EMRFD Message Archive 14007

Message Date From Subject
14007 2017-06-13 10:11:36 itsbeen29years Recommended IF bandwidth for CW (Cohn) Crystal Filter
I have constructed a Cohn filter using  cheap 4.032 MHz crystals, and 270 pF capacitors.  The crystals were matched quite closely.

My question: what is a recommended CW Ladder Filter bandwidth?  400 Hz, 500 Hz?

My results show I have a 294 Hz -3dB bandwidth for a 4 crystal model ; and similar bandwidth, but smoother shaped, lower insertion loss, for a 3 crystal model.

With the 4 crystal model, I can get a 1.7 kHz bandwidth at about -60 dB.   Perhaps this is going to be too sharp a filter.

I used a test setup, with the Poor Ham Spectrum Network Analyzer (PHSNA) to generate these curves, assuming a 200 Ohm input and output resistance.    

I guess I can try it out, and see for myself, but wanted to get some advice.


14012 2017-06-13 14:34:04 Bill Carver Re: Recommended IF bandwidth for CW (Cohn) Crystal Filter
It depends on your operating style and your ears, Nigel.  People can point things out, and give their opinions, but your ears are the final arbiter of the filter width.

From a practical standpoint, if it is the only filter in the receiver, getting too narrow makes it time consuming to search and find signals. that said, I've build several analog CW/RTTY receivers with 300 Hz filters that felt reasonable on 40m and 20m CW when operating over a 50 KHz span.

Once you've found/zeroed in on a signal, the time-domain response (group delay) of the filter can become a limiting factor on how narrow you will want to go. It is possible to go as narrower than 50 Hz at 25 WPM if the group delay is flat. I made an 8 pole linear phase filter at 4 MHz and recorded a QSO for a Dayton presentation. It had less than 100 Hz bandwidth and nobody in the audience even came close to guessing what the 3 dB bandwidth was because it had flat group delay to -30 dB attenuation.

Cohn filters have delay peaks at the edge of the filter. CW sounds best when the signal is tuned precisely in the center. The few Cohn filters I've made sounded OK to me at 300-400 Hz bandwidth with 4 poles, but I'd get several opinions from other people.

Bill W7AAZ

14047 2017-06-19 09:09:29 wb8yyy_curt Re: Recommended IF bandwidth for CW (Cohn) Crystal Filter

Bill has nailed it -- your filters may be nice enough for use.  funny thing that my commercial rig has adjustable DSP, and I rarely adjust it from 300 Hz.  Ditto for my K2, although I have 3 narrow choices I tend to also keep it around 300 Hz.  Someone else might prefer something slightly wider - no huge issue.  Possibly your real-world termination might widen it very slightly - so I suggest trying what you have made - as without changing xtals your only adjustment would be the shunt capacitance between crystals.  Enjoy the result of your work.