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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?