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 DK1AG __________________ AXTAL GmbH & Co. KG www.axtal.com 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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