[time-nuts] What type of Crystal?
Bernd T-Online
BNeubig at t-online.de
Sun May 31 19:44:46 UTC 2009
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
DK1AG
__________________
AXTAL GmbH & Co. KG
www.axtal.com
Ed Palmer schrieb:
> 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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