[time-nuts] Allan variance by sine-wave fitting

[Tom Van Baak]

> Yet another reason to nuke the battery and the A2 board. 

Yes. Then again, the effect is very minor when you look at the PN and ADEV plots. It falls into the category of "look how sensitive a TimePod is" more than "look how bad a 5065A is". And remember it's just a warning lamp, with a toggle switch to reset the blink.

> a PIC-DIV pull compared to the 1 pps section of one of these old beasts (Cs or Rb)? 

Right, a PIC divider is much lower power, but if you're driving a 50R load that's still a lot of current. I suspect this is one reason why high-end standards use 10 or 20 us wide pulses and not 50% duty cycle square waves for their 1PPS outputs. Same power but 10 us is 50,000x less energy than 0.5 s. I've stopped using squares waves for 1PPS around here.

BTW, a trick for blinking LED's -- use two of them out of phase: one that the user sees on the front panel and one that is blacked out or hidden inside. A flip-flop (Q and /Q) or even a set of inverters is all you need. The current draw thus remains constant in spite of the blinking.

/tvb

----- Original Message ----- 
From: "Bob kb8tq" <kb8tq at n1k.org>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Monday, February 26, 2018 11:51 AM
Subject: Re: [time-nuts] Allan variance by sine-wave fitting


Hi

Yet another reason to nuke the battery and the A2 board. 

It is amazing just how small a signal can mess things up at the levels involved in
a good frequency standard. The old “when in doubt, throw it out” mantra may be
a good one to keep in mind relative to a lot of add on features…. how much does
a PIC-DIV pull compared to the 1 pps section of one of these old beasts (Cs or Rb)? 

Lots to think about. 

Bob

> On Feb 26, 2018, at 2:20 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> 
>> 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.
> 
> /tvb
> 
> ----- 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
> 
> 
> Hi
> 
> 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. 
> 
> Bob
> 
>> 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.
>> 
>> /tvb
>> <5065a-adev.png><5065a-pn.png><5065a-tdev.png>
> 
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