DMTD board

[thinkfat]

as ch_scr said, the Q and Qbar outputs are always inverted with respect to each other, so there will be definitely a DC current through the mixer if the input signal is disconnected.

I think a series capacitor is a wise idea. I am thinking about using even two capacitors, one for Q and the other for Qbar, to keep the connection as symmetrical as possible. Maybe this is unnecessary?

Wikipedia says ECL gate current is limited to avoid driving the output transistors into saturation and avoiding slow switch-off. I'd therefore say it's not necessary to do any current limiting or DC blocking.

[tkamiya]

I just learned Mini-circuits will sell insulated spacers for SBL-1.  I am requesting price quote and will update this post once I get it.

P/N B14-045-01

I got a quote.  40 cents each.

[notfaded1]

I have two nice Datum / FTS 1130 10MHz osc that I want to test. They were pulled from some Telecom freq references. I have not found any data on these but they are in the same package as the high end Datum 1000 osc.

I did some more digging.  No data sheet unfortunately, but better than nothing.

https://web.archive.org/web/19990502022103/http://www.datum.com/prod_fts_1130.html

It sure looks a lot like the 1000B I have Ed from the Datum 4040A.  What hardware did these FTS 1130 come from?

Bill

[Mrt12]

On a previous page I said that I am making a similar PCB with three channels.
For reasons of symmetry, I now expanded it to four channels :-)
Attached is my layout of the RF part. As one can see it is actually two identical DMTDs which can be used either as two independent DMTDs or as a multi channel mixer. So it is actually a QMTD ;-)
In terms of crosstalk, I don't know how bad this layout will be, but I plan to use a PCB with a solid ground plane and fill unused space with ground such that the isolation should be optimal.

[chuckb]

I have two nice Datum / FTS 1130 10MHz osc that I want to test. They were pulled from some Telecom freq references. I have not found any data on these but they are in the same package as the high end Datum 1000 osc.

I did some more digging.  No data sheet unfortunately, but better than nothing.

https://web.archive.org/web/19990502022103/http://www.datum.com/prod_fts_1130.html

It sure looks a lot like the 1000B I have Ed from the Datum 4040A.  What hardware did these FTS 1130 come from?

Bill

The oscillators were in a master osc module for some rack mounted Stratum 2 Telecom equipment. I just salvaged the osc and SMA coaxes then tossed the rest of the module many years ago. The IEEE proceedings stated they were developed for a Datum FTS 3380 Telecom Timing Signal Generator. Whatever that is.

Each osc has two independent 10Mhz outputs, which is nice. Ten years ago I opened it to figure out which pins got power and ground. I also noticed that the pcb had provisions for two more buffered 10Mhz outputs. I may have to open it again and add some more buffers and coax connectors to the PCB. I will take pictures if I do that.

It looks like they modified/selected the normal FTS1000 osc for 3x better temp co and aging. So I have high hopes for their performance. We will see.

I will probably use my little HP 5316B Counter (100ns single shot resolution) for this testing. I added a small Rb and PS inside to replace the normal osc many years ago. It makes for a small powerful package. It hasn't gotten much use in the last few years. And it's much quieter than the HP 5370B (20ps single shot resolution) counter.
 

[thinkfat]

The whole idea behind the DMTD system is to not need a 20ps counter in the first place

[tkamiya]

Um....  I'm thinking we should leave this thread to topics that are related to DMTD and Corby's design.  It'll be kind of hard to follow in few months otherwise.

[notfaded1]

No problem the dual DMTD board looks pretty neat if it would work.  I don't think I've seen a 4 channel one before.  Although Corby said he had an 8 channel counter so anything is possible.  Seems like you could use two of the original design though couldn't you?  I assume Corby and others have done this for exactly the same reason.  The way I understand it the two references need to be very close to each other in stability so the measured deviation can be corrected for by dividing by the square root of 2.  Many of the videos I've watched the person used two identical/similar reference oscillators to test the DUT.  Bill Wriley did say it could be extended to M clocks.  I don't fully understand negative variances but it's supposed to be a sign the method is failing.  To try and better understand how this works I've been reading this old paper that has a chapter called Decomposition Of Multi-Clock Comparisons.  For some reason this paper seems to explain things better than some others.  To be honest though I'll be happy if I can get one DMTD working well.

Bill

[Mrt12]

If it's interesting for someone: I made some progress with the layout of my quad mixer. In the picture attached is one half. They are almost perfectly symmetrical, I even tried to match the trace lengths between the channels.
I never did a PCB layout with ECL gates before, and I have been warned it will be "extremely difficult", but I don't know to what degree this is true. Therefore I took care to minimise every track's length which has to do with the ECL gates.

@Corby: you posted on page 1 some numbers about the noise floor of your DMTD. But I remember that you said it was "too good". Why? I see that your design uses one OpAmp after the mixer. I decided to do so as well, but I am not sure how much it will affect the noise floor. Can you say something about this?

[Mrt12]

No problem the dual DMTD board looks pretty neat if it would work.  I don't think I've seen a 4 channel one before.  Although Corby said he had an 8 channel counter so anything is possible.  Seems like you could use two of the original design though couldn't you?  I assume Corby and others have done this for exactly the same reason.  The way I understand it the two references need to be very close to each other in stability so the measured deviation can be corrected for by dividing by the square root of 2.  Many of the videos I've watched the person used two identical/similar reference oscillators to test the DUT.  Bill Wriley did say it could be extended to M clocks.  I don't fully understand negative variances but it's supposed to be a sign the method is failing.

Bill

Yes you divide by sqrt(2) if the two DUTs are very similar to each other.
A bit simplified: you take a load of measurements and then you calculate the variance. The variance you get is the sum of the variance of both DUTs (remember, in the ideal case, the offset oscillator drops out).

I don't know how I can enter math here, so I write it like so: the variance of your measured data is

sigma_meas² = sigma₁² + sigma₂²

where sigma² is the variance, and sigma₁ and sigma₂, respectively are the stabilities of your individual DUTs. Now if both DUTs are "the same", e.g. two LPRO-101 against each other, you have

sigma₁ = sigma₂ = sigma₁² = sigma₂² = sigma_dut²

and therefore you can write

sigma_meas² = sigma₁² + sigma₂² = 2 * sigma_dut²

and now you don't want to know the variance, but the standard deviation AKA allan deviation, so take the square root

sigma_meas = sqrt(2 * sigma_dut²) = sqrt(2) * sigma_dut

For the three cornered hat, the methodology is very simple as well. This time you observe three oscillators, so you kind of have three measurements, each of which has its own variance (sigma_xy is the variance when DUTx is measured against DUTy):

sigma₁₂² = sigma₁² + sigma₂²
sigma₁₃² = sigma₁² + sigma₃²
sigma₂₃² = sigma₂² + sigma₃²

and now you can do the math and solve this equation system because you have three equations and three unknowns (the variance of each DUT). For example, take the first equation, subtract the second one and add the third one. You get

sigma₁₂² - sigma₁₃² + sigma₂₃² = sigma₂² - sigma₃² + sigma₂² + sigma₃² = 2 * sigma₂²

Note, on the right hand side, how all DUT variances have dropped out. All what remains is the variance of a single DUT, and on the left hand side, this is what you measured. So you can figure out the unknown variance of one DUT, provided you take at least three different measurements.

The problem with the negative variance arises from numerical problems. Because your measurement contains noise etc., your variances may not behave exactly as they should and the whole thing breaks apart. For instance, with the Riley DMTD, you can only measure two DUTs at a time against each other. So, if you want to do three cornered hat, you measure 1-2 on day 1, 1-3 on day 2 and 2-3 on day 3. However, the DUTs might change slightly during this time, you don't observe them at the same time and this is where the numerical problem may arise.
I would expect that if you had a multi-channel DMTD, where you can measure all three DUTs at the same time, the numerical problems will be less.

[ch_scr]

If it's interesting for someone: I made some progress with the layout of my quad mixer. In the picture attached is one half. They are almost perfectly symmetrical, I even tried to match the trace lengths between the channels.
I never did a PCB layout with ECL gates before, and I have been warned it will be "extremely difficult", but I don't know to what degree this is true. Therefore I took care to minimise every track's length which has to do with the ECL gates.

@Corby: you posted on page 1 some numbers about the noise floor of your DMTD. But I remember that you said it was "too good". Why? I see that your design uses one OpAmp after the mixer. I decided to do so as well, but I am not sure how much it will affect the noise floor. Can you say something about this?

With so many channels you might want to include some passive resistive splitters like the system that was linked before in the thread. (If you have space left on the pcb)

[thinkfat]

If it's interesting for someone: I made some progress with the layout of my quad mixer. In the picture attached is one half. They are almost perfectly symmetrical, I even tried to match the trace lengths between the channels.
I never did a PCB layout with ECL gates before, and I have been warned it will be "extremely difficult", but I don't know to what degree this is true. Therefore I took care to minimise every track's length which has to do with the ECL gates.

The absolute delay between the channels is not all that important, as long as it is constant.

[ch_scr]

If it's interesting for someone: I made some progress with the layout of my quad mixer. In the picture attached is one half. They are almost perfectly symmetrical, I even tried to match the trace lengths between the channels.
I never did a PCB layout with ECL gates before, and I have been warned it will be "extremely difficult", but I don't know to what degree this is true. Therefore I took care to minimise every track's length which has to do with the ECL gates.

The absolute delay between the channels is not all that important, as long as it is constant.

I wondered if instead of having a phase shifter on one channel input a variable trigger level would work? Would have to be edge sensitive tho, so something a bit more clever that a voltage level pot on the negative input of input comperator would be in order. This to-be-concieved solution certainly has more potential to mess things up than some coax-in-a-box...

(If one were to put a divide-by-2 flipflop after the comperator, the signal would always be 50/50 on the pulsewidth, no matter what the input signal was or what the "phase shifter" did to it. Whole shebang would always be running at half the input frequency (and offset))

On another note: Thermally coupled input gates? To better synchronize their thermally induced delay differences?
Maybe clamp a strip of 2mm aluminium over all smd cases?

[5065AGuru]

Mrt12,

Hi,

All the more sophisticated dual mixers I have Do not have that "too good" performance.
After cleaning up some stuff the new noise floor is just about where I expected it.
The fancy ones use more than one stage of Opamps.
One thing to do to get a close stable phase match is to use low TC caps on the op amp stage and match them.
Also match the parallel resistors in that stage and use low TC on them.
On my NBS unit I replaced the original 100K resistor with .01% 1PPM and the capacitor for the 1HZ setting with selected PPS caps.

Cheers,

Corby

@Corby: you posted on page 1 some numbers about the noise floor of your DMTD. But I remember that you said it was "too good". Why? I see that your design uses one OpAmp after the mixer. I decided to do so as well, but I am not sure how much it will affect the noise floor. Can you say something about this?

[thinkfat]

Mrt12,

Hi,

All the more sophisticated dual mixers I have Do not have that "too good" performance.
After cleaning up some stuff the new noise floor is just about where I expected it.
The fancy ones use more than one stage of Opamps.
One thing to do to get a close stable phase match is to use low TC caps on the op amp stage and match them.
Also match the parallel resistors in that stage and use low TC on them.
On my NBS unit I replaced the original 100K resistor with .01% 1PPM and the capacitor for the 1HZ setting with selected PPS caps.

Cheers,

Corby

Corby, how do you select PPS capacitors for matching TC?

@ch_scr: that might be a drawback of a differential signal path. You have to match even more components, not only between the channels but also for symmetry of the positive and negative half of the amplifier. I guess the IF filter is probably not that critical, but definitely the components that set the amplifier gain.

PS: my take on the DMTD with differential IF

[5065AGuru]

I only matched the capacitance of the caps! I relied on the very low TC intrinsic to the PPS caps.

Cheers,

Corby

[tkamiya]

Corby,

I'm amazed by amount of excitement your project has generated.  Thanks!

[Mrt12]

Corby,
you said

After cleaning up some stuff the new noise floor is just about where I expected it.

so what's the corrected noise floor then? do i need just more gain if I want to go towards the 1e-13 range at 1sec?

