EMRFD Message Archive 1153
Message Date From Subject 1153 2007-11-06 08:05:50 Alan Melia Overtone crystals
recently, feel free to fire me at an archive.
I have a need for a few off, single frequency filters in the 80 to 120MHz
range and wish to use overtone crystals in a ladder configuration. I was
sure I have seen an article somewhere detailing some examples of VHF filter
use but I am unable to find them. Does one simply measure the resonance of
interest and derive motional parameters in the same way as for fundamental
crystals (QEX and RadCom)? I am sure I saw a article with some detail which
may save me a lot of effort in case I am trying to achieve something that is
not possible.
Thanks and Cheers
Alan G3NYK1154 2007-11-06 08:20:12 Leon Re: Overtone crystals
1155 2007-11-06 09:44:49 john lawson Re: Overtone crystals
they manufacture crystals for commercial use and
amateur use. In the old days the ARRL almost used
them exclusively for crystals for projects for QST and
the Handbook. I'm sure they can steer you in the
right directiionwith your questions as a last resort.
John
1156 2007-11-06 10:12:07 Wes Hayward Re: Overtone crystals
Welcome aboard. Hope that we can be of assistance.
Sure, there is nothing at all wrong with using using overtone
crystals for filters. Indeed, some of the narrowest filters I've
ever built used overtone crystals. I managed a bandwidth of one
part in a million, but the crystal Q requirements were extreme. That
was fun, but less than practical.
Be sure that the scheme you use for measuring the crystals operates
at the overtone frequency. You can't use, for example, the simple
G3UUR oscillator that is presented in EMRFD, for that will end up
with fundamental mode operation. There can be very poor
correlation between fundamental and overtone frequency.
Measurement of Lm can probably be done at the fundamental. I suspect
that the ideal evaluati1158 2007-11-07 07:13:48 Alan Melia Re: Overtone crystals
they are a bit expensive. Because of the manufacturing process bulk purchase
of the same frequency is often worthwhile. I initially had a problem with a
3 stage ladder that didnt seem to perform correctly....would you believe it
turned out to be an open circuit capacitor ! and because they are obviously
quite low values....it took a while to find.
The application by the way is in direct frequency synthesis of low phase
noise signals for local oscillators at microwave frequencies.
Ok on the Osc method not really being applicable in this area. Thanks Wes, I
rather expected to have to set a process like was described by Jack
hardcastle in a QES a few years back and feed data into a spread sheet.
Multiple resonances (overtones) do seem to catch a lot of people out. I did
some measurement of fundamental v Ot frequencies in the 50 to 60MHz range
and found that there was no general "equation" the OT frequency was about
40 to 60kHz low of 3 times the fundamental looking at several dozen samples.
I have found some oscillators that activate both fundamental and third
overtone simultaneously and lead to "birdy" at the difference frequency.
This is not normally noticed unless you are building a LF converter ! Also
"logic-block" oscillators (inverters) driving (good quality) crystals so
hard in the 4-5MHz fundamental range that they were predominantly
oscillating at their overtone 12 to 15MHz.
Thanks for the responses I look forward to learning a lot more as a member
of the Group.
73 de Alan G3NYK1169 2007-11-20 08:33:09 Mike Brainard Re: Overtone crystals