematic diagram of an oscilloscope with an electronic-optical indicator.

Block 2 is an adjustable stabilizer for the speed of rotation of the motor shaft, made on transistors T4, T5. If the load on the motor shaft increases (friction in the bearings, disc vibrations), the rotation speed drops. As a result, the current through the resistor R25 increases, the bias voltage at the base of T4 increases. This causes an increase in the collector current of the latter, and hence the base current T5. The resistance of the emitter-collector section T5 decreases, the voltage on the motor increases, and the rotation speed is restored to Reducing the load causes a reverse process.

When the supply voltage changes, the current through the chain D2, R24 maintains such a mode T4, T5, in which the voltage on the motor remains constant. The sweep frequency is set using variable resistors R18 "Smooth" and R19 "Rough".

The calibrator is made according to the scheme of a symmetrical multivibrator with a variable pulse repetition rate. The amplitude of the input signal is measured by comparing it with the amplitude of the voltage from the calibrator. When determining the frequency, the device is used as a null indicator.

The power supply provides voltage: 280 V, 12 V and 6,3 V. The 12 V voltage regulator is made on a T6 transistor and a D3 diode according to a typical circuit.

Construction and details

The circuit of the device is mounted on three printed circuit boards (Fig. 4-6) made of foil-coated fiberglass or getinaks with a thickness of 2-3 mm.

Unit 1 PCB

Unit 2 PCB

Unit 3 PCB

The case of the device is made of a corner 10X10 mm. Its walls are made removable. On the front panel (see Figure 7) there is an electric motor and a nine-pin lamp socket.

Fig. 7

The screens dividing the body into three compartments are made of 1 mm thick sheet metal. To avoid accidental electrical short circuits, the screen walls are pasted over with thick paper. The false panel is made of 2 mm thick getinax and is attached to the front wall with four screws.

Attachment to the electric motor axis, clamping nut and reamer disk are shown in Figure 8. On one side, the disk is pasted over with black paper (for packaging photographic materials), and 44 radial slots 0,3 mm wide are cut into it. The device is equipped with a rotary handle made of steel tube Ø 0 mm.

Fig.8. Nozzle, clamping nut and disc.

The power transformer Tr1 is wound on a core Ш16Х24 mm. Winding I contains 1750 turns of wire PEV-1 0,15, 11 - 1950 turns of PEV-1 0.C 111-170 turns of PEV-1 0,35, IV-54 turns of PEV-1 0,25.

The winding of the inductor Dr1 is wound on a core ШЗ X 6,3 and contains 500 turns of wire PEV-1 0,15. Fixed resistors R16 and R30 - MLT-1, R25 are 45 cm of PEV-1 0,1 wire wound around the body of the MLT-0,5 resistor with a resistance of at least 50 ohms. The remaining resistors are MLT-0,25 or ULM. Variable resistors: R40 - • SPO-0.5-V, the rest - SPO-0.5-A. Electrolytic capacitors С5, СЮ, С11, С12, С13, С14-К50-6; C2, C3, C6 - IT-1; C1, C9 - EM. Capacitors C7, C8, C15, C17, C19 - MBM; C16, C18 - K10-7B. Capacitors C7, C19 are designed for an operating voltage of 300 V. Naturally, all parts can be replaced with others with operating voltages not lower than those indicated in the diagram.

Transistors MP41 can be replaced with MP39 - MP42; MP26B - on MP26, ML26A; MP38 - on MP35, MP37; P214 - on P213 - P217, P201 - P203.

The 6EZP lamp can be replaced by 6E2P by connecting the control grids together.

The DRV-0,1 electric motor can be replaced by any small-sized DC motor with a supply voltage of 10 V. In particular, on the DP-13 - a toy micromotor. It must be placed in a mild steel screen with a thickness of 0,5-1 mm and amortized.

As Dr1, you can use a ready-made transformer from a transistor radio.

Switches B1 - B5 - microtoggle switches MP3-1, MP-7. The body of the device must be connected to a common "positive" bus and grounded. Particular attention should be paid to the shielding of the input and output circuits of the amplifier.

Setting

To set up the oscilloscope, the following instruments are required: an avometer, a sound generator, an electron beam oscilloscope.

They check the installation of the rectifier and, having connected block 2, turn on the device. The modes of the elements should not differ from those indicated in the diagram by more than ± 20%. When turning the R23 knob, the height of the luminous sectors of the lamp should change from zero to maximum. If this cannot be done, select the value of R21. Then set the R18 engine to the left position according to the diagram and, turning the "Frequency coarse" knob, control the voltage on the electric motor. It should vary from zero to maximum (for a given motor).

The speed of rotation of the electric motor is checked with a disk installed on its axis.

Then they start setting up block 1. The disk is stopped, switch B1 is set to the "1: 1" position, and an oscilloscope is connected to the collector TK through a capacitor with a capacity of 0,1 μF. A signal from the ZG with a frequency of 400-1000 Hz and a voltage of 100-200 mV is fed to the input of the device. At the output of the amplifier, the signal amplitude is 7--8v. Otherwise, it is necessary to select the values ​​of resistors R5 and R13. Then, by switching B1 to the "1:20" position, using the resistor R2, they achieve that the input signal level is attenuated by 20 times. It remains to check the overall gain. The signal from the ZG is reduced to 10 mV, and the adjustable device is set to the maximum sensitivity mode (B1 in the "1: 1" position, the R3 slider is in the upper position according to the diagram). The sector height on the EOI screen should increase by 2 mm at the initial sector height of 5 mm. This corresponds to a sensitivity of 200 mm/V.

Gradually increasing the frequency of rotation of the disk with the "Sweep coarse" knob, one achieves an image on the screen of the full period of oscillations of the input signal with a frequency of 400-800 Hz. Fine-tuning is done with the "sweep smoothly" knob. It remains to connect block 3, calibrate it, and the device is ready for operation.

First, they check if there is a generation. B4 and 85 are closed. The height of the luminous sectors of the EOI should increase sharply and not disappear in any position of the R40 resistor slider and the VZ switch.

Then, a signal with a voltage of 80-100 mV is fed from the sound generator to the input of the oscilloscope. The disk will stop, B4 is open. The "Gain V" knob sets the voltage level so that the EOI sectors almost converge. Close B4. Smoothly change the frequency of the CG, find the position when the luminous stripes on the screen diverge sharply. This happens when the frequencies of the generator and the calibrator coincide. Selecting the values ​​of the resistor R39 and capacitors C15-C18, set the limits of frequency change 100-1000 Hz, 1000-10000 Hz (first and second ranges) and apply intermediate divisions to the frequency scale of the calibrator.

Next, a frequency of 1000 Hz with an amplitude of 1 V is set on the ZG. The height of the luminous sector is set equal to 10 mm with the "Gain Y" knob. ZG is disconnected from the input of the device. B4 is open, the disk is stopped, and the slider of the resistor R32 is in the upper position. A shielded wire connects the output of the calibrator to. amplifier input "U". The calibrator is tuned to a frequency of 1000 Hz and turned on. If the sector height is different from 10 mm (voltage amplitude 1 V), the resistance of the resistor R31 is selected. By changing the value of the output voltage of the generator, intermediate divisions are applied to the scale of the resistor R32.

Now we not only set up an oscilloscope, but also learned how to use it.

Author: V. Prokhorin, pos. Chernogolovka, Moscow region; Publication: N. Bolshakov, rf.atnn.ru

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