[time-nuts] Systematic noise

[Tom Van Baak]

Magnus,

Two things to check.

1) Is the bump you see at all correlated to the relative frequency between the two oscillators? I have seen this when comparing two ULN, for example. You can make the bump move left or right depending on the beat frequency.

2) Some oscillators have separate oven and oscillator circuits that make the following experiment easy: perform a phase noise measurement with the oven power on as usual, and then a second measurement with the oven power off. True, the frequency will start to drift within tens of seconds, but this should not spoil the P/N plot, or even the ADEV plot below tau a few seconds.

/tvb

----- Original Message ----- 
From: "Magnus Danielson" <magnus at rubidium.dyndns.org>
To: "Time-Nuts" <time-nuts at febo.com>
Sent: Tuesday, February 12, 2013 4:09 PM
Subject: [time-nuts] Systematic noise


> Fellow time-nuts,
> 
> Again I feel that it is good that I do not trust the samples I have been 
> given, but start by measuring them. Again I have found a sample where I 
> have systematic noise from the oven controller. Looking at the frequency 
> plot, I see triangle shaped frequency modulation and the period gives a 
> bump just above 1 s (guestimate around 1.6 s). The ADEV has a flat floor 
> at about 2.5E-11 except for the bump with a few multiples which is 
> expected. Otherwise fairly clean and the phase noise seems to pan out at 
> around -157 dB, but I haven't looked at the 0.01 to 1 Hz spectrum where 
> I now expect the "fun" spike(s) to be.
> 
> Will be fun to see how the all-nighter measurement looks, as the house 
> heating cycles should crunch in if it is too sensitive.
> 
> Will swap oscillator and make a reference measurement tomorrow.
> 
> I use a pair of BVAs as reference, and they give me about 2 decades of 
> margin for the ADEV, but since I measure them against each other, it's 
> nice to get a floor measurement (actually, since it is the sum of the 
> two, it will be about twice any of them).
> 
> Anyway, the interesting part was that I found this systematic noise from 
> the oven controller. I've found that once before. That part didn't get 
> selected, as it was clearly non-stable linear oscillation mode, where as 
> this is clearly controller steering having this systematic effect. 
> Expecting a digital controller to be found in there. Will be interesting 
> to see of there is long term systematic effects from the house 
> environment. The warm-up was almost entirely classic crystal warm-up drift.
> 
> The vendor of the part is not mentioned out of discretion.
> 
> The lesson to be learned is that oven controllers can create systematic 
> noise, and you need to figure out if you can live with it or not. Some 
> modern parts can expose such behaviours, but have a perfectly working 
> oven controller otherwise.
> 
> Cheers,
> Magnus