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
FreeBSD committer      | BSD since 4.3-tahoe
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:

Hi Carsten

Thanks for the very interesting article. As my DMTD uses an audio ADC,
the 10MHz inputs first have to be down mixed to the selected audio IF.
This probably will add to long term instability.
Although in the article "Measurement of Phase Difference Using DSP
Algorithms by Non-Coherent Sampling" by Michal Krumpholc and Milos
Sedlacek it is explained that DFT is not an optimal choice for a DMTD, I
decided to use DFT. This because a bit true simulation showed the impact
of the spectral leakage to be below the performance ambition and using
all samples reduced noise. Also a PLL is used to sync the sampling with
the reference input to reduce the worst impact of the non-coherent
sampling.
In your "DSP for Sine Wave Reference Signals " method you only have one
NCO, how do you avoid the non-coherent sampling and spectral leakage
problem with two non coherent inputs?
In my measurements there is a clear difference in noise between using
coherent inputs (e.g. single clock send to both inputs) and non coherent
inputs, even if the frequency difference is in the order of 10 micro
Hertz, but this could be due to the noise from the DDS used to generate
the dual signals with very small frequency differences. As I have no
access to very stable clocks the DDS is the only tool I can use for
stability measurements.
Erik.

On 24-10-2022 10:10, Carsten Andrich wrote:

On 10/23/22 10:19 PM, jeanmichel.friedt--- via time-nuts wrote:

One thing to watch out for, though, is at the transition from ADC clock
to CPU clock domains, you will have some sort of synchronizer, and you
have to watch out for either metastability or an "off by one" kind of thing.

Perhaps an exception is a classic double buffer in hardware, where ADC
loads a dual port memory with its clock, then CPU unloads the buffer,
while ADC fills the other.  But you still have the uncertainty in the
dual port mechanism.

On 10/24/22 1:33 AM, Tom Van Baak via time-nuts wrote:

for what it's worth, we've just measured a bunch of GNSS band pass
filter/LNA combinations and they run about 30-40 picoseconds/C delay tempco

Luckily the results with the semi rigid coax cables is a bit better then
that.
Also mechanical stability improved a lot.
http://athome.kaashoek.com/time-nuts/DMTD/stability.PNG

The big excursions where caused by opening the door to the room.
The second fairly stable half was measured with a closed door and no
people present.
The input signals where 0.01 Hz apart. The leakage between the two
inputs probably caused 1 ps phase oscillations every 100 seconds.
Erik.

On 24-10-2022 16:25, Lux, Jim via time-nuts wrote:

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

<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/

https://www.miles.io/TimePod_5330A_user_manual.pdf

<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

On 10/24/22 12:49 PM, John Miles via time-nuts wrote:

We didn't measure it separately for our L-band LNA/BPF system, but I'd
be willing to bet that it's the filter that dominates the time delay
tempco.  Changes in L and C with temperature are well known, and changes
in tuning will change the delay.

In our system we're looking for uncertainties << 1ns, over 0-40C
temperature range, so 30 ps/K is a big number.

Hi

I think you will find that a lot of these GNSS gizmos have ceramic or SAW filters
in them. That’s likely not true for a device that covers from above L1 down to below
L5 in one gulp. It is the likely approach for an L1 or L1 / L2 sort of device. It’s almost
certainly how a “telecom” antenna is done.

Bob

On 10/24/22 4:59 PM, Bob kb8tq wrote:

Our receivers are L1/L2, but yes a discrete LC filter.  But SAW devices
have a noticeable tempco too.  30 years ago I used to assume a 100ppm
box for 100 degree change, with a parabolic curve, but I can't remember
if that was Si or LiNbO3 .

Hi

A couple hundred ppm over 100C would not be out of the question for
ceramic. Since you have a parabola on the SAW, it’s going to depend on
just what range the 100C covers. Even there > 100 ppm is not unusual.

Bob

Hi Erik,

spectral leakage only occurs with the DFT due to its implicit
rectangular window applied to the input samples. The appeal of the
digital down conversion (DDC) implemented with the NCO is that coherence
does not matter and spectral leakage does not occur. It's simply a
perfect, digital implementation of the down mixing you perform in the
analog domain. Its purpose is to shift the frequency of the signal to
enable reduction of the sample rate to reasonable levels (a few kSa/s
instead of >= 25 MSa/s) via decimation. The decimation can also be seen
as averaging, so this approach also uses every available sample.

Additionally, the use of complex down-conversion enables straightforward
phase estimates. Relative to the sampling clock by taking the phase
angle of the complex samples of a single channel. Between two channels
by taking the phase angle of fraction of two channels' complex samples.
If you unwrap the phase angles and apply linear regression, you can
compute the average frequency difference.

Best regards,
Carsten

On 24.10.22 15:35, Erik Kaashoek wrote: