Author Topic: DMTD board (Read 84779 times)

nctnico · « **Reply #425 on:** February 20, 2021, 02:01:05 am »

Quote from: tkamiya on December 30, 2020, 06:09:18 am

I think there is a major error (on my part) in acquisition screen.

The setting is DMTD 10MHz + 10Hz, 10 readings per second.
Data type is time interval.
Input frequency is 10E-6
Scale factor is 10E-7

Late to the game... If it is a Keysight counter then there can be a problem when using time interval measurements. On the Keysight 53230A for example the interval loses accuracy when it becomes negatives (wraps around). If you measure the interval (from edge to edge) from channel 1 to 2 then 2 should always come after 1. When measuring 1PPS signals using a Keysight 53230A I make sure that the result can never become 'negative' because measurements over nearly a 1 second period do not have picosecond accuracy.

Edit: checked some results; in case of a wraparound on the 53230A the error can be in the order of 200ns.

tkamiya · « **Reply #426 on:** February 20, 2021, 02:37:16 am »

Since then, I dumped HP51132A. It drops a count when it's about ready to wrap around. I'm using Cory's DMTD counter described elsewhere.

tkamiya · « **Reply #427 on:** March 07, 2021, 05:35:45 am »

During this long thread, 3 port and 4 port DMTD setup was suggested. I'm very curious.....

Mr. Riley's famous small DTMD is a combination of 2 port versions. What was suggested in this thread, if I wasn't mistaken, are true 3 and 4 channel where each channel gets hetrodyned via offset oscillator. From here, I'm a bit foggy. How do compare the result? 4 port version will end up 6 combinations thus we'll have 6 different TI results. Now what??

Would someone who tried these share what you found out?

jpb · « **Reply #428 on:** March 07, 2021, 11:37:55 am »

A three port would allow three different DUTs to be compared in a three-cornered hat way giving the individual Allan deviations (if it all worked out) or at least allowing them to be ordered to get the best.
This assumes the noise is not correlated so the measured Allan variance (square of the Allan deviation) is the sum of the squares of the two separate Allan deviations.

Four ports would allow the reference oscillator itself to be measured against itself which would give an indication of the noise introduced by the system.

artag · « **Reply #429 on:** April 08, 2021, 02:08:18 pm »

I got a few of the PCBs made to experiment with : more than I shall need.
Happy to send them out (I've got 6 or so) for free to UK readers.
PM me.

Mrt12 · « **Reply #430 on:** April 15, 2021, 09:40:46 am »

Hallo all

I have not participated in this thread for a while and just checked today what I have missed in the meantime ;-)
there are some interesting results here. But I wonder whether someone has also tried the ECL gates, like MC100EL16 or so, instead of the MC1650?
I am stil having the phase fluctuations on my design, and I still wonder whether the ECL gates are the cause of these problems or the mixers or the mixer filters.

The ECL gates should have a 0.7ps Jitter, so I am not sure which noise floor I could expect from them.

tkamiya · « **Reply #431 on:** October 30, 2021, 08:50:01 am »

Is anyone still playing with this? I have been sitting on mine for quite some time. I still can't get my head wrapped around roll-over situations. Then a thought occurred to me.... Roll-overs are nothing less than reaching counter's highest value and restarting from zero. Then why can't I post process this, and every time it rolls over, add the highest value and create non-rolled over value. Keep doing it..... Then I can have my data without roll over. Arduino is probably capable of handling this since only 2 data comes out every second.

If I wanted to fill a missing data, I think I can post process this and fill-in-the-gap automatically as well, like someone did in head.

I just wanted to know if anyone made significant headway in this project.

jpb · « **Reply #432 on:** October 30, 2021, 04:49:02 pm »

Quote from: tkamiya on October 30, 2021, 08:50:01 am

Is anyone still playing with this? I have been sitting on mine for quite some time. I still can't get my head wrapped around roll-over situations. Then a thought occurred to me.... Roll-overs are nothing less than reaching counter's highest value and restarting from zero. Then why can't I post process this, and every time it rolls over, add the highest value and create non-rolled over value. Keep doing it..... Then I can have my data without roll over. Arduino is probably capable of handling this since only 2 data comes out every second.

If I wanted to fill a missing data, I think I can post process this and fill-in-the-gap automatically as well, like someone did in head.

I just wanted to know if anyone made significant headway in this project.

