How inaccurate is a 555 or 7555 REALLY?

You could hand match R and C to potentially get that, but then it'll change with temp and other parameters. So practically you won't achiveve 0.1%

If it's just a fixed frequency you want then use a crystal oscillator and divider. That can be made as small as or smaller than a 555 solution.

Alternatively use a tiny microcontroller in an SO-8 or maybe SOT-23 package. Internal oscillators are only 1% accurate or so, but if you use an external crystal you'll get what you want. Use software timing or a PWM to give you the frequency you want.

Dave :)

TLC551, SOIC-8. Not really small.

Expecting much better than ~1% over reasonable temperature changes, supply voltage changes etc. is probably optimistic. Not only the chip, but capacitors and other parts will change.

You could step the 1.5V up to 3V using a switching regulator and then use a CMOS chip or a microcontroller with a crystal timebase, and avoid the calibration.

Best regards, Spehro Pefhany

actually the good old 555 is stable to .005% per degree C so your first task to get .1% is to keep it in the range of +/- a few degrees C. then you have to turn your 1.5v into the minimum operating voltage of 4.5v, easy enough to do with a maxim charge pump chip which is tiny and smd and needs only a couple of small caps. then you have to have your resistor and capacitor be cermet and polystyrene so they will have a good enough tempco. if you do all this you could easily get .1% out of a 555. the chip is the least of your problems with an RC time constant, the problem is temperature coefficient. you can get extremely low tempcos by combining positive and negative tempco components to get a single one with almost zero tempco.

i agree this is a job for a microprocessor. you can get a PIC in an SO-8 package and to get .1% stability or 1/1000 you would need something like a watch with a stability of 1 second every 16 minutes. a pretty easy thing to do, as you can see. when you want precise stable timebase related functions a computer is the easiest and best way to do it.

-bob

metal film

polyphenylidene sulphide

Fat chance. You'd have to stqabilise the voltage as well as the temperature - the divider inside the 555 is a series of three voltage-sensitive diffused silicon resistors.

I spent a month or so back in 1974 on exactly this problem, and discarded the 555 very early on.

For a individiual capacitor - the temperature coefficient of wound capacitors depends on the mechanical tesion in the dielectric, and the tolerance on te temperature coefficient is all over the place.

Polyphenylidene sulphide sufrace mount capacitors are not wound (as far as I know) and do better, but they still don't hack it at the 0.1% level.

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.

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.

the last poster made it sound like getting .1% accuracy out of a microprocessor based system was impossible. he needs to go look at the accuracy of a $2 digital watch again. 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?

-bob

All the ones I see have 20, 30 or 50 ppm accuracy.

Equates to 0.002%, 0.003% or 0.005 % tolerance.

..... Phil

.002% would be good enough for me.... but i don't know where to find examples on making a tiny quartz crystal divide down to 420hz in a tiny package (I don't have more than about 3/4" box to fit things into.) or any other component oscillators that can reach 420hz with the least number of components.

Any basic circuit diagrams online that someone can point me to would be greatly appreciated! Sine wave output preferred, but I think square will do if that ads complexity (I bet it would). Again please remember I'm a novice with minor experience :)

Thanks so much, ~Brendan

The size and performance criteria you have come up with make it a job for an expert.

....... Phil

"mri_bob"

They DO use temperature compensated LC oscillators and even then the temperature drift is still so bad that in nearly all cases an "automatic frequency control" ( AFC ) circuit has to be used to keep the radio on station.

Modern hi-fi FM receivers use frequency synthesisers and so have the accuracy and stability of the crystal used in that circuit.

....... Phil

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

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.

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.

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.

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.

That is a seven bit divider - which you can set up with a single

http://www.ortodoxism.ro/datasheets/philips/74HCT40103D.pdf

which is availalble in a 10mm by 6.2mm surface mount package - rather smaller than your 19mm square target space

If you poke around the standard crystals available off the shelf from a broadline supplier like Farnell you should be able to find something gets you closer to 420Hz -

Dividing by 23810 calls for a 15-bit divider - two 74HCT40103 parts in series, or a single programmable logic part which you would be able to find in a 44-pin square package - the first example I came up with occupied a 12mm square., which is about the same as a pair of surface mount 40103 parts. Phil Alison should be able to show me up by finding something smaller.

