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    Crystal switching?

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    Tom(ADMIN)
    AMS Forum Owner

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    Crystal switching?

    Post  Tom(ADMIN) on Tue Jan 17, 2012 10:40 am

    Can anyone tell me, is it possible to fit some sort of device to my Robbe F-14 tx to allow me to switch between crystals without having to physically remove the back case, remove crystal 1 and replace it with crystal 2?

    I use the F-14 with crystal 1 on my Neptune and crystal 2 on my Deep Quest submersible, and it would be much easier just to be able to flick a switch to change over.
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    Hermann
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    Re: Crystal switching?

    Post  Hermann on 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)

    Tom(ADMIN)
    AMS Forum Owner

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    Join date : 2010-11-05
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    Re: Crystal switching?

    Post  Tom(ADMIN) on Tue Jan 17, 2012 9:26 pm

    Hermann...I thank you for the explanation.

    But...I have no idea what it all means! I'm totally and utterly confused by it all...by thank you for taking the time to answer anyway.

    I now have a headache.... :-)


    Regards,


    Richard.
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    Hermann
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    Re: Crystal switching?

    Post  Hermann on Thu Jan 19, 2012 4:43 pm

    Sorry Richard,

    I didn't want to cause confusion so I hope the headaches will vanish soon.

    Let me give a more simple explanation: If you install more components within the oscillator circuitry of your transmitter (that is the section that controls the frequency of your transmitter) then the transmitting frequency may be shifted (detuned) a little due to "parasistic" effects. As a result transmitter and receiver don't match any more completely and the control range can be much less (or control may even fail).

    Kind regards
    Hermann

    Tom(ADMIN)
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    Join date : 2010-11-05
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    Re: Crystal switching?

    Post  Tom(ADMIN) on Fri Jan 20, 2012 9:08 am

    Hello Hermann,

    All is well now, the headaches have vanished...

    I don't know why it never occurred to me earlier, but all I need to do is to fit both subs with the same frequency crystal, that way the transmitter will be able to operate either boat without ever needing to open the back case.

    Simples!


    Richard
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    Hermann
    AMS member

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    Re: Crystal switching?

    Post  Hermann on Sat Jan 21, 2012 4:30 pm

    Well Richard,

    just as Joe Cocker sings: "...the best things of life, are the simple things..."


    Best regards
    Hermann

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