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Thread: trf radio design question

  1. #1

    Default trf radio design question

    Greetings from NJ, USA. This is my first post.

    I have been looking at some TRF input stage designs for 10MHz WWV and stumbled across this one and was wondering how the author of this might have arrived at the values he selected for the input stage tank circuit.

    Please see the attached image.

    Looking at L1 and CV1:

    With CV1 at 10pF to 40pF capacitance and L1 somewhere less than 4.1uH since it is to be tapped, assuming the full 4.1uH of the inductor, I come up with a resonant frequency range of roughly 12.4MHz to 24.9MHz and far higher with the reduced inductance at the tap.

    Looking at L3 and CV2:

    With L3 being 3uH and CV2 being 10pF to 70pF, I come up with a resonant frequency range of roughly 11MHz to 29MHz.

    With the input capacitance of the mosfet considered at say around 7pf, with L3 being 3uH and CV2 now steering 16pF to 76pF, I come up with a resonant frequency range of roughly 10.5MHz to 23MHz, but still not what I would consider to be targetting 10MHz "on paper."

    I can't tell for sure, hence my question here, but my guess for the moment is the tank circuits at this stage are not being used for band pass, but perhaps just to attenuate lower frequencies while attempting to minimize filter insertion loss?

    For reference, this complete project can be found by googling qrphomebuilder 10 mhz wwv receiver experiments. I mention this because in a subsequent stage of this receiver an xtal band pass ladder filter is also used which I am slightly more comfortable with in my understanding. It is only the RF input stage tank circuit values that are still perplexing me.

    Thanks in advance for any responses.
    Chas


  2. #2
    Super Moderator 5B4AJB's Avatar
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    Those values will ultimately depend on the antenna used.

    Frequency stability is quite dependent on the antenna in a TRF...

  3. #3

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    Quote Originally Posted by 5B4AJB View Post
    Those values will ultimately depend on the antenna used.

    Frequency stability is quite dependent on the antenna in a TRF...
    I actually did consider that for L1 and CV1 since they appear to connect directly to the 50-ohm antenna coax. The antenna reactance could explain what that first tank circuit expects to work with.

    But as for L3 and CV2: They should be chosen to be more closely a match to 10MHz right?

    I am wondering if perhaps the builder's Gate1 of FET Q2 added more parallel capacitance than I am expecting. If it is 15pF or more, then that cap should just be able to get 10 MHz. Maybe I am misreading the datasheet. I am assuming the specified input capacitance of 6pF is what should be there at that point in the circuit parallel to the tank.

    Thanks for the reply.

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