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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Programmable Multi-Tone generator wanted
Summary: Two switched-capacitor filters will generate two sines
Message-ID: <11059@milton.u.washington.edu>
Date: 13 Nov 90 22:40:22 GMT
References: <15621@slice.ooc.uva.nl> <JON_SREE.90Nov13012237@world.std.com>
Organization: University of Washington, Seattle
Lines: 29

In article <15621@slice.ooc.uva.nl> ropg@ooc.uva.nl (Rop Gonggrijp) writes:
>
>
>   For a phone switch I am currently developing I am looking for a chip that:
>
>   Will generate two sine-wave tones simultaneously
>
>   Is computer programmable (height of tones, and preferebly amplitude)
>
>   Works from 100 to 3000 Hz
>
	A switched capacitor filter (the National Semiconductor MF10
for instance) can generate sines from a digital programming frequency.
So, if you can generate programmed-frequency square waves, clock
one half of an MF-10 and you'll generate your sine waves.  See
AN-307 (National Semiconductor Linear Applications Databook) for
an example circuit.
	To control the amplitude, note that at one point in the
feedback loop (circuit of Fig. 7, op cit) the signal is a square wave;
feed this square wave into the Vref port of a four-quadrant DAC,
and take the DAC output to drive the filter, and you have both
programmable amplitude and frequency.  Or, just run a few bits
worth of R/2, R/4, R/8 etc. resistors to switches to ground,
and when you ground them, instant programmable attenuation.
	Takes two DACs for the two amplitudes, a programmable counter/
timer for the clocks that determine the output frequencies, and
an MF10 dual filter chip.  Total parts cost circa $12.

	John Whitmore