Re: Simple spur fix

Warren Allgyer
 

It turns out I did have time for one pass of spur measurement. Here are screens for each of 17 through 10 meters. Conditions of the test: Audio input: 100 mVrms, 1.5 kHz tone. RV1 adjusted in each case for 2 watts output.

So far it looks really good with the eBay filter in place of R27. No other changes.

Yet to be verified: 100 mV audio drive, without the filter in place, gave very unacceptable IMD performance. It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing that will indeed have to wait. Also, we simply have to listen to the recovered voice quality and decide if it is adequate. Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

Overall, very promising as a cure for the spurs.

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