Erik Kaashoek via time-nuts writes:

At a 1ps level of interest, that is not a well-founded expectation.

In particular look out for power-supply variations, which will transpose the trigger point for your "digital" clock-signals.

PS: Do not ignore mechanics either: At the speed of light 1ps takes you only 0.3mm.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

The down converted input signals are converted to digital using an ADC. The
rest is DSP. No digital circuit triggering timers. Can the clock of the MCU
still have an impact? For sure the clock of the ADC can have an impact.

The mechanics are very nasty. No touching of the table with the DMTD on it.
Worst are the coax cables. BNC connectors are terribly unstable.

Is 1 to 10 ps to be expected in the electronics? Or should I invest in semi
rigid coax?
Erik

On Sun, Oct 23, 2022, 21:58 Poul-Henning Kamp phk@phk.freebsd.dk wrote:

Erik Kaashoek writes:

I think you are at the level where you have to treat everything as
suspect, and through well designed experiments try to isolate where
to spend money.

Some randomish input:

1. Take photos of the setup of /all/ your experiments, so you
   later can figure out which ones were affected by the magnetized
   screwdriver or whatever.

2. Make your circuit/experiment constant power.  Having separate
   "idle" and "measure" modes, forces you to wait for thermal
   balance every time you start a measurement.  (One of the few
   things HP only got /almost/ right with the HP3458A)

3. Characterize your sensitivities.  Modulate the power-supply,
   see what happens.  Module the temperature, see what happens.
   kick the table, see what happens.  Turn the experiment 90, 180 and
   270 degrees around as many axis as you can/dare, see what happens.
   Run the experiment in full light and darkness, see what happens.
   If you dont know your sensitivities, you dont know if your
   readings are real or noise, even if they reach statistical
   significance.

4. Pre-owned fridges and freezers make good cheap shielded thermal
   chambers, if you do not plug them in.  If the experiment produces
   too much heat, run constant temperature water through the coils.
   If there is space, add a couple of bricks to add thermal impedance.

And good luck :-)

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi

When you blow cold air over this or that, you likely create a temperature
gradient. Sensitivities of various parts are actually higher to gradients
than to temperature offsets. It’s typically a stress / strain relation that relaxes
out in steady state.

In addition to ( as noted above ) suspecting everything in the circuit. You
also have to suspect the measurement technique……

Bob

I realized when completing http://jmfriedt.free.fr/ifcs2021.pdf that the only
clock that matters in Software Defined Radio is the ADC clock which timestamps
each and every sample, from which subsequent digital processing can recover the
acquisition time. The digital processing system can be asynchronous, buffered, pipelined
but the latency between acquisition and processing will not matter in an open
loop analysis of the radiofrequency data. In the cited work we mistakenly believed
initially that the CPU clock had to be steered, before realizing it was only the clock
referencing the ADC (and the FPGA) that mattered.

Best, JM

Hello,

PTFE cables have quite a phase variation vs temperature, with the worse
slope just around 20ºC. Better alternatives are the Huber+Suhner CT
cables. The catalog shows some nice curves on phase variation vs
temperature:
https://literature.hubersuhner.com/Technologies/Radiofrequency/ct-product-family-en/?page=1

Best regards,

Javier

On 10/23/22 22:15, Erik Kaashoek via time-nuts wrote:

Hi Erik,

only the ADC clock should matter and the used ADC should be of the
simultaneous sampling type. If it's not, its multiplexer may have a
detrimental temperature-dependent effect on the phase measurement.

I've implemented a fully digital DMTD using USRP N210 with LFRX
daughterboards [1]. To analyze stability of the system itself, I
compared a split 10 MHz signal. Over the course of 4 days, the measured
standard deviation was 359 fs [1, Fig. 11]. I don't have temperature
measurements available, but the lab wasn't air conditioned, populated,
and diurnal difference between two SRS FS725 was clearly observable
(another measurement not in the paper).

The high stability could be explained by the N210's dual-channel ADC
that directly sampled both 10 MHz signals. I believe, temperature
differences between the preceding analog components (most importantly
the LFRX daughterboard) probably have a very limited effect on account
of the negligible relative bandwidth of the measured 10 MHz signals'
true frequency (a few (dozen) mHz vs. 10 MHz). If the 10 MHz were
down-mixed to a few Hz in the analog domain, the relative bandwidth
would increase substantially. Of course, that's just an educated guess.
I did not investigate temperature stability when I wrote the paper.

Best regards,
Carsten

[1] https://arxiv.org/pdf/1803.01438.pdf

On 24.10.22 07:19, jeanmichel.friedt--- via time-nuts wrote:

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-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

[2]
http://ww1.microchip.com/downloads/en/AppNotes/AN3526-Dual-Reference-Noise-and-Stability-Measurements-with-the-53100A-PNA-DS00003526A.pdf

On 10/23/2022 9:05 AM, Erik Kaashoek via time-nuts wrote:

Hi Javier,
The PTFE cables have been replaced with semi-rigid coax cables and the
stability, both mechanical and temperature, have improved.
Thanks.
Erik.
On 24-10-2022 8:02, Javier Herrero via time-nuts wrote: