RetiredHAL

Hi, what would be an allowable error margin for a Xtal Oscillator. I took one out of an old Battery wall clock and tried to feed it into a 4060 chip to see if I could get a 2Hz "tick".
To my surprise it was way off, Yet gave good time, when it ran. The mechanism was the Xtal ,some very small electricals under a Glob of pink and a 2cm coil with an "armature" to drive the analogue movement. I could not find any trimming capacitors to tune the movement.
My question is this? Is the armature acting as a "slug" to tune it?
My second question? What accuracy can I expect if I buy a 32768 Xtal?

Cheers RetiredHAL

Nigel Goodwin

For it to be accurate you have to load it correctly, you should have a variable capacitor to adjust it to be exactly correct. Any correctly designed circuit should allow to adjust it both above and below 32768Hz.

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KMoffett

I too wonder about the accuracy of the clock circuits once they're out of the clock. I took one apart that was entirely mounted on a metal framework ... xtal/battery contacts/black blob/driver coils. I cut away the coils and battery contacts so that all that was left was a small "H" shaped frame with the xtal and blob. I diode OR'ed the output to get a 1 second pulse. I also did a divide by 60 circuit to get a 1 second pulse off of the 60Hz power line. I put electronic counters on each of these, started them together, and let them run for three days. The xtal clock counter was about 45 seconds ahead of the power line counter. I have all kinds of these clocks at work and at home, and only need to reset them for Daylight Savings or a dead battery. So their normal accuracy seems much better than that. I'd like to try another one and leave all the parts intact, or find ones that are on phenolic PCBs, and do the same thing to see how they compare. Calls for a trip to Goodwill/Salvation Army/Savers.

Ken

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RetiredHAL

Thank You Nigel, for your quick reply. I understand about the variable cap to tune the Xtal AND circuit. This is evident on the 4060 Datasheet and other documentation. My point being that there was no evidence of a trimming cap in the clock movement unless it was under the glob of Pink paint which I shall carefully scrape away tomorrow. The blob of pink is about 4 mm round. it also means that this being a cheap mass produced clock a lot of manual intervention would be required to produce this item (clock movement driver). And it means it's frequency changes once it is out of the clock.

Cheers
RH

Nigel Goodwin

The original clock was probably designed using a circuit accurately tuned to the exact crystal used? - with either fixed capacitors, or fixed capacitance built-in the PCB. Presumably the 'blob of pink paint' is a COB IC (Chip On Board), basically an unencapsulated IC fixed directly to the board, then encapsulated with the 'blob'

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RetiredHAL

Nigel, I suspect you are right about the COB. The part I tested included the PCB and blob and Coil , but not the armature as it sort of "fell" apart and I was unable to find it after it fell under my workbench in the garage! (sawdust) Hence my question about the armature being part of the tuned cct. (acting as a Slug?, at least that is what we called the screw inserts in coils)
Like KMoffett, I shall visit some Salvation Army places to buy some more defunct kitchen clocks for testing. I am intrigued now as to what causes the change in frequency.

Cheers RH

Diver300

Some circuits are adjusted by fiddling with the divide ratio, rather than adjusting the crystal to the "correct" frequency.

http://ww1.microchip.com/downloads/e...Doc/39747d.pdf

Page 176 (confusingly labled page 174 in text on the page) shows how one microcontroller does it.

audioguru

You cut away metal parts of the clock so you reduced the capacitance of the crystal's wires to the frame which was grounded. Then of course the clock ran faster.

The mechanical pendulum must swing slightly slower than the electronics so it can be physically pushed to the correct frequency.

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RetiredHAL

Hi, Audioguru, I could not scrape off the "Blob" to see what was underneath.
I agree with your explanation that the Xtal should run faster, if I strip the mechanism of the extra metal (extra capacitance).
My observations were that the period of oscillation was 30.8 uSecs. This means that the Xtal/COB/coil pack (without Clock) resonated at ~32.467 Khz. Not 32.768 Khz or more as expected (30.517 + uSec). Is my maths correct? (It's been a long time).
I have ordered a 32.768 Khz Xtal to see if the scope has a timebase problem. I could only purchase a 10 Mhz Xtal (I live in a small country town and there is only a handful of ready components) and made up a Collpit oscillator (discrete components) and measured the frequency. The scope showed exactly 0.1 uSec period for the oscillation (so it appears that the timebase is correct for that frequency range). I hope to give you some more info when I recieve the 32.768 Khz Xtal later this week and see whether the timebase on the scope is out.
NB, I can't repeat the experiment as the original Xtal is now defunct. (it will only take so much abuse from my soldering iron, etc).

Cheers
RH

audioguru

Your oscillator measured nearly 1% slow. Your measured time is 0.017% slow. A huge difference.

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ronsimpson

How are you measuring the oscillator frequency?
Do not use an o-scope to measure frequency. (+/-5%)
Do not use a scope probe on an xtal. It adds 20pf.
In the case of the 4060 measure the frequency at one of the higher Q outputs.

RetiredHAL

Hi, I've been to the "Opportunity Shops" and purchased some battery wall clocks at a ridiculously low price. $3 for two clocks. I dismantled both and apart from the Clock Hands there was no metal. ALL plastic. I probed the running movement (iow, the movement mechanism was running without hands) and found exactly the same time 30.8 uSecs as before (slow).
I shall have to do a further test on a 4060 ic to see if my scope is out or whether I am loading the cct with my probe (tek p6109 12pF). I agree that 12pF is significant, but I would have thought that if I measured at the output of the emitter of the amplifier, I would be isolated enough to allow an accurate time measurement.
NB, I KNOW that the Xtal must run at its resonant frequency (especially 32.768 Khz, as they are used in hundreds of millions of kitchen wall clocks). So this is only an academic problem trying to understand where the error is. ( I am leaning toward my test equipment or my application of the test equipment) .

Nigel Goodwin

A scope generally isn't accurate enough for measurements like that, +/-5% timebase accuracy is probably about as good as you get.

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ronsimpson

You should never count on you scope for accurate measurements. I think your scope is off by 1%. (only 1% that is really good) Use a frequency counter! Last time I calibrated my frequency counter with WWV the internal temperature controlled oscillator was off a little. 10,000,000.2hz

32,768 is the number! It divides down to 1Hz with a very simple ripple counter. Like the 4060. A divide by 32467 is much harder to make.

mvs sarma

Instead of doubting on metal parts influencing the frequecny drift, why not take a measurement of the master crystal frequencyon the test pin of the concerned IC.
Most cheap clocks would not have been tuned, neither they have any arrangment for such fine tuning.

PS: while i was typing , Ronsimpson has posted his idea.

He is very right that the best quality frequecny counters need calibration form a known standard source. afterall wall clock mechanisms, do definitely need adjustments.

I have seen synchronus clocks and these derive time from national standard transmissions at atomic standard clock sources.

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Regards,
Sarma.