Schematic cap value

Discussion in 'Homebrew and Kit Projects' started by KB3KSA, Jan 14, 2021 at 3:11 AM.

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  1. KB3KSA

    KB3KSA Ham Member QRZ Page

    I got this from the "Sprat Pixie File"; it's one variation of the Pixie transceiver. Schematic shows a 60pF (C2) cap switched in when keyed. But...

    What the is value of the other Colpitts cap (C1)? Is that supposed to be 0.001uF?

    And why switch the 60pF and not the 1000pF? The 60pF isn't going to produce much of difference when keyed...Going from 1000pF to 1060pF?


    1-13-2021 10-05-13 PM.png
     
  2. K6CLS

    K6CLS Ham Member QRZ Page

    the text at the top says "700Hz offset on transmit"; C2 60pF is removed from the oscillator when the 2N3906 base is grounded by the key (TX).
     
  3. KB3KSA

    KB3KSA Ham Member QRZ Page

    What is the value of C1?
     
  4. KP4SX

    KP4SX Premium Subscriber QRZ Page

    It says .001 :) 60pf certainly will create significant shift. Trying to key the .001 wouldn't work. It is fixed to gnd in both T and R.
     
  5. KB3KSA

    KB3KSA Ham Member QRZ Page

    I had mocked up the oscillator and couldn't get much offset with a 60pF, but I was going from grounding the crystal to grounding it through 60pF. Must be my misinterpretation of the switching part of that circuit.

    I assume 001 means uF?
     
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Yes. =1000pf.
     
  7. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Being connected to the collector of the PNP keying transistor, how would C2 reach ground through that circuit (trick question)?

    Anyway, here is a corrected version of W1FB's Pixie:

    w1fbpix.gif
    Which I found here: https://www.qsl.net/we6w/text/pixie.html

    73, Mark
     
    KB3KSA likes this.
  8. KB3KSA

    KB3KSA Ham Member QRZ Page

    Thanks, Mark. Yeah, I eventually realized that the PNP was switching IN the power supply voltage. I haven't experimented with capacitors enough, I didn't realize what effect that would have. Interesting. Hey, at least I was suspicious about which cap was being switched. :)

    Thanks for the link.
     

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