> Another question, has anyone here used an HP 3575A Gain Phase Meter > (1Hz - 13MHz) to set their Rubidium to match the GPS sourced 10 MHz > clock? Would that method be more accurate to line the Rubidium up > than using a 12 digit frequency counter clocked off of the GPS? How useful is just a scope? Trigger on one signal, look at the other, wait a while to see if it drifts. It's probably easier to notice changes (drift) if you look at both signals and align them so you don't have to use delayed sweep. I'm not sure how stable delayed sweeps are, but it should be easy to test. Just look at the triggering signal and delay out a half cycle and see if that's stable. Handwave... If I can see a 1 nanosecond drift, that's 1E-9 per second. (Old scopes aren't that good, but I said I was handwaving.) So I have to wait 1000 seconds (17 minutes) to get 1E-12. In this context, how stable is a GPSDO? If I used a Cesium for the sync input, how much fuzz would I see on a GPSDO output if I watched it wander back and forth for a day? -- These are my opinions, not necessarily my employer's. I hate spam.

Gentlemen, Using oscilloscopes to measure relative phase has been widely used for a long time. The setup appears in many H-P Op/Srv manuals in the adjustment procedure section to calibrate the instrument timebase to a shop standard. But, it's not a useful technique for settings more precise than 1 or 2E-10 in the best situation. The process degrades badly as the 'scope sweep speed increases past 10ns/div since a 10MHz sinewave slope is approaching a rather flat line. Watching 2 rather flat lines for precise zero crossings isn't as simple as it seems. In addition, any noise/drift or outright shifts in vertical position obscure the result & aren't unusual during a 16 minute window. Delayed sweep operation just adds more uncertainty, so you end up waiting for the phase difference to drift to the point that both traces are displayed in the same window. But now, they won't stay there for 16 minutes, unless they're already VERY close to matching. The whole process demands that you walk your way down to locking up the 2 sources. It's more painful than it sounds! Regards, Pete Rawson

From: "Pete" <peterawson@earthlink.net> Subject: Re: [time-nuts] Setting Rubidium to match GPS source Date: Fri, 28 Sep 2007 09:07:29 -0600 Message-ID: <000e01c801e1$4da7efa0$0200a8c0@BASE1> Pete, > Using oscilloscopes to measure relative phase has been widely > used for a long time. The setup appears in many H-P Op/Srv > manuals in the adjustment procedure section to calibrate the > instrument timebase to a shop standard. > > But, it's not a useful technique for settings more precise than > 1 or 2E-10 in the best situation. > > The process degrades badly as the 'scope sweep speed > increases past 10ns/div since a 10MHz sinewave slope is > approaching a rather flat line. Watching 2 rather flat lines > for precise zero crossings isn't as simple as it seems. In > addition, any noise/drift or outright shifts in vertical position > obscure the result & aren't unusual during a 16 minute window. This is why you gain up both channels more and more. This converts them more into vertical lines again and you can zoom in more. However, my experience is that evenetually the 1/f phase-noise will become so apparent that the average motion due to frequency error becomes harder to detect by eye. The method is thus still limited. We have better methods available (i.e. various TIC systems). > Delayed sweep operation just adds more uncertainty, so > you end up waiting for the phase difference to drift to the > point that both traces are displayed in the same window. > But now, they won't stay there for 16 minutes, unless they're > already VERY close to matching. The whole process demands > that you walk your way down to locking up the 2 sources. > It's more painful than it sounds! Can be. The delayed trigger window as such only require a short-term stability and for 10 MHz we are talking about up to 100 ns. Just as with a time-error estimator for a TIC. Cheers, Magnus