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The first SSB radio I had that used an audio filter to accommodate CW was a Ten Tec Argonaut 509. It also used audio derived AGC similar to the audio AGC kits used in the uBitX. That approach is less than ideal because it allows signals anywhere in the SSB passband to affect the received signal. However, It was still effective and performed well at Field Day with QRO rigs nearby. Don't completely dismiss the simpler approaches just because a different method might offer advantages.
The most reliable means to accommodate AM, FM, SSB, and CW bandwidths is with an xtal filter for each mode operating at the same IF frequency. It just isn't that difficult. In the case of the uBitX we were planning to build a second filter for CW and best to build it for the IF already in use. Diode steering can select the filter and that can be controlled from the software if desired.
If a second filter is to be on a different IF frequency we can use both at the same time with a variable frequency conversion oscillator that shifts the signal from the first filter and passes it to the second filter so that only a sliver of spectrum (CW bandwidth) emerges from the second filter (a windowing effect). That is a more reliable form of passband tuning. Using just a little bit of shift has the effect of narrowing the SSB pasband a bit too so it is not just a CW modification. We were going to build a second crystal filter anyway, weren't we? In any case, the audio filters will improve the performance of the radio even with more IF filtering. The radio gets more complicated and the expense increases along with size. We could go from something that fits a shirt pocket to something the size of a Volkswagen beetle! Choose the tools you really want and leave the rest for somebody else:)
I have been using my V3 with an audio filter and it works well. The audio filter is tuned to the same audio frequency as the sidetone and CW offset.
bark less - wag more
On 9/20/20 11:42 PM, Jerry Gaffke via groups.io wrote:
Here's an couple posts regarding various ways to add a narrow band filter for CW.