From: John Popelish
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Subject: Re: Cheap low frequency impedance bridge?
Date: Fri, 11 Oct 2002 01:48:44 GMT
NNTP-Posting-Date: Thu, 10 Oct 2002 21:48:44 EDT
Bill Sloman wrote:
> I've got a component at work that looks like a resistance in series with a
> capacitor, and I've got to monitor the capacitance fairly precisely to look
> for degradation in use - the device is set up so that the resistive
> impedance (which is what we are interested in) almost dominates the
> impedance at the measurement frequency, but the capacitance can get eroded
> in service, and we are going to have put numbers on this.
> The measurement frequency range is 2kHz to 200kHz, and we'll probably
> monitor down to rather lower frequencies if we can, to boost the reactive
> impedance to more or less match the resistive component.
> So I need an impedance bridge. I don't need - and couldn't pay for - an RF
> mpedance bridge going into the MHz range.
> The resistive impedance range runs from to 10R to 10k, but the region up to
> 100R is the most interesting. The capacitance runs from about 1uF to about
> 100uF, and the higher values are frequency dependent (just to make life
> interesting and to exclude LCR measuring gear that doesn't give you control
> over the excitation frequency).
> The excitation voltage should be about 5mV rms - we've used more (up to
> about 100mV) without seeing any gross problems, but more precise
> measurements may need the lower drive level.
> We've done some preliminary work on an HP 4192A bridge in a nearby
> university lab, at vast expense. The HP4192A was just what I wanted, but
> Agilent don't make it any more, and my boss couldn't afford to buy it if
> they did.
> Anybody know of anything cheap that offers frequencies up to 200kHz? I used
> a signal generator and an oscilloscope for the intial development, a couple
> of years ago, but for this work we really need something rather more
> Bill Sloman, Nijmegen
Used General Radio 650A and 1650A impedance bridges go pretty cheaply
on EBAY all the time. I don't know how good they are at 200 kHz, but
they work well over the audio range, and at least have calibrated
dials. Don't know how much sensitivity they give at 5 mv excitation,
but I think you can amplify the signal that goes to the null meter for