Hi All,
I have a bit of a problem that I'm trying to sort out.
Basically, we have had a small number of crystal failures and the
crystal manufacturer claims that this is due to us overdriving the
crystal. So I am trying to repeat their measurements and adjust
component values to meet their, and our, requirements.
The problem is that the maximum specified drive level is 1uW and the
maximum crystal ESR is 3.5k. So the maximum rms crystal current is
about 17uA. In the past (different crystals) I have used a current
probe but that only operates down to about 20mA/division on the scope.
Adding more turns would increase the series inductance and upset
things. I can insert series resistance and use FET probes, but while a
1k series resistance will give me a reading, it is not much above the
noise level (I'm getting about 12mV of noise in my minimum bandwidth
of 20MHz). Also, it's close to the ESR value so is probably upsetting
the readings. If I use passive probes, I get less noise, but the
capacitance of the probes upsets the circuit and gives me false
readings - and is possibly the cause of me damaging my origonal
crystal sample.

The basic crystal spec is 455kHz, 1uW max, ESR =3D 3.5k max, 1.4k
typical, 4pF loading.

Any ideas? I'm even thinking of using an AM radio receiver with a
455kHz IF to see if I can get some sort of amplification, but will
need a bit more thought as to how to connect and calibrate it.

I have a good  range of test equipment available, including some RF
specific gear such as signal generatrs, spectrum analysers and VNAs
etc although it's mostly aimed at VHF and above frequencies.

Richard P
--=20
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