EMRFD Message Archive 347

Message Date From Subject
347 2007-01-16 01:02:55 neomag_magneo Crystal Ladder filters above 30 MHz
Hi group,

I have a question concerning the applicability of appr. 33 MHz crystals
in ladder filters. I found a batch of abt. 200 pieces in a local flea
market (by NOKIA Radioamateurs' Club) and purchased those assuming
overtone operation and the fundamental frequency being abt 10 MHz.
Preliminary testing revealed, however, that the 33 MHz is more like
the fundamental frequency, at least that was the only one to give any
oscillation. Before carrying out any further tetsing and
experimentation: has anyone experience on 30 MHz+ crystals in ladder
filters. I do not assume any ultranarrow bandwidths but would be happy
to achieve 3 - 6 kHz, a roofing filter or AM filter for BC bands.

Best regards

Heikki (OH2LZI)
348 2007-01-16 13:56:50 Wes Hayward Re: Crystal Ladder filters above 30 MHz
Hello Heikki, and group,

I don't have any direct experience with crystal with a frequency this
high, especially fundamental mode parts. I have examined some third
overtone crystals and have built some simple filters with them.
Frequencies ranged from 10 to 25 MHz. Everything simulates just as
expected once you have the correct motional components in place for
the model. It becomes more difficult to get the filters tuned at the
higher frequency. You may have to build a special oscillator to
characterize them. I had to do this in order to evaluate some 25 MHz
third overtone crystals that I picked up at one time. Once you have
the parameters, everything is just like lower frequency filters. A
moderate bandwidth of a few kHz should not be difficult at 33 MHz. I
would recommend starting with a simple filter with just 2 or 3

Let us know what you find.

73, Wes

349 2007-01-16 15:02:05 Sam Billingsley Re: Crystal Ladder filters above 30 MHz
351 2007-01-16 23:48:16 neomag_magneo Re: Crystal Ladder filters above 30 MHz
Hi all!

Thanks for the info and encouragement... It is good to know that 50
MHz ladder filters do exist even in commercial equipment, looks like a
Cohn filter. Before designing/building anything I will have to measure
and characterize the crystals, unfortunately I am now quite busy with
other things, so this needs to be postponed a while. If/when I come up
with some reasonable results I will inform the group of my findings.


Heikki (OH2LZI)
352 2007-01-17 16:53:22 topossibilities Re: Crystal Ladder filters above 30 MHz
The following site may be of interest.

It has a method and calculator for determinating crystal parameters as
well as a calculator to design up to a 8 pole ladder filter.



Lots of books, parts and kits for sale at: