[time-nuts] HP 5372A vs. 5370A
Ed Palmer
ed_palmer at sasktel.net
Wed Feb 9 21:49:32 UTC 2011
SAIDJACK at aol.com wrote:
> Hi Ed,
>
> excellent email. You raise the SNR of this forum!
>
Thanks Said. It's rare that I have a chance to contribute. Many of
these discussions are way over my head, but I'm learning.
>
> I have tried to measure the CW-12 output myself with phase noise and Allan
> Deviation equipment, and the constant cycle jumps prevent these from giving
> sensible results. Both PN and ADEV plots look awful, many orders of
> magnitude noisier than even an average oscillator. Glad to know that your 5372A
> could get the job done.
>
> btw: jumping cycles 200 times a second is not disciplining an oscillator,
> it's numerically controlling a frequency. Thus the CW-12 cannot be
> considered a GPSDO due to the massive 8ns phase jumps.
Absolutely correct. Navsync doesn't call the CW-12 a GPSDO. They
explicitly say that the 10 MHz is a numerically controlled oscillator.
They even have an app note on cleaning up the output. I was curious to
see if I could use it as a 'new, improved' version of the Jupiter GPS
receiver with it's 10 KHz output. Haven't gotten around to it, but I
believe others have.
Ed
> But for many apps such as
> driving a microprocessor clocks this may be good enough, even though one has
> to be careful that their internal PLL's can lock to such a noisy (jumpy)
> source. Of course the number of corrections per second should have a very
> strong correlation to temperature as the TCXO used on the CW-12 drifts over
> temperature.
>
> bye,
> Said
>
>
> In a message dated 2/9/2011 13:14:16 Pacific Standard Time,
> ed_palmer at sasktel.net writes:
>
> The CW-12 also has a 10 MHz output. I used the Histogram Time Interval
> function to measure the periods of 100 Million cycles with a resolution
> of 200ps. It took less than 30 seconds to measure, process, and display
> the results. The results showed that there was a normal distribution
> around 100 ns and a second normal distribution around 92 ns. The
> difference is approximately the period of the internal clock (120 MHz).
> This told me that the 10 MHz is kept on frequency by occasionally
> shortening the period by one cycle of the internal clock. For my unit,
> this happens about 200 times a second. I know this by the ratio of the
> number of long periods to short periods. This behaviour explains why
> Navsync warns that this output needs to be cleaned up before using it as
> a frequency reference.
>
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