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

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

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

....... Phil

i did not make up the .005% spec for the 555. It is on the first page of National's data sheet. the poster said he wanted .1% and i said the 555 would not be the controlling factor.

as for prototype/production issues, the guy said he wanted to make one, he didn't say 1000. your experience in a production environment is irrelevant.

i said crystals were good enough. i don't know why you are arguing that point? you seem to be misstating my position on that one just so you can disagree with me.

i said .1% is a minute and a half a DAY and a $2 watch is better than that, then you rant about needing to go to a lot of trouble to get .5 seconds per day and say that is .01% Well, there are 86,400 seconds in a day so .01% is 8.6 seconds. You are off by over an order of magnitude and you are berating me saying i don't know what i'm talking about?

and you do not need automatic frequency control in a radio to keep it from drifting 100 kHz. nor do you need a PLL. a cheap simple fm ham receiver with no AFC will not drift that much by an order of magnitude. to quote you: "Try to understand the technology you pontificate about before you claim to be getting real"

bob

"mri_bob"

Temperature drift: 0.015 % *per degree C* for the astable mode.

Drift with Supply: 0.3% per volt.

The 555 will certainly be a *major source* of frequency drift.

You MUST allow a for at least a 25 degree C change in temp - ie from 10 C to 35C.

So, even with *perfect* Rs and C sand a rock steady supply, temp drift will be 0.375 %.

A well made, variable 100MHz LC oscillator will drift by around 5 kHz per degree C.

However, once the gear has warmed up and if the room temp is stable, there is no temp change going on !!!

If there is NO temp change, then there is of course NO frequency drift !!!

This the DUMB error you are making.

....... Phil

Ignoring the insufficent supply voltage, the basic inaccuracy of the

please let's go back to my original answer and let me explain what i was trying to say. when the guy said he was a novice and wanted to build something with a 555 that was stable to .1% the FIRST thing I said was that he would have to keep it temperature stable to a few degrees to have any chance, and that if he did that it would not be the 555 that would prevent it from being done. i was trying to help him to understand the variables involved in the analog implementation by analyzing the best that might be done with common analog parts, then I said that there were several sources of .1% error that would add together and so it would be very difficult to do what he wanted. I then told him that a crystal based microprocessor circuit would be a better choice.

bill then attacked my response, ignoring my first statement about needing to control the temperature tightly. he said watch crystals were not good enough to get .1% accuracy. I pointed out that a cheap watch was easily accurate to .1% because that was a minute and a half a day. he proceeded to come up with an argument against watches being that accurate by grossly miscalculating the seconds in a day.

my last statement that a 100 MHz FM radio would not drift 100 kHz without AFC is true from my experience with older radios with no AFC after they have warmed up. I should not have made the statement about "get real" and i apologize. I was reacting to what i felt was an unjustified attack against my attempt to explain to a novice the various sources of error when trying to get an analog circuit stable to .1% I did not mean to insult anyone.

-bob

"mri_bob"

So totally impractical in the real world.

No temp change = no frequency drift.

Frequency stability of oscillators is a non trivial topic.

..... Phil

If you can afford to use a 3V lithium cell instead, there are a few microcontrollers in SOIC-8 package that have a 32.768 kHz xtal oscillator option. Divide by 78 will get you 420.10 Hz. While you can't avoid the 30 usec jittter, you can improve long term accuracy by dividing by 79 once in a while (use a phase accumulator to add up phase error until you get a whole cycle)

Unless you find yourself a crystal oven and keep it at a steady 70C, or build yourself a proper temperature controller around a Peltier junction - the one I published managed

by

Sometimes. I don't get too worried about such allegations until they put down their own numbers.

Not always true. Inductors have a nasty tendency to shift inductance as the DC current through the coil changes, even if you keep the temperature constant (whichcan be a little difficult if you are changing the heat dissipated in the coil).