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These are a variation of the min-loss cohn filters. As only the coupling capacitance is varied without varying the terminating impedance, we must expect high ripple at all setting except one. What does this mean? It means that tha filter.will exhibit ringing and phase delays.
A better option would a smooth Butterworth response with minimum ringing at a fixed frequency. You can vary the bfo for shifting the audio tone. A 400 hz bw will be narrow enough and yet offer a brightness that we miss in more aggressive designs.
Hello dear OM,
His tunable quartz filter also seems to meet your requirements. A friend of mine is building such a filter, but I don't know if he succeeded, yet. I think it's worth to have a try...
Thierry / F1HSU
Dear uBITX enthusiasts.
recently my uBitx transceiver has been finished. I find this project ufb because it is an excellent learning platform for me.
As with many others, a number of modifications are on the agenda now after I installed the uBitx into a case.
One of these modifications concerns the construction of a CW filter.
Up to now mostly audio filters have been described, whereby active filters are described with operational amplifiers or DSP filters with microcontrollers. I can understand that these two solutions are easy to integrate into the transceiver.
Looking at the circuit diagram of my CW transceiver HB-1B from Youkit I noticed that this transceiver uses a quartz filter whose bandwidth can be continuously changed with capacitance diodes (synonms are varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode). A quick search with Google gave this inofficial link to the circuit diagram of the HB-1B: https://matfyz.estranky.sk/fotoalbum/hb1b.-.html
I didn't find much information about this type of SSB/CW crystal filter. But I am impressed by the acoustic result of the simple circuitry of my HB-1B and wonder why this alternative has not been discussed in the uBitx modifications so far.
TenTec has patented this filter: https://patents.google.com/patent/US5051711A/en, which describes the passband curves of this filter.
On a German website (http://funkamateure-dresden-ov-s06.de/index.php?article_id=174 ) I found very recent measurement results on the bandwidth of these filters, which match the data from the TenTec patent.
Based on this little information the possibility to build a tunable quartz filter seems to be very interesting. Since I only deal with electronics as a hobby, I would like to know if there are any arguments against this filter.
And: does anybody know how it performs when compared to the audio filters?