Re: How inaccurate is a 555 or 7555 REALLY?


<bill.slowman@xxxxxxxx>


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 said no such thing.

If you must quote me out of context, use my words and not your own.



Phil claims that better than +/-0.1% watch crystals
are also available,


** Phil actually said that standard watch crystals are made to 0.002 %
accuracy or 20ppm at 25C.



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

They are pretty awful.


** Yep, 20ppm = 52 seconds a month drift.

Only if the crystal is frequency trimmed and the watch worn on the wrist can
time keeping get seriously accurate.



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.


** Errr - 0.01% = 8.6 seconds per day .

0.5 second per day = 5.8 ppm or 0.00058 % !!!!



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.



** AFC is not necessary with synthesised FM tuners and IME simply not
sed - standard crystals are plenty stable enough with around 1 ppm drift
per degree C.

Even radio scanners work fine with narrow band FM ( 25 kHz wide) at UHF
frequencies without AFC.



........ Phil



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