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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Freq. and Cap. meter questions
Summary: Simple frequency/voltage converter; a PLL chip.
Keywords: digital meter.
Message-ID: <6932@milton.u.washington.edu>
Date: 31 Aug 90 04:52:09 GMT
References: <33045@unix.cis.pitt.edu>
Organization: University of Washington, Seattle
Lines: 33

In article <33045@unix.cis.pitt.edu> fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) writes:
>
>	Greetings. I got my simple (+/-1999 mV) meter to work
>	just like the "real thing"
>
>	Now, I would like to add a frequency counter and, perhaps,
>	a crude (key word) capacitance meter.
>
>	Is there a chip or hybrid that will convert frequency
>	(hopefully linear) to volts (or ohms)?

	Try a phase-locked loop; for few percent accuracy, you
can use a CD74HC4046 (the data sheet claims 0.1 %; that seems
perhaps optimistic to me), with a single resistor and capacitor.
This includes a high-impedance preamp, works to circa 20 MHz, and
runs on a few milliamps of +5v.  Cost under $2.

>	Also, the capacitance meter seems to be a "can of worms"
>	if one is to make one from discrete components - is that
>	true? 

	Not necessarily: get a clock and a counter; set up an R/C
timer with a good accurate timer chip (XR320 from Exar, or LM322
from National, is more accurate than a straight '555); drive the counter
with the (constant) clock and gate it with the timer output.
This scheme is actually simpler than the A/D converter your DMM
has in it.
	Alternately, hook the capacitor into a PLL chip (like the
CD74HC4046), and observe the output voltage when the PLL locks
onto a fixed frequency.  This produces a voltage proportional
to C.  Change the fixed frequency for range-switching.

		John Whitmore