On Tue, Dec 19, 2017 at 10:16 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
Since the uBitx does not bother with a diplexer, all frequencies beyond the filter passband see a high impedance at the input and output terminals of the crystal filter. So the easy way to add a CW filter is to just put a second filter operating at a different frequency in parallel? with the 12mhz crystal filter.? Change the frequency of the Si5351's CLK1?into the second mixer to switch filters. Using fairly low frequency crystals for the CW filter like the 1.8432mhz Farhan suggests will tend to give a narrower passband than the 12mhz crystals of the SSB filter.? The 45mhz roofing filter of the uBitx is narrow enough to reject CLK1 that's 1.8432mhz away, and also knocks out anything that might go through the undesired crystal filter.? ?Sneaky!
For a similar scheme, Paul M0XPD has a Sprat article about his parallel IF SSB/CW rig: . I thought his blog post was a revelation when I first saw it and I am very excited to see that his topology is an option in the uBitX design. See also Paul's blog post about his trip to Dayton and more materials at . I think I remember watching a video of his presentation, but I can't find it online.
When my uBitX arrives on the slow boat, I'll probably initially use a 4SQRP HiPerMite audio filter to provide narrower audio for CW, but the possibility of building up a crystal filter on a scrap of PCB from 5 or 10 MHz crystals and just tacking it in parallel with the existing filter is extremely appealing. Kudos to Farhan's dual-conversion design and the Si5351 for making this kind of versatility possible.
