> From:         Harold Hallikainen[SMTP:harold@HALLIKAINEN.COM]
> Sent:         Thursday, January 15, 2004 12:48 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC:] want to make a LC meter with pic.

> Some of the inexpensive C meters I've seen have a wien bridge oscillator
> generating a sine wave. This drives one of the capacitor terminals. The
> other capacitor terminal drives an op-amp current to voltage converter (op
> amp with resistor from inverting input to output, non-inverting input
> grounded, input applied to inverting input directly). Capacitor current
> with known voltage and known frequency is proportional to capacity. So,
> they just use their AC voltage measurement capability to measure the
> voltage at the output of the current to voltage converter.

> They can't tell if it's a capacitor, a resistor, or an inductor, but it's
> a simple way of measuring a capacitor if it is indeed a capacitor.

> If you want to get fancy, you could generate a sine wave with a lookup
> table and D/A to drive the capacitor. Then, instead of just measuring the
> RMS current, do fast D/A samples of the current (as indicated by the
> current to voltage converter). Do a complex number division (using either
> polar or rectangular notation, whichever works out best), to get Z=V/I . Z
> can then be represented as R+jX . Knowing the frequency, you can convert
> from X to inducatance or capacitance and model the unknown impedance as
> L/C in series with R or L/C in parallel with a different R.

> Harold

An analog method would be to generate sine and cosine waves with
an analog oscillator which had quadrature outputs. Mixing the current output
with samples of both input phases will give you DC voltages which tell you
the real and imaginary parts of the impedance (actually the admittance).

John Power

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