Clocks

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Hello,

does anybody know about clock accuracy  - I need a very stable clock to
synchronize 2 devices via a RF connection. They must exactly have the same
clock, at this time I'm using a 100 MHz clock generator and a Spartan-3
fpga. But for my intended purpose it's not accurate enough. That means, on a
scope my generated data bursts of each device with a interval of 3 us are
"running away" (that ones those aren't triggered). The 3 us intervals differ
perhaps in half a ns or something.
Is there a practicable solution for such a problem? Does a DLL with clock
mirroring eliminate the problem?

Thanks in advance,

Stefan



Re: Clocks
Stefan,

DLL with clock mirroring would eliminate a skew between the clocks, but if I
understand you correctly, you are facing not a simple issue, especially
because you have 100 MHz and you want to use RF connection as a link. That
is, you have to embed certain information in the outgoing data, such as the
information about your local clock.

The common way to do this is to run, for example, 50-bit wide counter with
100 MHz clock in one end and transfer the value of the counter through the
data to another end. In the another end, there is the same counter and this
value is compared with the one embedded in the data traffic. The difference
between the two gives you an estimation about the difference between the two
clocks.

The rest depends on the device that you are using for clock generation and
its crystal pullability or frequency range that you can use.
If your clock would have been much slower, there is a way to do this using
very high-speed clock & phase shift technique.

If you could give a few more details..........

Hope this helps.
Vladislav


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Re: Clocks
Hello Vladislav,

the problem is that I cannot transfer data via my RF link. (in the end this
is the aim...) I just transmit bursts of a 200 Mbit signal with a interval
of 3 us. These bursts contain data for a spread spectrum calculation, and
are only used with a analog correlator. To transmit data, I invert the bits
from this burst and after that on the correlator the output is -1 (instead
of 1 before).
And for this purpose, I have to synchronize these 2 bursts. I just tried to
use a DCM for clock generation, with no effect.
Really annoying... I got so far in this project and now this problem...

Any suggestions?

Regards, Stefan





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Re: Clocks
Stefan,

My knowledge of SSC is pretty scarce, but from what I do remember, the
problem you are facing is not trivially solvable (I hope I am wrong here)
unless can obtain a direct or indirect information about the clock, which
you are using as a sampling clock, correct? (and this is the problem!)
Because if this is not a sampling clock...

Again, I hope I am wrong.

Vladislav

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Re: Clocks
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I'm aware this is not an analog forum... Squaring the
signal would give 200MHz. You could phaselock your
squared oscillator to the incoming stream.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

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