Block of quartz filters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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The weakest point in the transceiver is the block of quartz filters, or rather the selectivity in the adjacent channel and the suppression of signals outside the passband. Gain The IF amplifier's cous is approximately 60 dB. In the ideal case, the attenuation of the main selection filter (FOS) - a quartz filter - outside the passband should be at least DD1 = 140 dB, and we have only 70 - 80 dB, thus the real selectivity of the radio receiver is not ensured. In the ideal case, if DD3=100 dB, and KusUCH=60 dB, then it is necessary to have an attenuation of 100+60=160 dB outside the filter passband.
In order to somehow ensure such a parameter, it is necessary to turn on at least two 8-crystal quartz filters in series, compensate for the attenuation in the second filter and perform the installation very carefully - shield and solder all connections, filter covers, partitions, etc. Filters are arranged "in a line": In the second filter, you can install a relay to narrow the bandwidth - CW mode.
Matching of quartz filters and attenuation compensation in them can be done using a simple circuit, fig. one.
Ris.1
The gain in the matching circuit is controlled by selecting the value of the capacitor C * in the source circuit of the field-effect transistor. This stage shall not be covered by the AGC circuit.
And yet, it should be noted that the transmitter power amplifier, to put it mildly, is not very linear and it is better to replace it with a more advanced one. It is desirable to improve the filtering in the output stage by installing a low-pass filter with a cutoff frequency of 32 MHz or, much better, band-pass filters.
Author: A.Kuzmenko, RV4LK, Ulyanovsk; Publication: N. Bolshakov, rf.atnn.ru
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