Re: How inaccurate is a 555 or 7555 REALLY?


mri_bob wrote:
of course crystals need to be trimmed to get them exactly on frequency, but
they do stay there. When was the last time you heard of a digital watch
that could not keep time to better than a minute and a half per day? .1%
accuracy and stability is easy with a crystal system.

as for capacitors, a cheap film capacitor is about 200 ppm per degree C so
if you are keeping it within the 5 degrees you need to get .005% out of
the 555 you will have .1% stability right there. you can do a lot better
than a cheap film capacitor.

as for the voltage, i said you needed a voltage converter to run the chip,
and these are regulated. the 555 uses 2/3 of the supply voltage as the
comparator switch point, so it has a lot of immunity to power supply
voltage changes.

Dream on - the 2/3 ratio depends on the diffused silicon resistors, and
they drift around with voltage.

you can see that there are several variables with the 555 that are close
to .1%, so by the time you add them up it would be difficult, but the last
poster obviously has never combined parts with different tempco directions
to get one that is better than either. i have.

May be you have done it with one resistor and one capacitor. Try doing
while paying attention to worst case tolerances, which is what you have
to do for even small volume production, which is what I was being asked
for back in 1974.

the last poster made it sound like getting .1% accuracy out of a
microprocessor based system was impossible.

Rubbish. anything with a 10MHz crystal will do 0.1% without any effort.
As Phil Allison has pointed out, getting 10MHz crystals stable to
better than 0.1% over their temeprature range is just a matter of
paying more money. Phil claims that better than +/-0.1% watch crystals
are also available, and he's rarely wrong on that sort of detail -
mymemory suggestsed that there was a tendency to rely on the human
wrist to keep the temperature range at the crystal within a fairly
narrow range.

He needs to go look at the
accuracy of a $2 digital watch again.

They are pretty awful. The slightly more expensive watch I use needed a
session with a good frequency meter to pull the crystal onto 0.5
seconds per day (+/-0.01%), but that was some twenty years ago.

And if you could not get an RC time
constant to stay stable to .1% your 100 MHz FM radio would drift 100 kHz
and you would be on another station every time the room changed
temperature. let's get real, huh?

Ever heard of automatic fequency control? Getting 100hkHz accuracy out
of an FM radio depends on dividing down the local oscillator to
somethihg like 10MHz and using a phase-locked loop to keep the local
oscillator locked to a decent crystal - once you have found the station
you want, the system then locks the local oscillator to the station
with some kind of automatic frequency control circuit, and the
frequency control voltage does change with room temperature. Try to
understand the technoligy you pontificate about before you claim to be
getting real.

--
Bill Sloman, Nijmegen

.

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