Variable bandwidth crystal filter

Some time ago we featured a design from Michael N2ZDB that used a Jones filter design from TenTec to produce a variable filter passband for his BITX40.

There has been more recent interest on the IOGroups BITX20 list in variable bandwidth filters in the µBITx.

Karl-Heinz DF9RU completed a build of a µBitx transceiver and found it to be an excellent learning platform. He has been toying with a variable CW filter.

Up until now audio filters have largely been adopted by members:  either active filters with operational amplifiers or DSP filters with microcontrollers.  Karl-Heinz acknowledges that these options represent easy solutions for integration into the µBITx.

Karl-Heinz has a CW transceiver HB-1B from Youkit. This transceiver has a quartz filter of variable bandwidth.  The bandwidth can be continuously changed using varicap diodes.  The circuit diagram of the HB-1B can be found here.

Karl-Heinz was impressed by the acoustic result of the simple circuitry of his HB-1B and wondered why this alternative had not previously been used?

TenTec has patented a filter design which describes the passband curves of this filter.  A German website  also documents results on the bandwidth of these filters, which match with data from the TenTec patent.

Thierry F1HSU suggests taking a look at Tasa’s site for a tunable quartz filter  : http://yu1lm.qrpradio.com/bp%20yu1lm.htm

Ashhar Farhan VU2ESE suggests that these filter designs are merely 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 settings except one. What does this mean? It means that the filter will exhibit ringing and phase delays.

A better option would be a smooth Butterworth response with minimum ringing at a fixed frequency.  You can vary the BFO for shifting the audio tone. A 400 hz bandwidth will be narrow enough and yet offer a brightness that we miss in more aggressive designs.

Wes wrote a paper on this on his website www.w7zoi.net.
Ted, KX4OM reminded us of the SSB6.1 transceiver, which employs a tuneable SSB filter with tuning diodes in place of the capacitors in a min-loss configuration. The rig is a basic SA612-based design which can be found here:

Allison KB1GMX suggests looking at this design as well.  It is not new and a bit tricky but works best with lower frequency crystals.

Reference

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