[time-nuts] Noise Floor

[John Miles]

> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of
> kb8tq at n1k.org
> Sent: Wednesday, March 25, 2020 9:46 AM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Noise Floor
> 
> Phase noise looks pretty good. ADEV has some weird "stuff" going on. Time
> to start tearing the board apart to see what's wrong with it?
> Maybe there's some noise in those resistors .... let's go !!!!
>

Resistor noise won't normally show up on plots at this level, but it does look like there is some kind of periodic disturbance near 1 Hz.  Could be a thermal artifact due to convection currents on the board, or possibly a case where large undamped LC components have been used in a power supply filter or bias network.  

I'd be more likely to suspect crosstalk from a nearby 5 MHz source that isn't quite on the same frequency, which should be easy enough to rule out.  Poor cable shield integrity can also encourage this type of artifact to appear, even in the absence of an obvious interferer.

On the phase noise plot, the 53100A thinks it's an instrument spur and is removing it.  It's very unlikely to be one, but this process will never be 100% reliable, especially in short-duration measurements.  You can see a suspiciously-flat segment centered at about 0.9 Hz, which often indicates that a spur is being suppressed automatically.  The stability data doesn't undergo any automatic spur detection or removal, so when you see ripple in the ADEV plot that doesn't correspond to an obvious spur on the PN plot, it's a good idea to either let the measurement run longer or set the 'Spur min offset' value to 10 Hz or so.  

-- john