crystal oscillator frequency

[Johnny Chang]

can a crystal's oscillating frequency be changed by changing the
capacitance across the crystal to something other than what the
manufacturer specified?

will it be stable?

 

[Robert Lacoste]

can a crystal's oscillating frequency be changed by changing the
capacitance across the crystal to something other than what the
manufacturer specified?

Yes if it is used in parallel resonance mode. You can then theoritically
tune it down to the serial resonance frequency (a variation of a couple of
KHz for a MHz range crystal) by increasing the loading capacitor. No (or
very little) if it is used at its serial resonance. But 99.9% of the
CMos-based crustal oscillators are parallel mode.

will it be stable?

No problem.

Friendly yours,
Robert
www.alciom.com
The mixed signal experts

 

[Fred Bloggs]

can a crystal's oscillating frequency be changed by changing the
capacitance across the crystal to something other than what the
manufacturer specified?

will it be stable?

Yes but it cannot change much, 500ppm is considered a large deviation
for AT and SC cut crystals.

 

[Leon]

can a crystal's oscillating frequency be changed by changing the
capacitance across the crystal to something other than what the
manufacturer specified?

will it be stable?

You can also use a series inductor to tune it above the marked
frequency. 'Pulling' crystals like this is a technique often used by
radio amateurs to cover part of a band with a single crystal.

Leon
--
Leon Heller
Amateur radio call-sign G1HSM
Yaesu FT-817ND transceiver
Suzuki SV1000S motorcycle
[email protected]
http://www.geocities.com/leon_heller

 

[MooseFET]

Yes if it is used in parallel resonance mode. You can then theoritically
tune it down to the serial resonance frequency (a variation of a couple of
KHz for a MHz range crystal) by increasing the loading capacitor. No (or
very little) if it is used at its serial resonance. But 99.9% of the
CMos-based crustal oscillators are parallel mode.

You can also shift it by putting a capacitor in series with the
crystal. This leads to a better oscillator in most cases. The
circuit looks like this:

Y1
C3 !-!
From amplifying device -------+-----!!----! !----+--- To amplifying
device
! !-! !
=== C1 === C2
! !
GND GND

The ratio of C1/C2 is constant so that amplifier needs a nearly
constant gain. Changing C3 changes the total capacitance seen by the
crystal.

No problem.

"No problem" may be the wrong answer here. How stable do you need the
resulting oscillator to be? When you pull a crystal, you do suffer
some reduction in stablity because the capacitor's value matters more.

 

[Phil Hobbs]

You can also use a series inductor to tune it above the marked
frequency. 'Pulling' crystals like this is a technique often used by
radio amateurs to cover part of a band with a single crystal.

Leon
--
Leon Heller
Amateur radio call-sign G1HSM
Yaesu FT-817ND transceiver
Suzuki SV1000S motorcycle
[email protected]
http://www.geocities.com/leon_heller

You have to be really really careful about that, though...if the
inductor is too large, the tank will resonate with the crystal being
capacitive. In general there are two frequencies where a given crystal
will have a given capacitive reactance. Also the crystal doesn't have
nearly as high a rate of change of reactance with frequency out there,
so the resulting Q is much lower.

Cheers,

Phil Hobbs

 

[Joop]

You can also use a series inductor to tune it above the marked
frequency. 'Pulling' crystals like this is a technique often used by
radio amateurs to cover part of a band with a single crystal.

Leon

A series inductor tunes it DOWN in frequency. A series capacitor tunes
it up.

One group that explored the obtainable range describe their stuff
here:
http://www.amqrp.org/projects/NB6M contestvxos/VXO Experiments.html
For more examples search for "wide range VXO".

Cheers,

Joop