Homebrew Crystal Filters

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Stan Vittetoe:
I recently completed work on a homebrew 80-10 meter SSB transmitter/receiver (it shares a common VFO, but has separate front ends, IFs, and filters). I built 9 MHz crystal filters for both the transmitter and the receiver IF, but I have been dissatisfied with the performance of the filters.

If I have a narrow filter in the receiver, the audio is poor. If I widen the bandpass, the selectivity suffers. I see a similar problem on the transmitter. I am using a Flex Radio 1500 to view the transmitter output, and when the transmit audio is satisfactory, I see part of the lower sideband being transmitted. If I try to tighten up the passband, the audio suffers.

I know that it is possible to construct effective crystal filters, but my results are not impressive. I don't have sophisticated test equipment--only a scope and my Flex Radio. I am beginning to believe that I should scrap the homebrew filters and buy a commercial 8-pole filter with a nice shape factor. I have used homebrew crystal filters in simple QRP CW rigs before, and they performed satisfactorily. I just don't know if the average builder such as myself can hope to construct effective filters for a SSB transceiver. Has anyone had experience with homebrew crystal filters similar to mine?

Peter Chadwick:
There have been a number of articles published on home brew crystal filters. If you want to build a filter at 10MHz, I have a load of 10MHz crystals that were rescued from the dump when a company lab closed. For 50 crystals, you will need to pay postage from the US (I'll mail when I go to Dayton), or you can collect (or get a friend to collect) from the IARU booth (in the ARRL area) at Dayton. Or just make a suitable donation to the ARRL Frequency Defence Fund.

73

Peter G3RZP

Dale Hunt:
Quote from: WA0VPR

...
If I have a narrow filter in the receiver, the audio is poor. If I widen the bandpass, the selectivity suffers. I see a similar problem on the transmitter. I am using a Flex Radio 1500 to view the transmitter output, and when the transmit audio is satisfactory, I see part of the lower sideband being transmitted. If I try to tighten up the passband, the audio suffers.




There is no solution to this, because there are two conflicting requirements.
If you want "full" wide bandwidth audio, you can't pass it through a narrow
filter.  If you have good low frequency response then there isn't enough
filtering to remove the carrier and part of the opposite sideband.   That's due
to the limitations of crystal filters built with real world parts.  You can't get
a lot of audio through at 400Hz while suppressing the carrier (only 400Hz
away) and the opposite sideband (800Hz away) with a typical filter roll-off.
(Asymmetric filters may be better in this regard than symmetric ones.)

For good audio "punch" you want to emphasize the speech frequency range
that contributes the most to communications effectiveness - that usually
means filtering out the low frequency components.  The point is to be
effective, not to sound "natural".  That allows you to put more power into
those portions of the audio spectrum that contribute to intelligibility at
the far end.  The typical 2.1kHz bandwidth for SSB intentionally sacrifices
some audio range for better selectivity.

For SSB to sound good the BFO frequency has to be carefully positioned relative
to the filter passband (in both the transmitter and the receiver), or you clip too
much on the high or low end of the audio.  The narrower the bandwidth, the more
important this is.


Meanwhile there is a lot of information on homebrew crystal filters available from
folks such as G3UUR, W7ZOI, and others.  Here is a useful summary page that
might point you in some useful directions:

http://www.qrp.pops.net/crystal-tester-2010.asp

With some fairly simple measurements of crystal parameters and some math you
can build pretty good filters, especially making use of some of the software that
is also available.

Gregory J. Beat:
Larry Benko, W0QE has this article on Crystal Bandpass Filters
http://www.w0qe.com/Projects/crystal_bandpass_filters.html

Cohn-type crystal ladder filter
http://epic.mcmaster.ca/~elmer101/cohn.html

INRAD sells two 9.0 MHz crystal filter kits (#350, #351) for DIY filter builders
These low cost filter kits include four specially cut and color coded crystals along with five surface mount capacitors. Center frequency is 9000 kHz and impedance in and out is 200 ohms.

600 Hz, 9000 kHz 4-pole crystal filter kit
http://www.inrad.net/product.php?productid=239&cat=149&page=1

2400 Hz, 9000 kHz 4-pole crystal filter kit
http://www.inrad.net/product.php?productid=240&cat=149&page=1

Kerry Power:
It's possible to design very good crystal filters but it takes some effort; the crystal parameters are important.

Many simple projects use a few crystals of unknown specification; sometimes it works, sometimes it doesn't, sometimes the builder is unsure.

Crystal parameters can be measured with simple equipment; a signal source and a detector.  You have those in your radio and 'scope.  Suitable attenuators will embed the crystal-under-test in a 50-ohm or 12.5-ohm environment (see the K8ZOA paper referenced below).

The G3UUR method doesn't even require that, just a simple oscillator, but it does require a frequency counter.

These methods are somewhat tedious if you are sorting large batches of crystals; better equipment speeds the process but is not essential.  It's usually necessary to measure a lot (20 - 50) of crystals to sort them into reasonably-matched sets for filters but, if you want good filters, that's what you have to do.

Jack Smith K8ZOA has written the best paper on crystal measurement that I know; go to http://www.cliftonlaboratories.com/Documents.htm and find the Crystal Motional Parameters paper.

A fixture similar to Jack's is excellent for the series-resonance method; the G3UUR oscillator is simple to build if you want to try that.

Filters can be designed once you have the parameters; the free program from the AADE site is a good one but there are several others available for downloading.

It's perhaps counter-intuitive but good narrow filters (CW bandwidths) aren't difficult to make; the wider SSB filters are quite difficult.  The essential reason is the small frequency difference between the series resonance and the parallel resonance of a crystal; you can only make a filter with somewhat less width than that difference.

The difference increases with crystal frequency which is one reason for using 9 or 10MHz crystals instead of lower-frequency ones.

I've looked in my computer files and found a few articles that are available on the 'net;

www.arrl.org/files/file/QEX.../QEX_Nov-Dec_09_Feature.pdf

http://www.giangrandi.ch/electronics/crystalfilters/xtalfilters.shtml

http://www.w0qe.com/Projects/crystal_bandpass_filters.html

Patient searching using various search terms will find a good deal more.

I've found working with crystals to be fun; I've worked my way through all the measurement methods described by K8ZOA, from simple series-resonance to the final graduation to VNAs.




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