# [time-nuts] What type of Crystal?

Mon Jun 1 10:53:03 EDT 2009

```Thanks for the info Bernd.  This is exactly the type of data I was
hoping to get.  I also have a 10544A that starts out about 1 - 1.5 KHz
low and it never made any sense to me because I thought it was an AT
xtal.  Now I can check through my junk box oscillators and fill in some
blanks.  Thanks!

By the way, I had a message disappear a week or so ago.  I wonder what's
going on?

Ed

Bernd T-Online wrote:
>
> 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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```