Variable uBitx IF bandwidth question

Gary Hanson
 

I apologize if this topic was covered in an early message string, but I have only been actively reading the list for a couple of weeks and noticed a couple of posts about it.


In the mid-1990s, I built the MXM transceiver kit and it used a 100 ufd or 150 ufd panel-mounted air variable capacitor to vary the IF band width of a four crystal Cohn filter where all the caps were the same value. If I remember correctly (at my age that isn’t always a safe assumption), the IF band width would vary from about 100 hertz to 500 hertz, maybe as much as 1 kHz.  Sorry, but I am out of town and can’t check the schematic.


Based on some of my early QRP kit building experience, the change in capacitance you could get from varying the voltage was fairly limited.  Wasn’t  the shift around 25-50ufd? Also, the change wasn’t linear across the entire range, though you could restrict the voltage and keep in a linear range.


My question is this:  Are there varactor diodes today with enough capacitance range to shift from a 2 khz bandwidth SSB signal to a 500 Hz CW signal?


I have not done the math to calculate how much capacitance shift would be required and in what range (...and not sure I even remember where to start - - so little time, so many dead brain cells).  I assume it would need to go from the current 100ufd caps upward by at least another 200ufd  I think my White Mountain 20 meter SSB transceiver from the 1990’s used around 330ufd in the 8 MHz SSB filter.


Also wonder if we could use a polyvaricap instead of diodes?


I agree with Bill that this would be a pretty easy solution if the right varactor diodes could be found.


Gary, KJ5VW



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