On 15 Mar 2011 at 15:52, Richard Prosser wrote:

> 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.
>=20
> The basic crystal spec is 455kHz, 1uW max, ESR =3D 3.5k max, 1.4k
> typical, 4pF loading.
>=20
> 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.
>=20
> 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.

Sounds like a tricky little problem. Try an Instrumentation Amplifier acros=
s your=20
current sensing resistor to buffer the signal? A quick look at the Burr Bro=
wn INA118=20
datasheet (because it was handy) seems like it would be close but not quite=
, eg.=20
common mode input impedance 10^10 Ohms would be ok, but common mode input=20
capacitance typ 4pF probably not ok. Pushing bandwidth too, at gain of 10 =
=3D=20
500kHz.

Perhaps there are others with better specs?

--=20
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz


--=20
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