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From: firstname.lastname@example.org (john jardine)
Subject: Re: Cheap low frequency impedance bridge?
Date: 11 Oct 2002 07:25:27 -0700
NNTP-Posting-Date: 11 Oct 2002 14:25:27 GMT
"Bill Sloman" wrote in message news:...
> 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
You've got a real stonker of a measuring problem here. I make the D
values ranging from '8'(2kHz-1u-10r) to '.000008'(200kHz-100u-10k) and
consequent phase angles of 83deg to an incredible 46udeg.
The HP4284 'LCR meter' (D down to .000001) or the HP4278 'C meter'(D
down to .00001) could make a stab at it but at enormous cost. The fine
phase(time) resolution needed, implies a lot of precision digital
signal processing and consequent expense.
Perhaps the best option is to restrict the test range to say a 'Q' of
1000 max and go for an older second hand HP LCR meter, say £1500 or a
HP4342 'Q' meter for a few hundred pounds.(Even then Q meter has a
lower limit of 22kHz +inductor Q problems.)
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