I've not really started yet. I have populated most of a board but then found I was a capacitor short and need to order one but I've had no time lately (I only have a chance to do it at weekends and family commitments have meant I've had no time) but I hope to get back onto it soon.
Roll over is, as you say, just the phase essentially going through two pi so, assuming random phase shifts are much less than this, it should be possible to post-process. I guess you also need to account for negative as well as positive phase at times but it should all be possible to do in post processing - it might be easiest just to do it on a pc once the file is saved.

tkamiya · « **Reply #433 on:** October 31, 2021, 07:55:47 am »

I didn't think of it as >2pi but you are right. I was thinking more in terms of delta(T) that if t(2) - t(1) is significantly different and going in opposite direction, add t(1) to t(last). I'll have to rethink this, I guess. The final algorithm might end up with complicated scheme....

I have had my time standards turned off for quite a while to save on electricity (as I wasn't using them). I turned them all back on again yesterday. I'm going to have to wait a months or so to establish equilibrium again.

capt bullshot · « **Reply #434 on:** November 15, 2021, 12:10:09 pm »

I've done some frequency difference measurement experiments some time ago, including a DMTD contraption. It's got nothing to do with the original posters DMTD board here, I've started my experiments independently of this thread.

Just for the records, here's a link to what I've done: http://wunderkis.de/tnfb/index.html

Anyway, that above link is just FYI, as I may refer to it later.

I've noticed some of you were experimenting and thinking about different input stages for the DMTD. As a follow up to my previous experiments, I've built a rather simple "modular" DMTD testing system. It consists of the input stage, the mixer stage and the output (comparator) stage. As TIC I'm using the MCU part of this: http://wunderkis.de/tnfb/fdif2.html . Using todays microcontrollers, it's a rather simple job to make up a dual channel reciprocal frequency counter with a resolution of 12.5ns (as in 80MHz timer clock frequency). So I don't have any issues with logging and phase wrapping using this setup.

For the input stage, I've used "no input stage at all" (sinusoidal signal directly connected to the diode mixer inputs) and some "fast" comparators (MAX9012) that were by chance available in my stash of components.

Next is the mixer stage, I've used a Mini-Circuits diode mixer and a mixer made up using a simple XOR logic gate.

Final stage is the IF amplifier / comparator, two variants: a rather simple one and a cascade of limiter amplifiers. Component values were somewhat optimized while testing.

Attached schematic of these modules: https://www.eevblog.com/forum/metrology/dmtd-board/?action=dlattach;attach=1323681

As my intention was to measure the noise floor of different combinations of these modules, I've used the 10MHz and 100Hz offset frequency output part of this (same link as the above TIC): http://wunderkis.de/tnfb/fdif2.html I've used a simple transformer / hybrid based power divider to split the 10MHz into both inputs.

All measurements were done at 100Hz offset frequency, reduced by aggregation to 1Hz sample rate by the TIC. As both 10MHz signals and 10MHz + 100Hz offset reference were supplied from a common source (the 10MHz OCXO), I assume this setup basically measures the noise floor of the different circuit configurations.

Here's the results:

All results are within the 5e-14 to 1e-13 (Allan deviation @ 1sec) ballpark, with the "best" result provided by using "no input stage" and the multi stage comparator. Otherwise, using an XOR logic gate instead of the diode mixer gives rather similar results, slightly better with the simple output comparator.

I hope it's useful and I didn't make any gross mistakes

tkamiya · « **Reply #435 on:** November 15, 2021, 04:22:18 pm »

Wow, great work!

Where did you get that interesting perf board? I've never seen anything like that before.

capt bullshot · « **Reply #436 on:** November 15, 2021, 05:57:45 pm »

Nearly everyone who sees them, asks where I get them

- at some rare occasions I find a partially used one in the scrap box @work
- otherwise one can order them from Mouser: "BusBoards Prototype Systems", Type "SP3T-50x50-G".

tkamiya · « **Reply #437 on:** November 15, 2021, 06:13:52 pm »

This is the neatest thing I've seen! Perfect for RF experimentation! Thanks for the information.

Where do you work?

Gerhard_dk4xp · « **Reply #438 on:** November 15, 2021, 07:26:37 pm »

Perhaps we should have JLCPCB make a few, with 4 pads between the vias
so that SO-8 fit in between. :-)

I have made quite a catalogue of stamps: inv / n.i opamps, diverse regulators,
ecl / CML gates / FFs, oscillators.. Everything SMD, bottom side is GND always,
so I can solder them directly on a copper-clad board.

tkamiya · « **Reply #439 on:** November 16, 2021, 04:19:29 am »

I was actually thinking similarly.

