Re: Simple spur fix

Warren Allgyer

My implementation of the filter with a single transformer. I encapsulated the transformer and the filter with heat shrink and mounted it as a single component. As before the power levels are standardized to approximately 2 watts for easy comparison.

A capacitor is indeed needed between the low impedance transformer output and the center tap of the mixer transformer. I soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output.

The filter input is attached directly to the other R27 pad. I elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.

In my case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable audio drive level can be used. These traces were made using a 1 KHz tone at 30 mVrms, a level that is commonly used as the "high" level for microphone input in commercial transceivers and one that should be easily attained without increasing the gain of the audio input stages. It does, however return the radio to being susceptible to intermodulation distortion because the levels are increased back to closer to the stock configuration.

Maximum power levels are also restored to nearly the same as the unmodified radio...... not a good thing in my view because IMD was unacceptable at any SSB power level above about 1.5 watts in the original. I suspect that is the same with the transformer added but it needs to be verified. Also needing verification is the SSB passband. According to tests on the filter it should be much more flat with the transformer added.


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