555 Frequency Precision

P

panos v

Guest
Hello,

I am having some problem with the 555 type oscillator. I am testing various
brands and qualities of 555s and each one produces a different frequency
for
the same configuration of resistors (Ra & Rb) and capacitor (C). Even
between 2 ics of the same
brand (and batch) the frequency is different. In detail for a theoretical
frequency of 72KHz, i get
experimental values of 54MHz for "no-name" TTL-555s, 56.6MHz for "no-name"
LinCMOS-555s and 67.6MHz
for Philips TTL-555s. As you can see the difference is from around 20MHz
downto around 4KHz.
To sum up, is this normal or is there a problem with my circuit? Does this
mean the precision of an unreliable "no-name" 555 (that produces
inconsistent frequency) will degrade in the future due to wear
out from usage or to increased temperature? Is there an other economical
and fairly common option to produce stable and theoretically predictable
digital clock at 76KHz?

thanks in advance
@@
 
Another batch of 555 frequency mysteries?

I was hoping to see your feedback about the very similar query you
raised 5 days ago, subject '555 oscillator queer behavour'. For
example, so far you haven't shown us your circuit, or provided details
of what frequency measuring approach you use.

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
panos v wrote:

Hello,

I am having some problem with the 555 type oscillator. I am testing various
brands and qualities of 555s and each one produces a different frequency
Click to expand...
Yes. That happens.

The answer is on the data sheet IIRC. Yup.
http://focus.ti.com/lit/ds/symlink/ne555.pdf

Check 'initial error' on page 6. Can be 3% ( presumably +/- ) making 6%
difference between 2 samples possible. That's for a monostable too. You'll be
using it as an astable and the numbers look worse ( unspecified max ). Reading
between the lines, assume it'll be no better than 12-14% worst case.

Don't forget the tolerance on your Rs and Cs either !


Graham
 
In article <opsi30lwq734r7we@localhost>, panos v <2222@yahoo.gr> wrote:
Hello,

I am having some problem with the 555 type oscillator. I am testing various
brands and qualities of 555s and each one produces a different frequency
for
the same configuration of resistors (Ra & Rb) and capacitor (C). Even
between 2 ics of the same
brand (and batch) the frequency is different. In detail for a theoretical
frequency of 72KHz, i get
experimental values of 54MHz for "no-name" TTL-555s, 56.6MHz for "no-name"
LinCMOS-555s and 67.6MHz
Click to expand...
You got 56.6MHz from a 555 *holy cow*.

Before anyone else says it "Use a PIC" They have a built in oscillator and
can give you the needed frequency on an IO pin.

Now that that's out of the way:

and fairly common option to produce stable and theoretically predictable
digital clock at 76KHz?
Click to expand...
How stable? How predictable? How accurate?

76*32 = 2.432MHz. You can get 2.457 crystals off the shelf. The CD4060
or 74HC4060 would do nicely.
--
--
kensmith@rahul.net forging knowledge
 
In article <r7g4s0p5g9ufi7eo6mnpm26mnpdkam2e4o@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:

[...]
Don't forget the tolerance on your Rs and Cs either !
[...]
Graham

BUT! It will stay on frequency one set with a pot.
Click to expand...
True, unless you use a bad ceramic capacitor. Use an NPO or plastic
capacitor.

--
--
kensmith@rahul.net forging knowledge
 
In article <41C25DFA.2A5125C9@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
I make a point of designing out presets whenever practical ! *Someone*
is bound to
be unable to resist fiddling !
Click to expand...
... or forget to fiddle on the production line.


--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote:

In article <41C25DFA.2A5125C9@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
I make a point of designing out presets whenever practical ! *Someone*
is bound to
be unable to resist fiddling !

.. or forget to fiddle on the production line.
Click to expand...
I'm glad to see you're aware of that one too !

I've seen presets set in *very* odd positions occasionally. I have next to
no time for them. If you can't design a tolerant circuit you should learn
how to !

Hilarious example. A design I reviewed had a 5.1V zener reference diode.
Followed by a preset to trim out its tolerance. Did no-one consider using a
bandgap reference ? Probably cheaper in total cost too !


Graham
 
On Thu, 16 Dec 2004 19:14:48 -0700, Jim Thompson
<thegreatone@example.com> wrote:


BUT! It will stay on frequency one set with a pot.
Click to expand...
---
Not necessarily, since the timing components will have their own
tempcos which _aren't_ ratiometric.

Not only that, if the timing cap is electrolytic its leakage current
will also vary with temperature, throwing the frequency off.

For the OP's application, at the frequency he's working he probably
won't be using an electrolytic, but you never know...

--
John Fields
 
On Fri, 17 Dec 2004 04:54:34 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

In article <r7g4s0p5g9ufi7eo6mnpm26mnpdkam2e4o@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:

[...]
Don't forget the tolerance on your Rs and Cs either !
[...]
Graham

BUT! It will stay on frequency one set with a pot.

True, unless you use a bad ceramic capacitor. Use an NPO or plastic
capacitor.
Click to expand...
---
Not necessarily true, since the pot and cap will have their own
tempcos and the pot will be used like a rheostat.

--
John Fields
 
Until the temperature changes.

Back in 1975 my boss got interested in cheap oscillators, and I looked
at a bunch of RC circuits - the 555 got discarded very early. An
emitter-coupled multivibrator was a lot better, but even that wasn't
good enough - probably due to the temperature sensitivity of the Early
effect.

Bob Widlar's LM322 (of blessed memory) got rid of the worst problems of
the 555 by using an internal regulator to stabilise the voltage across
the oscillator circuit, but it still wasn't as good as the
emitter-coupled multivibrator (which probably came from Peter
Baxandall).

-------------
Bill Sloman, Nijmegen
 
Until the temperature changes.

Back in 1975 my boss got interested in cheap oscillators, and I looked
at a bunch of RC circuits - the 555 got discarded very early. An
emitter-coupled multivibrator was a lot better, but even that wasn't
good enough - probably due to the temperature sensitivity of the Early
effect.

Bob Widlar's LM322 (of blessed memory) got rid of the worst problems of
the 555 by using an internal regulator to stabilise the voltage across
the oscillator circuit, but it still wasn't as good as the
emitter-coupled multivibrator (which probably came from Peter
Baxandall).

-------------
Bill Sloman, Nijmegen
 
In article <8j25s091aqbdq896v8i6ujr1bm60tulo26@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:
[....]
Not necessarily true, since the pot and cap will have their own
tempcos and the pot will be used like a rheostat.
Click to expand...
Ok, add "don't use a cheap pot either" to the advice.

--
--
kensmith@rahul.net forging knowledge
 
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