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From: email@example.com (Bill Sloman)
Subject: Re: Cheap low frequency impedance bridge?
Date: 11 Oct 2002 14:22:01 -0700
NNTP-Posting-Date: 11 Oct 2002 21:22:02 GMT
Fred Bloggs wrote in message news:<3DA6E7D6.firstname.lastname@example.org>...
> 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
> LOL- this post is quite comic coming from THE individual who is always
> first to suggest the DDS for nearly *any* frequency agile application.
Comic? I find it frustrating in the extreme to have to go out and buy
or rent this hardware, but we want something that will be up and
working on Monday, rather than after a period of design and
The Analog Devices AD9854 DDS chip that produces tightly coupled
in-phase and quadrature outputs
is exactly what I'd love to be able to use - much neater (though
probably no more effective) than the scheme Win Hill set around two AD
> Why don't you take a look at the impedance measurement scheme of
> precision I/Q detection, the DDS is the ultimate plan for
> this-especially at your frequencies. It will have to use an embedded
> microprocessor or PIC though,- looks like the opportunity for you to
> finally learn this technology is upon you. You go, Bill...:-)
An embedded microprocessor or a PIC? How quaint - what I want for
Christmas is an excuse to buy a fast 14-bit A/D converter (I've got
data sheets for three squirrelled away) and a DSP chip (probably one
of the AD parts - I looked at the TI parts a couple of years ago, and
they didn't look as if they were going to be all that nice to program)
and do all the demodulation and a large chunk of the filtering in the
Tom Bruhns (who has given me useful advice on the Agilent products)
has said a little aboutwhat he's doing in this area, which would seem
to go a bit further down the DSP route, but I haven't thought far
enough through what I'd need for the job I'm doing to guess where I'd
want to end up.
I might even be able to sneak a precision ratio transformer (as
described in Rayner and Kibble) into the design.
Fred, it really isn't kind of of you to remind me of all the fun stuff
I could be doing if this were a instrument development problem rather
than a get-me-the-numbers-yesterday problem. Not that I can complain
too loudly - I've had my design fun on this project already, and I
plan to try and write that up for IEEE Transactions on Instrumentation
Bill Sloman, Nijmegen
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