Re: Crystal CW filter with Version 4 uBITX boards? #ubitx


Alex - PA1FOX
 

Hi Guys,

Been very busy with other things during the last couple of weeks so I've not been checking this group also.
Indeed, I got an email from Jim about the crystal filters.  And indeed I use two 10 xtal Cohn ladder filters in parallel, without any switching hardware. Shifting the IF does from 12 MHz to 4MHz does the trick. I am very satisfied with the performance, and promised Jim to make some video recording demonstrating it. My narrow filter is the same setup as the wide filter, using cheap 4 MHz crystals which were matched with a test oscillator. The capacitors were set to 68pF but that's a personal preference. I have not measured it with a spectrum analyzer, but it 'sounds' more narrow than my official INRAD filter in my IC730, which is 400hz. 

The difficult thing is the software part. You will need four calibration settings for the BFO oscillator. One for wide and narrow USB and one for wide and narrow LSB. This together with a variable sidetone frequency made it a real programming challenge. We don't want the RX frequency to shift when changing from wide to narrow both in USB and in LSB mode. I finally got that working, but my bitx is completely homebrew based on the original ubtix version. I am not using Raduino but a cheap Nano and Adafruit board and my firmware has been considerably modified (but not using the encoder for dial).

Back to the wide/narrow behavior, it's very pleasant to work with. I switch wide/narrow by short pressing the function button and tapping the key at the same time. The video will explain more.

73, Alex
PA1FOX

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