Must use 10MHz external clock for 1G data sampling?

[alex.]

I need to use the 1GHz sampling card to collect some data at 2 remote site simultaneously.

The sychronization of trigger will be done by GPS clock.

I saw in this kind of apllication the DAQ cards always fed with an external 10MHz GPS clock. To insure no drift in the sampling.

I just wander why this connect of 10MHz signal is needed. If it is a GHz DAQ card, the clock from itself should be already very accurate, and the measurement time alway not take long.

Why this 10MHz signal is injected?

Thanks.

[Norbert_B]

This Tutorial should answer your question.

hope this helps,

Norbert 

[alex.]

I have gone throught the document.

In case I have a 1GHz DAQ card, itself should have a very good oscillator inside. Why still need one from outside?

[Norbert_B]

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turbot wrote:
[...]very good oscillator[...]

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Very good is relative.....

I don't know which DAQ device you are using, but the oszillator for e.g. 6115 is specified to:

Base clock accuracy ......................................... ±0.01%

That means that the accuracy is 100 ppm. So two devices can diverge up to  0.012 secons per minute acquisition time. So after an hour, the difference can already be 0.72 seconds. And this value is true even if the device is properly calibrated!!

The  6608 has a drift of 75 ppb, which is less than 0.001 times in comparison to the DAQ device clock! So the drift in our example would be reduced to 0.00056 seconds per hour!

The 10MHz clock is used to build a PLL (phase lock loop) to synchronize the devices.

hope this helps,

Norbert

PS: I hope i didn't mistype any values when calculating the times using the Windows Calculator 🙂 

[alex.]

I aimed at the digitizer/oscilloscope with sampling rate from 300M to 2G, such as 5152 or 5114.

And the sampling time duration is not long, less <1S. But need to be accurate. I need to compare the signal's starting difference with accuracy of 50ns.

If their own clock is accuracy enough, it will be good. Or I need to find some GPS with 10MHz output also.

May I know how you convert and calculate them, thanks.