[time-nuts] Re: What phase variations to expect in a DMTD due to temperature fluctuations?

[John Miles]

The 1 ps/C figure is typical of most components other than magnetics.  Some
RF transformers clock in at 10 ps/C or more, and some don't seem to have
much of a phase tempco at all.  And as Javier points out, the choice of
cabling can have a big influence as well. 

 

To Eric's earlier point, there is nothing religiously bad about BNC
terminations in general, because you're not going to move the cables around
during the measurement.  Right? :)  If you are, then screw-on connectors
such as N, SMA, and TNC are obviously a better way to go.  But the best
advice remains, "Don't do that."  It's far better to maintain a
mechanically- and thermally-stable environment than to rely on individual
components to hold their (usually unspecified) values for you.

 

In production test we tend to see residual phase behavior like the attached
screenshots, which came from a 53100A but could also have come from a 5330A
or 3120A.  Here, the unit has warmed up for an hour, but there's still a bit
of initial drift due to slight internal temperature changes associated with
the measurement process.  Once those effects settle, drift is usually on the
order of 1 ps/day in a stable environment, as seen in the other screenshot.


 

The cited spec of "< 10 ps/hour after 2 hours, typically less than 2
ps/hour" is very conservative, usually by 10x or more.   We're leaving some
wiggle room for less-than-ideal conditions in the field, from temperature to
cabling.  You should see similar sub-picosecond drift figures from your own
DMTD, given similar conditions.  If not, there will be a reason, one that is
probably not too hard to address.

 

-- john

Miles Design LLC

 

 

> I'd say 1 ps / C is fine, but the 10 ps number seems high so you may

> want to identify the source of that. For some context look at the specs

> of a metrology grade distribution amp:

> 

>
<https://spectradynamics.com/products/hpda-15rmi-high-performance-distributi
on-amplifier-1-50mhz/>
https://spectradynamics.com/products/hpda-15rmi-high-performance-

 
<https://spectradynamics.com/products/hpda-15rmi-high-performance-distributi
on-amplifier-1-50mhz/> > distribution-amplifier-1-50mhz/

> 

> Its tempco spec is 1.5 ps/C, which sounds good enough for a professional

> timing lab to me. Even if you had 3 C modulation of temperature that's

> under 5e-15 @1000 s so it would be in the noise. And if better

> performance was required you can simply keep the room to 0.3 C instead

> of 3 C. That way everything in your lab -- cables, connectors, power

> supplies, oscillator references, measurement instruments -- improves

> accordingly.

> 

> Another example is John Miles' TimePod [1] and PhaseStation [2]. Note

> these are not spec'd explicitly in ps/C but rather ps/hour. I had not

> seen that before.

> 

> /tvb

> 

> [1]  <https://www.miles.io/TimePod_5330A_user_manual.pdf>
https://www.miles.io/TimePod_5330A_user_manual.pdf

> 

> [2]

>
<http://ww1.microchip.com/downloads/en/AppNotes/AN3526-Dual-Reference-Noise-
and-Stability-Measurements-with-the-53100A-PNA-DS00003526A.pdf>
http://ww1.microchip.com/downloads/en/AppNotes/AN3526-Dual-

 
<http://ww1.microchip.com/downloads/en/AppNotes/AN3526-Dual-Reference-Noise-
and-Stability-Measurements-with-the-53100A-PNA-DS00003526A.pdf> >
Reference-Noise-and-Stability-Measurements-with-the-53100A-PNA-

 
<http://ww1.microchip.com/downloads/en/AppNotes/AN3526-Dual-Reference-Noise-
and-Stability-Measurements-with-the-53100A-PNA-DS00003526A.pdf> >
DS00003526A.pdf

> 

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