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?

 

I thought your post was enough different from the original topic that I moved it here. This is the original thread - http://forum.allaboutcircuits.com/showthread.php?t=38508

 

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?

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

 

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

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.

 

I thought your post was enough different from the original topic that I moved it here. This is the original thread - http://forum.allaboutcircuits.com/showthread.php?t=38508

Thanks. I must have spaced.