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Balanced modulators of the transmitting path of transceivers are most often performed on diodes, transistors or microcircuits. The use of varicaps in these devices provides significant advantages, which are described in the proposed article.

A very interesting modulator for the transmission path of transceivers, especially economical direct conversion transceivers, is a passive modulator made on varicaps - semiconductor diodes with a controlled capacitance of the p-n junction. Being reactive elements, they almost do not absorb energy, so the modulator made on them is characterized by increased efficiency. In addition, it has a high impedance at the low-frequency input, which makes it easier to build a microphone amplifier. In this case, the modulator provides a significant amplification of the power of the low-frequency signal.

The literature describes schemes of balanced modulators on varicaps with balancing transformers [1]. The proposed modulator does not contain winding elements, since the balancing transformer is replaced by a high-frequency phase-inverted cascade on a transistor.

The modulator circuit is shown in the figure.

The device operates as follows. Varicaps VD1 and VD2 are connected in series between the emitter and collector of transistor VT1. The DC voltage available between these transistor electrodes also serves as a bias voltage for the varicaps. The variable resistor R5 is a balancing element of the modulator and divides this voltage approximately in half. When you change the position of the resistor slider, the bias voltage on one varicap increases, on the other it decreases. Adjustment ensures that the capacity of the varicaps is the same.

The values ​​of the resistors R1 and R2 of the divider in the base circuit of the transistor VT1 are chosen so that the voltage between the collector and emitter in the absence of a signal is about 7 V at a supply voltage of 15 V.

When an RF voltage is applied to the base of the transistor VT1, through a decoupling capacitor C1, the cascade operates as a phase inverter and equal but antiphase RF voltages are emitted on resistors R3 and R4, since their resistances are equal, and almost the same current flows through them (current bases can be neglected). In this case, the voltages on VD1 and VD2 are also the same, therefore, the output RF voltage is zero.

The modulating sound signal is fed through the decoupling circuit R6C4. With a positive half-wave, the capacitance of the varicap VD1 increases, and VD2 decreases, with a negative one, vice versa. The balance of the bridge formed by resistors R3, R4 and varicaps is disturbed. At the output, half-waves of the DSB modulated signal with suppressed carrier appear.

Without a balancing transformer, this modulator is free from the shortcomings of the latter (labor-intensive winding, inaccurate balancing, pickup, etc.) and has amplification not only at low, but also at high frequencies, since the input impedance of the transistor stage is significantly higher than the output. This allows you to connect the modulator to the local oscillator without additional buffer stages, and when using quartz stabilization, even combine the modulator with the local oscillator [2].

In this modulator, the inclusion of a balancing resistor R5 deserves attention. Usually it is connected between the power source and the common wire, but here it is connected between the emitter and the collector of the phase-inverted stage. This is done for the following reasons:

1. When the transistor mode changes, caused by a change in temperature and other parameters, the voltages on the collector and emitter change, which in a device assembled according to the usual scheme can lead to an imbalance in the cascade. In the proposed switching option, all voltages change synchronously, so there is no imbalance.
2. The balancing circuit does not need additional decoupling from the power source, which reduces AC hum, etc.
3. The adjustment is smoother, since the resistors R3 and R4 play the role of restrictive and reduce the limits of voltage change on the varicaps.

Resistors R3 and R4 are specially selected with low resistance in order, firstly, to reduce the output impedance of the modulator, and secondly, so that the difference in the output impedances of the phase-inverted stage at the emitter and collector outputs is less affected.

The output impedance of the modulator is of the same order as the reactance of the varicaps at the carrier frequency. From this it follows that at high frequencies it makes sense to use varicaps with a small capacitance, and at low frequencies - with a large one. For better carrier suppression, it is advantageous to use capacitance-matched sets of varicaps. Suitable, for example, sets designed for TV channel selectors (KV123A and the like with a capacity spread of no more than 3%).

Experimental verification of the modulator at a frequency of 5 MHz showed the following. When an RF voltage with an amplitude of 1,5 V was applied to the input, a DSB signal with an amplitude of 1 V was obtained at the output of the modulator with a carrier suppression of more than 40 dB. In this case, the LF voltage amplitude did not exceed 2...3 V. It should not be excessively increased so as not to enter the region of opening voltages on varicaps [3] and not cause non-linear distortions of the modulated signal (limitation of the envelope at the LF voltage peaks).

Literature

1. Polyakov V.T. Radio amateurs about the direct conversion technique. - M.: "Patriot", 1990, p. 129.
2. Polyakov V. SSB Signal Conditioner. - Radio, 1983, No. 3, p. 21.
3. Stepanov B. Features of the use of varicaps. - Radio, 2002, No. 9, p. 27, 28.

Author: M.Syrkin (UA3ATB), Moscow

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