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From: firstname.lastname@example.org (Bill Sloman)
Subject: Re: Cheap low frequency impedance bridge?
Date: 11 Oct 2002 14:55:36 -0700
NNTP-Posting-Date: 11 Oct 2002 21:55:37 GMT
John Popelish wrote in message news:<3DA62DF9.E6567ECA@rica.net>...
> 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
> > precise.
> > ----
> > 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
> more sensitivity.
There is a 650A on e-bay for $19.95, but I couldn't get any details on
it. My boss likes the price. A classical bridge could well be an
interesting option, but I might have to download Jim William's low
distortion oscillator from the Linear Technology web site if I wanted
a really good null.
Bill Sloman, Nijmegen
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