Pads on top side, ground plane on bottom, no plating on vias, and two rows of connected pads at top side and bottom side. Those connected pads will be a power bus. What I'm not sure is, how small I can actually go on pad sizes. I used to use 0.1" spacing single sided perf board all the time, and after few solder/unsolder sessions, they start to peel off. It was smart of the designer of aforementioned product to use square. They offer more adhesive area.

thinkfat · « **Reply #440 on:** November 16, 2021, 08:23:47 am »

Quote from: tkamiya on November 16, 2021, 04:19:29 am

I was actually thinking similarly.

Pads on top side, ground plane on bottom, no plating on vias, and two rows of connected pads at top side and bottom side. Those connected pads will be a power bus. What I'm not sure is, how small I can actually go on pad sizes. I used to use 0.1" spacing single sided perf board all the time, and after few solder/unsolder sessions, they start to peel off. It was smart of the designer of aforementioned product to use square. They offer more adhesive area.

And they are easier to solder, too, because it's way easier to connect the heat source to the pad.

capt bullshot · « **Reply #441 on:** November 17, 2021, 04:54:42 pm »

Here's a result of the DMTD I made, test setup is:

Internal reference synchronized to the PPS output of a simple GPS module (cheap standard ublox neo-6)
Input 1: True Position GPSDO
Input 2: One of my homebrew radio clocks, synchronized to PPS output of a random cheap GPS module (not the same as for the reference)

Internal reference and homebrew radio clock both use one of the cheap OCXOs that I bought from an ebay seller, see datasheet below.

As the microcontroller based TIC has two ratiometric frequency counters and a time difference measurement, it can continously log both differences of the inputs vs. the reference and the input difference in one run. Data are logged as frequency difference, not phase. I do import the frequency difference data into TimeLab using these settings:

ADEV plot by TimeLab ADEV values are well above the measured noise floor, looks plausible as long as I didn't make any systematic mistake. Maybe someone with more experience in this kind of measurement can have a look at the plots and evaluate the plausibility of my measurements.

This is the recorded raw frequency difference, showing an arbitrary excerpt of the logged time series data to get some visual idea of how these oscillators behave.
Yellow trace is PPS disciplined internal reference vs. the GPS disciplined radio clock
Green trace is internal vs. TruePosition GPSDO
Blue is radio clock vs. GPSDO

capt bullshot · « **Reply #442 on:** November 17, 2021, 06:44:47 pm »

Another result:

Ref and Input 2 as above, Input 1 connected to the 10MHz output of the HP53131A with Opt. 10 (HS Oscillator). So this is a free running oscillator measured vs. the PPS disciplined internal reference of my contraption:

So the 10811-60160 (which is the part number of the internal 53131A HS reference) shows better short term stability than the chinese OCXO in my radio clock.

capt bullshot · « **Reply #443 on:** November 18, 2021, 06:32:49 am »

Last but not least in this small row of measurements:

Now I've switched the internal reference from PPS disciplined to free running. MCU firmware basically keeps the EFC input constantly at the last value after switching off the PPS source, removing instability and noise introduced by the disciplining control loop. Compared against the HP53131A internal reference, both now were warmed up for about 36h. The 53131 relies on its native thermal properties, while I've put some "highly sophisticated" thermal shielding around my DMTD (just put it into a cardboard box, and added some thermal mass above the box. Room temperature was low, but halfway constant while the observation period.

So it's a comparison of two free running oscillators (blue trace) vs. the GPS disciplined radio clock OCXO (other two traces). HP53131A internal 10811 and DMTD contraption internal chinese OCXO apparently win. Wonder if I should increase the radio clock / internal reference PLL time constant? It's not clearly defined, as I've implemented some kind of auto on the PLL loop gain, starting with a rather fast loop and then slowing down, depending on PPS to Ref. phase deviation.

jpb · « **Reply #444 on:** November 28, 2021, 05:41:26 pm »

I've finally completed one of the two boards (kindly given to me by artag who had a surplus). I've recently switched to lead-free from lead solder and had terrible trouble getting temperature right and getting it to flow (especially on the surface mount parts) and also I was rather doubtful about the parts I'd had to acquire on ebay so I was very pleased when it just worked!

