A simple signal generator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple signal generator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The generator circuit is shown in the figure. Its work is based on the principle of shock excitation of oscillations in a resonant circuit. This principle is based, for example, the operation of the mechanism of the piano, piano. The sound in these instruments is caused by hitting a special hammer on a string tuned to a certain frequency.

Simple Signal Generator

In our device, the oscillatory circuit is formed by a coil L1 (or L2) and a capacitor C1. The role of the hammer striking the oscillatory circuit is performed by the generator of relaxation oscillations on the MTX-90 thyratron. The control electrode of the thyratron is connected to the cathode, that is, it works like a diode.

Such a gas-filled diode has a remarkable property. As long as the voltage on its electrodes is low (less than the so-called ignition voltage), it does not conduct electric current. If you increase the voltage, the diode "lights up" and conducts electricity. At the same time, inside, between the electrodes, the neon filling it will glow with red light.

Having caught fire, MTX-90 retains conductivity even at voltages lower than the ignition voltage. The difference between the ignition and extinction voltages can be quite large - 20-150 V.

In order to ensure intermittent ignition of the thyratron, capacitor C3 is connected in parallel with it. It charges through resistors R1 and R2 rather slowly, and discharges quickly through the thyratron. The current flowing through

resistors R1 and R2, cannot keep the thyratron burning. When the voltage across the capacitor falls below the turn off voltage, the thyratron will turn off. The capacitor will charge again. By changing the value of the resistor R2, it is possible to change the thyratron flash frequency from 600 to 2000 times per second...

Together with the capacitor C3, the capacitor C2 is charged and discharged. It is connected in parallel with C3 through an oscillatory circuit L1C1 or L2C1.

When the thyratron lights up, capacitor C2 is discharged through the circuit; damped electrical oscillations occur in the circuit. This process is repeated 600-2000 times per second. The frequency of natural oscillations of the circuit depends on the value of the inductance of the coil L1 (L2) and the capacitance of the capacitor C1. In our case, it varies within 150-415 or 520-1600 kHz, depending on the position of switch P1.

The device is connected to the receiver under study using a magnetic antenna, on the rod of which coils L1 and L2 are wound.

The device is powered from a 220 V AC mains through a rectifier. It is assembled according to a half-wave transformerless circuit. The use of a transformerless circuit is safe, since the device does not have output terminals connected to the network wires.

Construction and details. Structurally, the device is best designed using a housing, a variable capacitor, a magnetic antenna and a switch of some small-sized transistor radio receiver with a range of long and medium waves. All parts of the circuit are placed in the case. Thyratron MTX-90 should be placed so that the end of its cylinder is visible. The glow of the thyratron will serve as an indicator of inclusion.

Instead of MTX-90, you can use a dinistor, for example, KN-102D, KN-102Zh, KN-102I. The power of the device in this case will be greater, but to indicate the inclusion of the device, you will have to put a separate neon light bulb.

If we restrict ourselves to only one modulation frequency, then the resistor R2 can be omitted, and the required value of R1 can be selected when setting up the device.

It is better to take a ceramic or mica capacitor C3, with a capacity of 910-1300 pF, with an operating voltage of at least 400 V. The magnetic antenna and contour coils are taken ready-made from an industrial receiver. Variable capacitor C1 should have a maximum capacitance of 250-500 pf.

Device setup. You can tune the device using a transistor receiver with a magnetic antenna. The device is placed next to the receiver. The receiver is tuned to a frequency of 150 kHz (2000 m). The variable capacitor of the device is placed in the position of maximum capacity. By moving the coil L1 along the rod of the magnetic antenna of the device, achieve the maximum sound volume of the receiver. If it is very large, the receiver should be moved away from the device.

Before fixing the coil L1 on the rod of the magnetic antenna of the receiver, you should make sure that the frequency of the device corresponds to 150 kHz. To do this, detune the receiver to either side of 150 kHz. The volume of the signal at the output of the receiver in both cases should fall.

Then set the receiver arrow to the next calibrated division. By changing the capacitance of the instrument's capacitor, tune it to the frequency of the receiver. This frequency value is marked on the scale of the instrument. In the same way, the remaining divisions of the instrument scale are found.

Calibration of the medium wave scale should start at 520 kHz.

A properly tuned device should cover the range of at least 150-415 and 520-1600 kHz.

Working with the device when setting up the receiver. On instrument and configurable

receiver, turn on the appropriate band. Place the device as close as possible to the magnetic antenna of the receiver. Set the receiver capacitor to the middle position. By changing the setting of the device, make sure that the tone of its modulation is heard in the speaker of the receiver. If the sound is very loud, move the unit away from the receiver's magnetic antenna. By changing the device setting in the direction of decreasing its carrier frequency, adjust the receiver to it.

In this case, three cases are possible:

1. The receiver receives a frequency of 150 (520) kHz; its variable capacitor is in the position of maximum capacity - the inductance of the loop coil is chosen correctly.

2. With the maximum capacitance of the capacitor, the receiver is tuned to a frequency greater than 150 (520) kHz - the inductance of the circuit is small, and it should be increased.

3. The circuit is tuned to a frequency of 150 (520) kHz, not at the maximum capacitance of the capacitor - the inductance) of the circuit is large, and it must be reduced.

Within small limits, you can change the inductance of the circuit by moving (the coil along the rod of the magnetic antenna.

After tuning the low-frequency end of the range, check the setting of its high-frequency end. If, when tuning the receiver to a frequency of 415 (1600) kHz, the capacitance of the capacitor is not minimal, an additional capacitor must be connected in parallel with the loop coil.

Author: E.Tarasov

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