On Tue, 31 Aug 2004 23:05:53 -0400, " Uncle Peter"
wrote:

It will oscillate at the fundamental, not the marked 100 MHz overtone
frequency. At 100 MHz, I'd guess that's a 7th or 5th overtone cut.
Most likely 7th, 20 MHz is about the limit for fundamental crystals.

So how does one tell if the xtal is fundamental or overtone? Not for
xtals marked 100Mhz, obviously, but for much lower frequencies which
could be either..
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul Burridge wrote in message . ..
On Tue, 31 Aug 2004 23:05:53 -0400, " Uncle Peter"
wrote:

It will oscillate at the fundamental, not the marked 100 MHz overtone
frequency. At 100 MHz, I'd guess that's a 7th or 5th overtone cut.
Most likely 7th, 20 MHz is about the limit for fundamental crystals.

So how does one tell if the xtal is fundamental or overtone? Not for
xtals marked 100Mhz, obviously, but for much lower frequencies which
could be either..

Easy. Look for resonances near 33.3MHz, 20MHz and 14.3MHz. Or...put
it in an oscillator circuit which favors fundamental mode and see
where it oscillates.

Cheers,
Tom

"Paul Burridge" wrote in message
...
On Tue, 31 Aug 2004 23:05:53 -0400, " Uncle Peter"
wrote:

It will oscillate at the fundamental, not the marked 100 MHz overtone
frequency. At 100 MHz, I'd guess that's a 7th or 5th overtone cut.
Most likely 7th, 20 MHz is about the limit for fundamental crystals.

So how does one tell if the xtal is fundamental or overtone? Not for
xtals marked 100Mhz, obviously, but for much lower frequencies which
could be either..

I have used fundamental cut crystals on their overtones and overtone
crystals on their fundamental. NOTE, the frequencies will NOT be exact
harmonics/multiples. Somewhere I have information giving the typical
differences.


The crystal Colpitts is one sure bet. Stay away from circuits with
inductors and tuned circuits for a fundamental oscillator. Some IC
oscillators can give misleading results. They can pound the rock too hard.


here's some circuits thanks to GOOGLE:

This type is my favorite. Used in just about all Motorola channel elements
of Motrac through Micor and probably beyond.
http://homepage.tinet.ie/~ei9gq/tx_circ.html
Though seems to me, the lower cap should be 100 rather than 220pf, but they
are REALLY non critical. One page I found had them both at 1000pf.

Fig 7 looks the same:
http://www.northcountryradio.com/PDFs/column007.pdf



Discover Circuits also has a lot of circuits.
http://www.discovercircuits.com/O/o-crystal.htm


--
Steve N, K,9;d, c. i My email has no u's.

P.S. the Educypedia has lots of circuit ideas, in general
http://users.telenet.be/educypedia/e...osciltypes.htm



If you do your own search, THIS is NOT a Colpitts crystal osc...
http://www.designnotes.com/CIRCUITS/colpitts.htm