I need to change my software that talks to my counter and sort out a proper offset oscillator but as a preliminary experiment I tried using my Agilent 33522A.

If I had one channel as 10MHz split and the other as 1Hz offset then I got a difference of exactly 1 Hz (not surprisingly) and the ADEV at 1sec is around 2 to 2.5 x 10^-13 (see screenshot). The counter and the DDS are both connected to one of my two Star4+ GPSDO.

I thought this was a little unfair as a test so I split the output of my other Star4+ GPSDO and used the DDS at 9.999999MHz but I get a frequency difference of around 1.6 to 1.8Hz!!

If I measure the frequency of my Star4+ against the other one the difference is way down in the mHz (see screen shot) so this is a little puzzling. To get 1Hz I had to reduce the frequency on the DDS (again puzzling as I'd expect to have to increase it??).

Anyway, the ADEV at 1sec is then around 4 x 10^-13 so not too bad.

So using DDS (even a good one) seems to amplify frequency difference from < mHz to more like 0.6 Hz - something for me to investigate.

The ADEV figures make sense if the correlation eliminates one half the overall noise (or something like that - slightly arm-waving at this stage, I've not had time to do a proper analysis - just an initial play).

ch_scr · « **Reply #445 on:** January 06, 2022, 01:52:28 pm »

I think there is a problem with the input termination on the MC100EL16D like it is shown above.
To have the 50 $\$\Omega\$$ in between the two inputs will overload the V_BB pin, datasheet says to limit current on that pin to +-0.5mA.
I found this page showing how it is usually done.
They show this picture for differential 50 $\$\Omega\$$ input:

And add that: "Either input in Fig. 2A can also be driven by a single-ended AC coupled sine or square wave signal,
because such a signal would swing symmetrically with respect to the non-driven input which is now biased at V_TT."
I think V_BB could be used, but input coupling would have to be designed different to achieve high enough impedance.
Edit:
Maybe this could work?

kpjamro · « **Reply #446 on:** February 02, 2022, 03:37:41 pm »

Hi - really late to the party I guess.

I have skimmed through several hundred postings and I am a bit cross-eyed.

I would like to ask for a summary/reprise of the DMDT project please. This may help others just tuning in.

Is the design as originally posted considered to work OK? Are any changes recommended? Parts substitutions?

If it is largely OK as-is, then, what are the key files needed?
I have found:

"ts105v19_D.pcb" which I can open with ExpressPCB and presumably use to order boards
"dmtd.png" schematic - is this available as a .SCH file? Is it current?
"STUFFINGTOPSDMV1.jpg" top view with components
"IMG_20201015_094408.jpg" bottom components
I found BOM in a posting - is there a project file for Digikey or Mouser?

Thanks very much.

Ken

tkamiya · « **Reply #447 on:** February 04, 2022, 07:33:33 pm »

I and few others made this DMDT by Corby. It definitely works as advertised. As far as I know, there hasn't been unadvertised changes beyond what is posted here. There is also another thread for easy-to-make-and-cheap period counter that is perfect for this project.

As to what is the latest, I don't have file names, but I suggest reading thread backwards and find out which is the latest. Known changes are change of capacitors on back of the board for temperature co-efficient, and addition of 50 ohm register to the output. You can tell the latest circuit because said register is hand written in.

This creation inspired others to try and make variety of DMDT.

I buy from Digi-key. BOM didn't work for me, so I ordered everything by hand selecting. There are two hard to find new IC. Those came from eBay. DBM is of course Mini-Circuits. I used the same PCB vendor Corby used. I recall sending them a ZIP file for traces, drills, and (there is no silk screen).

kpjamro · « **Reply #448 on:** February 05, 2022, 02:25:11 am »

Who is Bert? and where can I find his boards?

kpjamro · « **Reply #449 on:** February 05, 2022, 02:38:07 am »

Thanks for the reply. I did make one of those propeller board timers.

I searched the entire forum and ts105v19_D seems to be the latest PCB. It has all the outputs in parallel instead of the 220 ohm resistor arrangement discussed and shown in a Kicad schematic by ch_scr (but without PCB files). The PCB would need some significant rearrangement to accommodate those resistors.

But - perhaps there is no more recent PCB file available. And I could not find a marked-up schematic. The only schematic of this is a PNG screen print taken from the ExpressSCH.

I guess I should try a DM to Corby - I hope he does not mind. This discussion format has no facility for managing files...

Regards


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Author Topic: DMTD board (Read 84779 times)

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