A simple compressor for a radio transmitter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Signal limiters, compressors
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To improve the efficiency of radiotelephony in amateur transmitters, it is advisable to use a device that compresses the dynamic range of speech - a compressor. The compressor is a bass amplifier with automatic gain control. The pages of the magazine "Radio" have already published descriptions of devices that compress the dynamic range of speech, but the authors followed the easiest path, simply applying a limitation of the signal amplitude. At the same time, the use of AGC makes it possible to compress the dynamic range of speech more efficiently and with less distortion.
The compressor diagram is shown in fig. 1. The principle of operation of AGC is as follows. Capacitors C2 and C4, blocking resistors R4 and R8, are connected to a common wire through the differential resistance of open diodes D1 and D3, which are silicon diodes D204, which have a pronounced dependence of the differential resistance on the position of the operating point on the characteristic.
Fig. 1
When a signal with a large amplitude is received at the compressor input, a closing voltage is applied to the diodes, their resistances increase; as a result, the gain drops. In the absence of a signal or with weak signals, the voltage across the diodes is small, their resistances are also small, the capacitors shunt resistors strongly. In this case, the gain is large.
Due to the fact that weak signals are amplified by the compressor more than strong ones, one should strive to increase the signal-to-noise ratio, since when using transistors with a large noise figure or ill-conceived installation, the noise in the pauses can reach a very large value.
The compressor requires virtually no adjustment. It may only be necessary to select the resistances of resistors R2 and R6, as well as supply a bias voltage to diodes D1 and D3. For this it is better to use a divider. By selecting the resistance of the divider resistors, you can find the optimal operating point.
The efficiency of the compressor is illustrated by a graph (Fig. 2).
Fig. 2
The expediency of including capacitors C2, C4 in the polarity recommended by the author raises doubts. Apparently, it is more correct to change this polarity, and apply the bias voltage that opens them to the diodes D1, D3.
Author: V. Leonov; Publication: N. Bolshakov, rf.atnn.ru
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