High performance balanced modulator-detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Modulators
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The world consists of paradoxes - discoveries are made by those who simply do not know that it is impossible to do this, and they do ... and discover! Maybe there is something in the idea presented here? Theorists! Find an explanation for the fact. And please be indulgent. RX3AKT.
To improve the efficiency of FET mixers in the passive mode, a rectangular shape of control pulses is widely used. A more effective way to increase intelligibility, in my opinion, is to use narrow pulses, when the duration of a single state is hundredths and even thousandths of zero duration. (Nicely said, isn't it?) To the ear, this is perceived as a boost in high frequencies. The intelligibility of the speech signal is sharply increased. The frequency response becomes more even. The balanced modulator-detector, Fig. 1, is assembled according to the well-known scheme of A. Pogosov (see the magazine "Radio" No. 10-81).
Fig.1 Balanced modulator-demodulator (click to enlarge)
The control circuit contains a crystal oscillator assembled on a DD1 chip, a frequency divider by 4 (aka a phase shifter) - on the DD2 MS and a phase discriminator on the DD3 and DD4 MS. A square wave signal from a 1 MHz crystal oscillator is fed to a digital phase shifter (divided by 4). Two antiphase signals with a frequency of 250 kHz are taken from its output. It is known that in the antiphase signal there is always some error in the phase difference associated with the unstable operation of the phase shifter, which is detected by the phase discriminator.
The signal selected by the phase discriminator, proportional to the error of the phase shifter, is the reference frequency for the balanced modulator-detector, assembled on the field effect transistor VT1. At the same time, the second input of the modulator receives a speech signal from the microphone amplifier, if we are talking about the transmission path. Or an IF signal if the circuit is used as an SSB detector (demodulator). Shown in fig. 1 balanced modulator-demodulator operates at 500 kHz and works well at half the local oscillator frequency. It is possible to double the oscillator frequency by using a 2 MHz quartz resonator.
The method described above can be proposed to be tried in relation to balanced frequency converters (mixers) of transceiver equipment by replacing microcircuits with microwave ones.
Author: M.Sattarov, settlement of Inozemtsevo, Stavropol Territory; Publication: N. Bolshakov, rf.atnn.ru
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