Removing crystal from simple transmitter

Discussion in 'General Technical Questions and Answers' started by SV3ORA, Jun 14, 2018.

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  1. SV3ORA

    SV3ORA Ham Member QRZ Page

    Hi,
    In this transmitter http://qrp.gr/easy40/ what will happen if I short circuit the crystal leads (i.e remove the crystal from the circuit and place a wire)?

    Will it still oscillate due to the collector LC?
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    No
     
    1 person likes this.
  3. KV4PD

    KV4PD Ham Member QRZ Page

    Why would you want to?
     
    1 person likes this.
  4. SV3ORA

    SV3ORA Ham Member QRZ Page

    Well, first to be tuneable. Then to avoid to have to have a crystal on hand.
    But the first question is, will it work at all?
    Then if it works, will that be any frequency stable at all to be usable? (it's a power oscillator)
     
  5. SV3ORA

    SV3ORA Ham Member QRZ Page

    Why is that?
    I mean, the LC is tuned at the crystal frequency after all and it is a power pierce (?) oscillator
     
  6. KV4PD

    KV4PD Ham Member QRZ Page

    OK, assuming for a moment that it might oscillate, but that doesn't mean it would be usable as a CW transmitter. Factoring in loading and thermal issues, I'd bet it's going to drift all over the place.
     
  7. KC9YGN

    KC9YGN Platinum Subscriber Platinum Subscriber QRZ Page

    Probably won't work. You might get some oscillation but without a crystal the frequency is going to wander all over the place. Crystal controlled oscillators are going to be more stable, have fewer harmonics and have less noise than LC alone. They're used even in PLL based VFOs
     
    KI4AX likes this.
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    I don't think it will oscillate and act as a VFO. The purpose of the small 7-25 pF variable capacitor appears to be to "smooth" the oscillations at start up, to prevent chirp. You adjust it so oscillations will start reliably but without chirp. Typically it would be a "set and forget" trimmer capacitor; some builders would substitute in a fixed capacitor once they knew the exact value that would be necessary with a particular crystal.

    To work in VFO for 40m with a practical inductor, we usually expect to see a value closer to 100 pF in a VFO.
     
    1 person likes this.
  9. VK3YE

    VK3YE Ham Member QRZ Page

    No it won't be stable. As a minimum a free-running VFO controlled transmitter needs three stages: A well-built and shielded VFO, a buffer (1 to 3 transistors) and the driver and power amplifier stages.

    A VXO is more stable and easier to build than a free-running oscillator. That's good for several kHz range. Or try a ceramic resonator such as you can get for some frequencies like 3.58 MHz. That could give about 100 kHz tuning range.

    This is a simple VXO tx.

    http://home.alphalink.com.au/~parkerp/projects/pro80cw.htm
     
    1 person likes this.
  10. SV3ORA

    SV3ORA Ham Member QRZ Page

    The transmitter/oscillator does not show any chirp at 12v (2W output with a 2sc166). However at 29v (8W output) it does show some chirp when the oscillator starts. I did not know that this might be due to the capacitor setting, I always set this capacitor for maximum output, noticing the current drawn.
    Trying to eliminate this chirp, gave me hair-pulling problems at the past, so it might worth it to try the capacitor setting to see how it affects it, thanks!
     

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