It sounds like you have the specs to attempt to model it in your circuit. On Tue, Mar 15, 2011 at 1:46 AM, Brent Brown wro= te: > 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. >> >> 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 =A0range 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 acr= oss your > current sensing resistor to buffer the signal? A quick look at the Burr B= rown INA118 > datasheet (because it was handy) seems like it would be close but not qui= te, eg. > common mode input impedance 10^10 Ohms would be ok, but common mode input > capacitance typ 4pF probably not ok. Pushing bandwidth too, at gain of 10= =3D > 500kHz. > > Perhaps there are others with better specs? > > -- > 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: =A0brent.brown@clear.net.nz > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .