[time-nuts] Allan variance by sine-wave fitting
Tom Van Baak
tvb at LeapSecond.com
Mon Feb 26 14:20:30 EST 2018
> at telling you it was sitting on top of a power transformer. It didn’t matter a lot
I did ten runs of various standards that were within a couple of meters of the bench; the TimePod did not move the entire time. Each standard had a different looking PN plot, so I'm pretty sure the 120 Hz spur we see is the 5065A itself, not something in the lab.
File http://leapsecond.com/tmp/2018b-Ralph-2-pn.png is attached.
Fun fact -- there's a wide spur at ~2 Hz on the 5065A phase noise plot. What do you think that is? On a hunch I opened the front panel and reset the blinking amber battery alarm lamp, and voila, that noise went away. Makes sense when you think of the power variations associated with a blinking incandescent lamp.
----- Original Message -----
From: "Bob kb8tq" <kb8tq at n1k.org>
To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Monday, February 26, 2018 7:00 AM
Subject: Re: [time-nuts] Allan variance by sine-wave fitting
One of the TimePods that I had access to in the past was particularly good
at telling you it was sitting on top of a power transformer. It didn’t matter a lot
which instrument the power transformer was in. For some weird reason it
was a good magnetometer at line frequencies. I never bothered to send it
back for analysis. Simply moving it onto the bench top (rather than stacked
on top of this or that) would take care of the issue.
As far as I could tell, it was just the one unit that had the issue. None of the
others in the fleet of TimePods seemed to behave this way. Given that they
normally are very good at rejecting all sorts of crud and ground loops, it was
somewhat odd to see.
> On Feb 26, 2018, at 7:13 AM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>> BTW: Do you know the cause of the oscillations in the 5065 vs BVA plot?
> The ADEV wiggles aren't visible with normal tau 1 s measurements. But since the TimePod can go down to tau 1 ms, when I first measure a standard I like to run at that resolution so effects like this show up. Once that's done, 1 ms resolution is overkill.
> In this case it appears to be power supply noise. Attached are the ADEV, PN, and TDEV plots.
> The spur at 120 Hz is massive; there's also a bit at 240 Hz; almost nothing at 60 Hz. When integrated these cause the bumps you see in the ADEV plot. It's best seen as a bump at ~4 ms in the TDEV plot.
> Note the cute little spur at 137 Hz. Not sure what causes the one at ~3630 Hz.
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