RF signal converter. Scheme, description

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RF signal converter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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This device allows you to shift the spectrum of the audio signal in frequency in one direction or another. With it, an adult can imitate a child's voice and vice versa. Working in conjunction with the built-in sub-low frequency vibration generator, it creates an echo effect that is as clear as when using a reverb.

The transducer can be used in puppet theaters, for sounding cartoons, in variety ensembles, etc. Very interesting sounds are formed when the transducer works with an electric guitar and other EMPs, while you only need to take into account that the frequency shift changes the tone of the EMPs.

(click to enlarge)

The input low-frequency signal is amplified by a linear amplifier on the A1 chip (see figure) and fed to the balanced modulator V3-V6. A sinusoidal RF voltage is applied to it from a quartz oscillator on transistor V2. In the absence of an input low-frequency signal, there will be no voltage at the mixer output (on coil L4). When a low-frequency signal appears, an RF signal enters the base of the transistor V7, containing mainly two frequencies: the total FHF + FLF and the difference FHF-FLF, the amplitudes of which are directly proportional to the amplitude of the low-frequency signal.

The electromechanical filter Z2 will pass only the Fhigh+Flow signal to the balanced mixer V10-V13. This mixer also receives voltage from a tunable oscillator made on transistor V17 (a buffer amplifier is assembled on transistor V15). At the output of the mixer, the C20R25C21 filter suppresses the total signal, and the difference - low-frequency - passes to the input of a linear amplifier on the transistor V14.

The frequency of the tunable oscillator is changed by the V18 varicap by applying voltage to it from the variable resistor R26. Assume that a signal with a frequency of 1 kHz is applied to the input of the set-top box. The crystal oscillator operates at a frequency of 500 kHz. At the output of the electromechanical filter, we obtain a signal with a frequency of 501 kHz. Assume that the tunable oscillator is tuned to 499 kHz. Then, at the output of the filter C20R25C21, the signal frequency will be equal to 501-499 = 2 kHz, i.e., twice as high as that of the input. If the tunable frequency is set equal to 500,5 kHz, then at the output we will receive a signal with a frequency that is half that at the input. By controlling the frequency of the tunable RF generator, it is possible to change the frequency of the low-frequency signal at the output over a wide range. In this case, a pitch shift of a person’s voice, the sound of a guitar string, or another signal applied to the input of the device from a microphone, pickup, etc. is formed.

The transducer operates in a manner similar to SSB sports radio devices, so more detailed information about it can be found in the relevant literature.

The converter also contains a sinusoidal vibrato generator on transistors V8, V9. By applying a vibrato signal to a V18 varicap of a tunable oscillator, you can obtain frequency modulation of the output low-frequency signal.

A well-stabilized and filtered voltage is required to power the converter.

Instead of the electromechanical filter (Z2) indicated on the diagram, you can use EMFDP-500V-9D. All coils are used ready-made - IF filters from the Sokol pocket receiver. For stable operation of the vibrato generator, you should choose a transistor V8 with a coefficient h21e of at least 300, and V9 - at least 60.

It is possible to set up the converter well only using instruments (LF and HF generators, wave meter, oscilloscope, RF voltmeter, etc.). First, the LIC3 circuit is tuned to the maximum signal on the L2 coupling coil and the wavemeter (or radio receiver) is used to make sure. that the generation frequency is 500 kHz.

The low-frequency signal is turned off and, by adjusting the resistor R6, the minimum high-frequency signal is achieved based on the transistor V7. If we now again apply a low-frequency signal to the input, then an RF signal will appear on the base and on the collector of this transistor, similar in shape to that shown in the diagram. Adjust the circuit L3C11C12 to the maximum of this signal.

Then the oscilloscope (or tube voltmeter) is connected to the EMF output and the maximum signal is achieved by selecting the capacitor C15. The generator output signal is reduced to zero, the resistor R26 slider is set to the middle position and the L6 coil is adjusted to the maximum signal on the L5 coil.

In this case, the frequency of the tunable generator must be such that the limits of its regulation are sufficient for practical use.

The last step is to set up the vibrato generator. To do this, the output of the capacitor C28, left according to the scheme, is turned off and a low-frequency generator is connected to the connection point of the capacitor C16 and the resistor R10. By setting the frequency to 10 ... 50 Hz and gradually increasing the voltage, a selection of resistors R14 and R19 achieve a symmetrical limitation of the signal at the output of the generator (on resistor R20). After that, the circuit of the capacitor C28 is restored. The frequency of the vibrato generator is set by the tuning resistor R11.

In the lower position of switch S2 according to the diagram, it is possible to "mix" the original signal into the converted signal. In this case, the level of the original signal is set by a tuning resistor R28.

When repeating the converter, you should follow the rules for mounting high-frequency devices.

Author: V. Ketners, Ogre; Publication: N. Bolshakov, rf.atnn.ru

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