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Wednesday, July 20, 2011

Doug Demaw and Vlad Polyakov

I was looking through W1FB's Design Notebook the other day and I came across the above schematic for a direct conversion receiver (page 111). Note the Polyakov detector. One strange thing though: Doug was running the oscillator at the operating frequency. I thought the big advantage of the Polyakov design was that you ran the oscillator at half the operating frequency (that's why it is sometimes called a "subharmonic" detector). Any ideas on why Doug did it this way?

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  1. I don't know why Doug did it that way but that circuit would make a great little test receiver for bench work.

    Make the VFO band-switching (or use a DDS), a MBT for the front-end selectivity, add some audio bandpass filtering and a speaker amplifier, voila, you'd have a "Vlad The Inhaler"!

    73.......Steve Smith WB6TNL

  2. Hello Bill,
    Thanks for the question.
    The question ”why?” Is always the most difficult question. I first answered another question. Are L2 and C1 really tuned to 7 MHz? For 7 MHz C1 must be adjusted to 54 pF. For 3.5 MHz C1 should be 216 pF. Further C2 must be adjusted to 7 pF to tune the oscillator to 7 MHz. (C2+100pF=107pF)
    So L2 and C1 tune for 7MHz and also the oscillator is tuned to 7 MHz.

    In this chapter Doug wrote about DC receivers, in which the oscillator runs on the receive frequency.
    So I think that Doug choose the most simple mixer.
    Both in terms of number of parts and to avoid a more complex bifilair transformer or extra potentiometer for balance. It's not the best mixer but.... As Doug wrote: This Polyakov mixer does not match the performance of a balanced mixer. Page 112.

    I used such a simple balanced mixer in my HDC14 Transceiver, which is doing great, with just 1 potentiometer more. Please look at the handwritten schematic on my website.

    So I think, Doug kept it simple.

    73, Bert PA1B


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