Re: Simple digital oscillators
- From: Terry Pinnell <terrypinDELETE@xxxxxxxxxxxxxxxxxxx>
- Date: Sat, 24 Sep 2005 08:49:03 +0100
Mac <foo@xxxxxxx> wrote:
>When I think about a cheap, quick and dirty digital oscillator, I think of
>a CMOS Schmitt trigger inverter with a feedback resistor and a shunt
>capacitor at the input: (use courier or similar for ASCII art schematic)
>
> R
> +--/\/\/\----+
> | |\ |
> +----| >o----+----out
> = c |/
> |
> GND
>
>But the frequency depends somewhat on the threshold and critically on the
>hysteresis voltage, both of which seem to be spec'd pretty loosely.
>
>AoE mentions this circuit:
>
> R1 C
> +-----/\/\--------+-----||-----+
> | | |
> | \ |
> | /R2 |
> | \ |
> | / |
> | |\ | |\ |
> +----| >o-------- +----| >o----+----out
> |/ |/
>
>(fig 5.30)
>
>AoE sings the praise of it for its low phase noise. But what kind of
>frequency spread would you expect to see? If you were trying for, say 25
>Hz, would you get 25+/- 5Hz?
That latter configuration is one I've used a lot over the years, with
4001/4011. Just dusted off my probable original source, R.M.Marston's
'110 CMOS Digital IC Projects for the Home Constructor'. Writing
around 1976 the author says:
"In practice, the operating frequency ... is subject to a change of
less than 5% over the production spread of transfer voltages, and to a
frequency shift of less than 2% with a 40% change in supply voltage.
Another advantage conferred ... is that of excellent thermal
stability. The operating frequency typically varies by only 1% over
the temperature range -40°C to +85°C."
--
Terry Pinnell
Hobbyist, West Sussex, UK
.
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