Balance modulator on varicaps. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Balance modulator on varicaps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Modulators

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In amateur shortwave equipment, balanced modulators based on semiconductor diodes, built according to a ring circuit, have found wide application. They provide deep suppression of signals, have a wide frequency range.

However, when generating an SSB signal using a filter method, these advantages are not used. Indeed, there is no need to suppress the modulating low-frequency signal, since the modulator is always followed by a narrow-band filter. There is no need for a broadband modulator.

On the other hand, the use of diode ring balanced modulators leads to unjustified complication of the circuit. The fact is that both inputs of the modulator are low-resistance, so you have to use cathode or emitter followers. In addition, in order to avoid non-linear distortion, diode modulators cannot be supplied with a signal whose value exceeds 100-150 mV. Given the losses in the diodes and balancing resistors, the output signal should not be expected to exceed 10-15 mV. Therefore, an additional amplifying stage is required after the modulator.

The figure shows a diagram of a balanced varicap modulator used in a tube-transistor transceiver (see "Radio", 1974, No. 8) and showed good results.

The capacitance of series-connected varicaps, together with the inductance of the primary winding of the transformer Tr1, forms an oscillatory circuit. Capacitor C3 serves to tune it into resonance with the input high-frequency signal. Resistor R5 regulates the bias voltage applied to the varicaps.

Balance modulator on varicaps

If the voltages on both varicaps are equal, their capacitances will be equal. Then the RF currents flowing through the primary winding of the transformer compensate each other, and there is no voltage on the secondary winding of the transformer. Resistor R2 is intended for the final balancing of the modulator.

When a low-frequency signal enters the input, the balance is disturbed. With its positive half-wave, the capacity of the D1 varicap increases, D2 decreases. With a negative half-wave, the capacitances of the varicaps change in the reverse order. Accordingly, the values of high-frequency currents also change according to the law of the modulating low-frequency signal, and a modulated high-frequency signal with a suppressed carrier appears on the secondary winding of the transformer.

Carrier suppression is quite deep. During a year and a half of operation of the transceiver, it remained within 50 dB.

Both modulator inputs are high impedance, so no emitter followers are required. With a HF signal of 1 V and a LF signal of 2,5 V, the output signal amplitude is 350 mV at a load resistance of 600 ohms. Nonlinear distortion is not noticeable.

Transformer Tr1 is wound on a ferrite ring core 50VCh-2 K7x4x1,2. If the modulator is designed for a frequency of 5,5 MHz, the primary winding should contain 38 turns of PEV-2 0,16 wire with a tap from the middle. The secondary winding contains 9 turns of the same wire. If the load of the modulator is high-resistance (field-effect transistor or lamp), the number of turns of the secondary winding should be increased.

Varicaps D901, D902 and KV102 with any letter index can be used in the modulator.

Author: V. Žalnerauskas (UP2NV), Kaunas; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Knots of amateur radio equipment. Modulators.

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