Re: Is push pull hf amp has higher spectral purity

Jerry Gaffke
 

Brian,

They were all busy looking at the harmonics, didn't think to check for that close in spur.
Would be interesting to see how strong it is on other rigs.

I believe the strength of the spur comes down to how strong that 4'th harmonic of the VFO is.
During transmit, the VFO mixes in after the signal passes through the crystal filter,
so a lousy crystal filter is not contributing to the spur.

Lesson learned:  Use high side local oscillators wherever possible.

Jerry


On Thu, Dec 6, 2018 at 02:56 PM, Brian wrote:
From memory, I measured the spur to be 35dB down on the 7.15MHz signal. Nobody else reported measuring it which surprised me since quite a bit was written about modding the LPF to meet fcc on harmonics.
The 12MHz xtal filter in my rig was very bad on the LF side, perhaps one xtal is slightly off frequency. I did consider building a new 12MHz filter but never got around to it. I have several home built filters at 8.86MHz but they are just a bit too big to fit in.
I decided to leave it as is and only use USB with digital.

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