Tuned transformer problem

Discussion in 'Wireless & RF Design' started by PRS, May 22, 2010.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I constructed an amplifier on a copper clad board. The input is to a tuned step up transformer which feeds a common emitter amplifier, and the output is to a tuned step down transformer. Both transformers are tanks tuned to a center frequency is 10 Mhz. I expected a signal at 10 Mhz with rapid attenuation. But, rather than rapid attenuation, I see a whole bunch of harmonic distortion when I vary the frequency. Any thoughts?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I suspect parasitic oscillation. Try installing a load (non-inductive) resistor on input secondary to reduce the Q and a small ferrite bead on the base lead.

    http://en.wikipedia.org/wiki/Parasitic_capacitance

    In the old tube days we would try to Neutralize the circuit. http://www.w8ji.com/neutralizing__amplifier.htm
     
    Last edited: May 22, 2010
  4. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I suspected the same thing. I tried decoupling the output from the positive supply line with a simple RC filter, but it didn't work. I'll give your idea a try. And thanks for the links. The second one looks quite valuable; I bookmarked it.
     
    Last edited: May 23, 2010
  5. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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