Hermann Tue Jan 17, 2012 8:35 pm
Hello Richard,
although crystal switching seems to be simple in imagination (arranging a bank of different crystals and getting access to a selected one via a (e.g. rotating) switch) it is not simple in practice. The reason is that additional devices like a switch will increase parasistic elements (e.g. stray capacitances and parasistic inductivities caused by the expanded wiring) will have an effect to your oscillator circuitry and will probably shift the transmitting frequency a little so that it may not fit any more to the receiver tuning frequency of the specific channel. Stray capacitances in the range of some 10 picofarads can already have an effect and they must be expected with common mechanical switches. So an extreme compact and concentrated switch arrangement would be required
Another method would be the use of semiconductor switches (pin diodes) directly attached to the crystals and operated with a DC bias voltage decoupled via low pass filter decoupling/ RF blocking. In this case a switch of even more dimensions may be possible as it will only be used to switch the DC-bias for the diode control.
Of course implementation of crystal switching will require some measurement equipment (e.g. frequency counter, spectrum analyser) to measure the effects on the oscillator circuitry and to prove proper operation.
I am sure that our electronic specialist John will confirm this. Probably he will give you more practical hints and perhaps even practical help.
Kind regards
Hermann
Last edited by Hermann on Tue Jan 17, 2012 8:36 pm; edited 1 time in total (Reason for editing : adding a missing paranthesis)
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