[time-nuts] ADEV query Timelab and TICC
bob at evoria.net
Sun Mar 19 23:30:39 EDT 2017
These look a lot like the 25ns pops I was getting when I first started generating the 1PPS output from the PIC. In my case, the PIC uses a PLL to multiply the external clock from 10MHz to 80MHz, which is then divided to 40MHz and used as an instruction clock. This gave an occasional early or late pulse, which was off by 25ns. I wound up fixing my problem by arranging it so that the timer used to generate the 1PPS was offset by 2 instructions (so that the timer fired between two successive 100ns pulses from the OCXO) and then gating the generated pulse with the OCXO so that the OCXO was the thing generating the actual 1PPS output. Of course, this could be something entirely different: For example, the quantization error on the 1PPS from the GPSDO as Tom mentions. But, in that case, it seems like there should be a lot more of them.
From: Tom Van Baak <tvb at LeapSecond.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sunday, March 19, 2017 10:04 PM
Subject: Re: [time-nuts] ADEV query Timelab and TICC
> I have sent a couple of files to Tom. They were taken simultaneously from
> an LTE Lite - one from the PPS and one from a PicDiv dividing the 10MHz to
> 1Hz. The glitches were on the PPS trace, but not on the PicDiv trace, so
> I'm fairly confident the TICC was working correctly.
Thanks for the raw data. It's very nice (2 hours 16 minutes = 8219 points). Everything looks fine with the exception of 8 glitches. These are sometimes obvious jumps in phase, which cause massive spikes in frequency. Two plots attached.
Almost every data point is within a few ns of each other. This is good. The standard deviation is a fraction of 1 ns. But once in a while there is a relatively massive phase jump. This is bad. Interestingly these 8 phase jumps all appear to be about 25 ns or a multiple of 25 ns in magnitude. The full list is (ns units):
25 * N ns is not random. So I think this is not a Windows problem, not a USB problem, not a TimeLab problem, not a TICC problem either.
It makes me wonder if this is a LTE-Lite problem. If Said or Keith from Jackson Labs is around -- is there anything on the LTE-Lite board that's close to 20 or 40 or 80 MHz? At this point I kind of trust Orin's data and I kind of trust the TICC. So when I see monster 25 ns phase jumps it makes me think there's a problem with the GSPDO board itself.
(Please realize that only on time-nuts may we can use the words "monster" and "25 ns" in the same sentence; the rest of the world has larger problems)
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