[SI-LIST] Re: Fundamental vs Overtone crystal
- From: "Mike Monett" <udi-4l1g@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, "Hill, John" <jhill@xxxxxxxxxx>
- Date: Tue, 11 Jul 2006 17:03:32 -0400
"Hill, John" <jhill@xxxxxxxxxx> wrote:
> Hello Ayan,
> Make sure there is no loop gain at the fundamental mode of the
> overtone crystal. Remember to oscillate all you need is positive
> gain and a loop phase of n*360 degrees.
> Use a VNA and open the loop to measure the gain and phase from
> below the fundamental mode of your overtone crystal to above the
> overtone frequency. Make sure there is no gain (with margin)
> anywhere the phase is near zero degrees (n*360).
> If there is positive gain at the fundamental mode of your overtone
> crystal and the loop phase hits zero degrees, it can bounce
> between oscillating at the fundamental mode and the overtone
> frequencies or maybe just oscillate at the fundamental mode of
> your overtone crystal.
> Best regards,
> John
Hi,
Just thought I should mention that high frequency and overtone
crystals have a low drive power level specification, and can be
damaged by operating at high power. You can't determine the power
dissipated in the crystal with a VNA, and using a current probe to
measure the current may affect the circuit. It may also lack the
sensitivity needed to measure the low current accurately.
One way to determine the crystal drive level is to model it in
SPICE. You can determine the input signal fairly accurately, and the
component values and strays can be measured reasonably well. The
problem is the long startup and settling time needed to do a
Transient Analysis to view the crystal and circuit waveforms.
Because of this, most designers were forced to use low-Q versions of
the crystal and inject a pulse to help the oscillator start. They
still had to wait for it to settle, which could take thousands of
cycles and take a long time in SPICE. Also, the low-Q version of the
crystal has a much higher motional capacitance, so the simulation
does not give the correct crystal current.
I have developed a method of starting the oscillator instantly at
the proper drive level. The method is very simple, has no effect on
the normal operation of the circuit, works with any high-Q circuit
modeled as LCR, is very easy to adjust, and allows you to run only
10 or 20 cycles in Transient Analysis. This is perhaps 4,000 to
8,000 times faster than previous methods. Here's the url:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Regards,
Mike Monett
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