Harmonic performance - SSB vs CW

Warren Allgyer <allgyer@...>

I am noting a significant difference in the harmonic suppression in SSB mode versus CW mode. 

The two signals are generated in a very different way. SSB is generated at 45 MHz and mixed down to the final transmit frequency by combining with the first oscillator in the balanced first mixer to make the SSB transmission. In CW the 45 MHz path is disabled and the first oscillator is simply moved to the transmit frequency. The first mixer is unbalanced with a DC bias to allow the first oscillator signal to leak through.

It appears the act of unbalancing this mixer produces harmonics, particularly high order odd harmonics that are clearly in excess of the -43 dBc requirement.

The screenshot shows  the harmonics of an 80 meter transmission at 10 watts. The purple trace is the CW and the blue trace is SSB. Signal source for SSB is a approximately -40 dBV 1 KHz tone with the level adjusted to match the CW carrier level. The SSB signal harmonic suppression is just adequate (green display line) however the CW signal is clearly non-compliant.

I am not sure what to do about it yet but wanted to raise the point. I have done a search on this board since I am new and I do not find this has been reported before..... perhaps I missed it?


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