[tkamiya]

....and what dictates the noise floor? 

I wonder such things as using semi-rigid line for input and output and securing such rigidly would improve noise level?  I plan to use N fitting for both in and out, then use sma-semiRigid-sma assembly for interconnect.  I also plan to secure it to chassis for longer runs.  I just don't trust BNC, and SMA is rather fragile for anything other than internal connections.  I'm afraid it will be just another possible cause for unrepeatable results.  After all, we are seating over less than nano-seconds variance.

[Gerhard_dk4xp]

AD8612 would be suitable, too, I guess. But I'm more concerned about the mixers. The MiniCircuits stuff is really nice, but I need to find a distributor that stocks them. Digikey doesn't have them at all and Mouser refuses to sell them in Germany 

The German distributor for Minicircuits is www.municom.de. They were here already
40 years ago and won't like it to be kicked away by Mouser.
You can get an interesting subset at funkamateur.de who is a NOS reseller, not a distributor.

Cheers, Gerhard

[5065AGuru]

Mrt12,

If you look back on page 1 you will see the plot "realnoisefloor". That is the corrected plot.

You can reduce the BW by increasing the value of the cap around the op amp.

If you reduce it to 10Hz then you can still use 10Hz or 1Hz offset frequencies.

If you reduce it to below 10Hz you can only use the 1Hz offset.

The lower the BW the lower the noise in the system and the lower the noise floor.

Also stable well regulated low noise power supplies are best.

The original NBS106 DMTD system used batteries, the later version (the one I have) has built in AC powered supplies.

I keep my DMTD units powered 24/7 so that they are mostly at thermal equilibrium.

I'm not sure how much you can buy with the simple dual mixer architecture?

For my regular DMTD units running them with 10MHz vice 5MHz inputs also buys a little better performance but unsure if that applies here.
Cheers,

Corby

[thinkfat]

Corby,
you said

After cleaning up some stuff the new noise floor is just about where I expected it.

so what's the corrected noise floor then? do i need just more gain if I want to go towards the 1e-13 range at 1sec?

I'd say the noise floor is mainly dictated by the ZCD. You want a low noise amplifier after the IF filter with moderate gain and small bandwidth and increase the bandwidth in additional stages to have a steep trigger signal eventually.

[thinkfat]

AD8612 would be suitable, too, I guess. But I'm more concerned about the mixers. The MiniCircuits stuff is really nice, but I need to find a distributor that stocks them. Digikey doesn't have them at all and Mouser refuses to sell them in Germany 

The German distributor for Minicircuits is www.municom.de. They were here already
40 years ago and won't like it to be kicked away by Mouser.
You can get an interesting subset at funkamateur.de who is a NOS reseller, not a distributor.

Cheers, Gerhard

I took a leap of faith meanwhile and ordered a few ADE-1 from China. Not sure what they actually sent, they are marked "ADE-1-24" and from the looks they are actually mixers

I've connected them to my AWG and feeding the LO and RF inputs with 10MHz and 10.000,010 MHz indeed gives the expected spectrum on the IF port. So they should be usable at least for an initial test.

[notfaded1]

AD8612 would be suitable, too, I guess. But I'm more concerned about the mixers. The MiniCircuits stuff is really nice, but I need to find a distributor that stocks them. Digikey doesn't have them at all and Mouser refuses to sell them in Germany 

The German distributor for Minicircuits is www.municom.de. They were here already
40 years ago and won't like it to be kicked away by Mouser.
You can get an interesting subset at funkamateur.de who is a NOS reseller, not a distributor.

Cheers, Gerhard

Probably the safest thing would be to just order them from Mini-Circuits.  They can ship to you from US or the UK.  For you probably UK would make the most sense.
https://www.minicircuits.com/WebStore/dashboard.html?model=SBL-1-1%2B

[Gerhard_dk4xp]

I have bought both at funkamateur.de and Municom (just recently 10 GHz stuff
for a commercial project). No problem whatsoever and no ado with customs
(as in 2 months).

Gerhard