John Wilkinson wrote:
On Sun, 11 Jun 2006 10:47:31 GMT, W3JDR wrote:
John,
Most overtone crystals are designed for operation on their series resonance.
There is also a parallel resonance, usually a few khz higher than the series
resonance. The fact that your oscillator is putting out a signal a few khz
higher than the frequency marked on the crystal is a good indicator that you
might be operating the crystal in the parallel mode.
You shouldn't have to resort to any 'tricks' like operating it in its
fundamental mode & multiplying, or putting frequency 'pulling' components in
the circuit. It should come very close to the marked frequency if you use
the correct circuit.
Can you refer to a circuit that's similar to yours so we can look at it and
recommend what might be wrong?
Joe
W3JDR
"John Wilkinson" wrote in message
...
Hi,
I need a good second LO at 44.545MHz.
I have a few crystals that I bought, that are say they are 44.545MHz, but
are really 44.548MHz. I tried pulling the frquency somewhat, but to no
great avail.
I have read that overtone crystals are hard to pull.
Does anyone know where I can get good quality crystals at this frequency?
Or how I may combine some easily available crystal frequencies to get
either 44.545MHz or 45.455MHz?
Best regards,
John.
Hi,
Thanks,
The circuit I am using is a simple JFET J310 with a tuned drain circuit.
The XTAL is connected from the gate to the drain via a 10nF cap.
Another was a colpits oscillator, with a 2N3904 transistor. The XTAL was
used in palce of the usual L and C.
Regards,
John.
The Pierce circuit is, I believe, only useful for crystals, and not too
useful at that.
The Colpitts will definitely want to see you using the thing in parallel
mode -- basically you're replacing an inductor with the crystal.
Check out the UHF experimenter's manual from the ARRL. The circuit that
they recommend has as it's core a basic grounded-base Colpitts, with the
wire from the feedback capacitors to the transistor emitter replaced by
the crystal:
.-----------o-----.
| | |
| | |
|/ C1 --- |
.-----| --- |
| | _ | |
| | | | | C|
| '----|| ||--o C| L1
| |_| | C|
| Y1 | |
=== --- |
GND C2 --- |
| |
| |
=== ===
GND GND
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
I don't remember the biasing (obviously this ain't going to work as is)
or where you take off your signal, but the idea is that L1, C1 and C2
form a regular old Colpitts at Y1's 3rd-overtone frequency. Y1 just
makes sure that the dang thing doesn't drift. What little work I've
done with the thing shows it to be very reliable about staying on
frequency and starting and all that cool stuff.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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