[time-nuts] What type of Crystal?

Bernd T-Online BNeubig at t-online.de
Mon Jun 1 02:53:22 EDT 2009

I am wondering why my post did not go through yesterday.
Here it is again:

For an OCXO you can determine whether it is an AT, BT or SC cut crystal
by looking at the frequency difference between warm-up and after.

Jim wrote earlier, that his "10544 osc is sitting about 1.5KHz
LOW at room temp and then increases in freq at warmup (OVEN
temp rising)". 1.5 kHz = 150 ppm @ 10 MHz.
1. A BT cut crystal has a second order tempco of approx. -4*10^-8 per
K^2 with reference to the turn-over temperature. Assuming an oven
temperature of around 85°C, makes a temp difference to room temp of abt.
60K:  (60K)^2*(0.04ppm/K^2) = 144 ppm = 1.44 kHz. This matches closely
to Jims measurement.
2. An AT cut crystal has a frequency vs. temperature response described
by a 3rd order parabola with its symmetry point around 25°C~35°C.
Without going into the math in detail: A cut angle with a UTP of 85°C
has an offset at 85°C compared to 25°C of about -45 ppm. This is much
less than Jim's observation, and the direction of the frequeency change
is opposite to the observed one.
3. An SC-cut crystal also has a frequency vs. temperature response
described by a 3rd order parabola with an inflection (symmetry)
temperature of around 95°C. But the SC-cut f(T) response has a much
flatter curvature than an AT-cut (see the HP magazine article cited
earlier). An OCXO with an SC-cut crytal operating at 85°C shows about
-18ppm offset at room temperature compared to the frequency at assumed
TOP of 85°C. This is a much smaller amount than Jim's measurement.

Therefore it is easy to conclude, that Jim's 10544 uses a BT-cut crystal.

Best regards

Bernd Neubig

Ed Palmer wrote:
>    The recent discussion regarding the type of crystal in the HP 10544A
>    brought this question to mind.  We're always coming across unknown
>    oscillators.  Usually we can figure out the pinouts and voltages.  Then
>    we can measure stability, aging, etc.  But are there any tricks to
>    figure out what type of crystal is in the oscillator?  How can you
>    detect the differences between AT, BT, SC, etc?
>    I think that AT crystals have a broader tuning range than SC and that
>    when warming up AT crystals tend to overshoot the final frequency and
>    fall back.  Are these generalizations correct?  Are there other tricks
>    to help differentiate the crystal types?
>    Ed

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