PA Time sequencer

The next job is the LNA transistor replacement, but the trouble is repeated already within next contest. Such problem is usually result of unsuitable time sequence of switching between receive and transmitting, where due to different delays (for example by mechanical movement of relay parts) may happen conflict between beginning of transmission (e. g. by push down of key), although the complete transmitting chain
PA Time sequencer - schematic

is not ordinary switch on yet, or in opposite within switch down from TX to RX, when antenna relay already going to RX, but due to another PA relay mechanical delays the TX is not calm down yet and last dot of TX CW power (or roger beep) may still be observed. Result of such unpredictable transition status is not enough isolation between RX and TX chain in the antenna relay (because this relay is just moving) and the peak of power transported across the switching antenna relay may easy destroy input transistor of LNA or even receiver front end. Even in case, when different delays in our TX chain were adjusted properly and such trouble was not observed for years, change of transceiver or transverter may create change of the whole chain delay and when we want (for example) use older PA together with new transceiver (which has not mechanical, but PIN diode RF switch inside), or even when the SW delays of transceiver is setup to QSK, the trouble of LNA damage or in the worst case even antenna relay may be destructed. Substantial part of solution of such unlucky situation is an installation of appropriate time sequencer into each final stage PA, which is used. Each important parts of TX/RX chain will be then controlled in certain predicted time steps, what ensures, that the above described conflict between TX and RX ways will be eliminated. And because such additional control circuit shall be installed into any present PA, it...

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