[quote="Nigel Goodwin"]
The crystal is tuned by altering the DIFFERENCE on the two sides of the crystal, so by adding to BOTH sides you're not really changing anything 8)
[quote]

Not so, it is the capacitance in parallel with the crystal which changes the frequency.
If you have 27pF at each side of the crystal, you will have 13.5pF in parallel with the crystal. (The capacitors are in series and centre tapped to 0v). There will also be the stray circuit capacitance.

27 pF at each side of the crystal is not the same as 39pF at each side. In the 39pF case, the frequency will be lower.

JimB

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But we can't trust radio RDS time data!
One independent national radio station I listen to (UK 100.5FM) used to transmit time data about 20 minutes out - this kept resetting my hi-fi RDS reciever clock.
I emailed them to ask why this could be; they never answered but their RDS data now no longer carries the time data :!: they stopped transmitting it.

There is an "old Chinese proverb", it goes something like ...
A man with two watches never knows the time :?

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Nigel Goodwin said Then JimB said My tests confirm what you say JimB, it did run slower with bigger caps on each side of the crystal. see my graphic below.
What's our teacher gonna say?

I'm wondering , maybe the caps are not not identical (5% difference on tolerence)?

Mr. Nigel?

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Never mind what the teacher says, he may learn something too.

The capacitors dont have to be identical, (say +/- 50 percent of each other, as a ballpark figure) they just have to give the correct capacitance to put the crystal on to the correct frequency.

JimB

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Always willing to learn 8)

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I have just finished making a Quartz crystal clock pulse generator for one of my clocks using a 32k Hz crystal.

It has just run for the last 3 days and has not lost or gained even a fraction of a second.

I set it up using a my old Maplin "Atomic" clock I made years back. This clock resyncs itself with an Atomic clock via radio every 60 seconds so is always correct. You can can get "Atomic" or Radio clocks for a few pounds now from Maplin/Argos etc. You can of course use "TIM" on your telephone.

All I do is sync the new clock with my ref clock then wait a day and recheck. I note how many seconds the clock is out then adjust the trimmer. Wait another day check and retrim again. Once you have got the clock running OK for a day just increase the time to 2 days or even a week. Don't forget by this time the adjustments on the trimmer will be very small indeed so I don't think with a 32K crystal this would be possible. The clock will also tend to drift summer and winter without temp. compensation so in the end you have to settle with a happy medium.

The setup takes a few days but in my opinion is the best.

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I like the key that is used to wind it. :lol:

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