On 28/02/14 22:51, Poul-Henning Kamp wrote:
A long time ago, I found out that the HP5370 is quite sensitive to qualities of the external reference signal and after playing around with it a bit, I decided to run my HP5370 from its own OCXO since that was both reproducible and eliminated what I suspected was the root cause. While playing around with John's new CPU board, and now having a bit more kit in my lab, I decided to revisit this detail. The setup I created is the following: 10 MHz GPS locked "lab-standard" feeds ext-ref on the HP3336. The HP3336 generates 10MHz/0dBm which feeds ext-ref on the HP5370 The same lab-standard also feeds the start input of the HP5370 which is setup to start-common, TI, 1k samples, output stddev. And then I step the phase of the HP3336 generated 10MHz through 0...360 degrees relative to the lab-standard. The result is the attached plot, where for every 18 degrees the stddev increases by 8-10ps, roughly 40%. This is evidently because the ext-ref on the HP5370 is multiplied to 200MHz, which is what drives the counter circuits. Another way to run this experiment, is to set the HP3336 to 10.0000001 MHz and log the stddev's over time while the two clocks sweep each other by in phase. Doing it this way can give you a plot of much higher resolution. And that scenario is where the trouble starts: If the HP5370 ref-in clock synchronous to the experimental signals, you will most likely be lucky, but sometimes you will not and the noise will be much larger. If the HP5370 ref is not synchronous, for instance running of its own OCXO, the two phases will sometimes conspire briefly and you get a few noisy samples, but the average will almost always be good. I have not tried to calibrate/trim the HP5370 to see what that does to these spikes, but it would be an interesting experiment.
I'm not a bit surprised. I tried using the normal HP5370 trimming routines, but I found that using my SIA3000 helped a lot on the 200 MHz synthesis chain.
Also, as I have told before the board doing the 10 MHz logic spews out a lot of 5 MHz with overtones, which is a simple mod away.
Would be interesting to see if you could trim these systematics down by tweaking the syntesis chain. Maybe some peaks is good indicators for a particular stage offset, which would be expected from the x5 followed by x4 multipliers with tons of filters. The 50 MHz tank would be a good prime suspect I would think.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
