DMTD board

[tkamiya]

I have not noticed any effect from WiFi or Cellular phones.  But, I did see impact from near-by 10MHz.  Basically, I have my lab wired with lab standard 10MHz from GPSDO.  Also, just about 1/2 of my stuff contains 10MHz OCXO.  There is no concrete proof that indeed, alone, was the problem but I saw something.

Careful wiring internally and externally of the box took care of it.

[Mrt12]

Dear colleagues

finally, I got all my parts from Mouser and I assembled now one of the two DMTDs I have on my PCB. I have exactly the parts that are in the schematic, so I use LT1007 and OPA27. For this high effort I put into the design, I expected way better performance ;-) perhaps I could have even used LM741 as OpAmps.

I tested the noise floor as follows:
HP 8341A, 10 MHz 6dBm output goes on a resistive splitter and goes to the DUT1 and DUT2 inputs (X5 and X7). The HP 8663A signal generator puts 10.00001MHz 0 dBm directly to the offset oscillator input X6.
The HP 5335A counter measures the time interval between the two outputs (X1 and X2, rising edge, auto trigger, it does not make a difference if I use 50 ohm termination or not on the counter).
The settings on the HP 5335A counter are as follows: Gate mode minimum, no delay - this outputs a TI reading 10 times per second.

I adjusted the cable lengths for DUT1 and DUT2 such that the time interval at the output became as short as possible (i.e. such that the zero crossings are close together). Currently I was able to adjust it to around 100 us. However I do not have semi rigid cables but just odrinary Huber+Suhner coax cables, but I arrange them on the table such that they are as stable as possible and don't touch anything during the measurement.

My Timelab settings are the default settings, with a scale factor of 1e-6 because I use 10 Hz beat note. I have attached my noise floor I get with this setup. It looks terrible! if we look at the "phase difference" it looks even worse.

Either my circuit is complete rubbish or I am measuring something wrong - but what?
Please don't tell me that this is because I don't have transformes at the inputs :-D is it?

I tried to repeat the measurement with 1 Hz beat note. There's not much of a difference.

Something is completely wrong here!

[jpb]

I've not built a DMTD system (yet) but I've done lots of experiments with mixing down with a single mixer and generally got similar disappointing results. I think the trickiest part is the zero crossing detection on the very low frequency beat wave - I think it is there that you should concentrate your investigation into your circuit.

Some of the old papers show doing this with multi-stage limiting amps.

My own planned approach is to use autocorrelation (sorry wrong spelling) using an audio interface which gets round the issue of trying to change very low frequency sine waves into nice square waves. A paper was published on this by a Chinese research group.

[tkamiya]

Based on my own result, I can suggest the following, if for nothing else, testing purposes.

Use OCXO detuned for 1Hz for offset oscillator. 
Use coax from DUT to DMTD and split the signal RIGHT AT THE INPUT
Use a counter you can talk SERIALLY and record the session via putty or some other terminal emulator and make sure to REMOVE preambles and garbage.
Play it back on TimeLab by importing it with 10E-7 factor

A lot of this SHOULD NOT have made any difference but it did for my setup.  I still don't have transformers at input.  I am not entirely sure if that is necessary either.

[thinkfat]

The phase difference plot is quite interesting, you seem to have 20-30 picosecond "jumps" that gradually get worse the further the measurement goes. In the beginning they're few and far apart, they gradually get more frequent until they dominate the plot. That doesn't look like flicker noise. It also doesn't look like the noise plots Corby has posted here.

For your measurement setup, I'd not try to exactly phase align the start and stop. How about going for a nice 90° phase offset?

[Mrt12]

The phase difference plot is quite interesting, you seem to have 20-30 picosecond "jumps" that gradually get worse the further the measurement goes. In the beginning they're few and far apart, they gradually get more frequent until they dominate the plot. That doesn't look like flicker noise. It also doesn't look like the noise plots Corby has posted here.

For your measurement setup, I'd not try to exactly phase align the start and stop. How about going for a nice 90° phase offset?

Yup I saw those needle like jumps also. However there could be a ton of reasons for these, I don't know yet where I should start looking!
These spikes could come from:

a) the HP 5335A. I have repaired it once. So far I didn't have any problems with it, but maybe it has an instability which is just so small that I didn't notice  it so far?
b) I have not locked the 10 MHz references of my test instruments together, this means: the two signal generators and the counter do not operate on the same reference. Could this make a difference?
c) I guess the HP 8341A has not a very good stability. Even though the data sheet of this signal generator doesn't specify stability, it specifies phase noise, and this is not very good. Maybe my 8341A is unstable? but again, I thought at least for the noise floor test this should really not matter!
d) maybe it was not a good idea to use switching regulators for the supply rails? I use TRACO TDN5 WI series, they operate at >100kHz and I have added an output filter with ceramic capacitors and inductors. I have tested the output voltages with a HP 34401A, 15.02V and -15.01V and 5.01V. That looks indeed really good.


I don't think there is a problem with the circuit itself, but it is rather a malfunction of some kind I'd say. If I compare my circuit to Corby's, the difference is in the input stage (MC1650 vs. more modern ECL gate MC100EL16). The OpAmp I use (LT1007) has lower noise; besides that I also use the LM311.

I have AC coupled my mixers. Could that make a difference? I used ceramic caps for that. Will try to remove them tomorrow and replace with shorts. However I don't really like the DC current through the mixers! Also my coupling caps to the ECL gates are ceramic. Could this be a problem? it is X7R 100nF. Shall I test C0G?

The fact that the spikes are rare at the beginning and become more frequent the longer I measure indicates that they could be related to some sort of thermal problem. Something in the circuit is cold at the beginning, and as it warms up, it produces the spikes.
I see that the DC/DC converter for the 5V rail is quite warm, even though it is specified for 1A output but it currently needs to deliver only 300mA. Maybe this could be a culprit. On the other hand it has an efficiency of 80%, so it dissipates around 0.5W, should get a bit warm I guess.

I'll try to measure the power supply rails tomorrow with the scope and see whether there is a lot of dirt. And also check without the AC coupled mixers. At the moment I am quite disappointed as I expected much better performance for this effort!

I bet it's the powersupply. Hopefully. This would be a place which would be easy to fix :-)

I'll post my timelab files tomorrow.

[5065AGuru]

Mrt12,

For initial testing use a good OCXO. Once you get a good noise floor you can try other sources.

Here is a PIX of what I use.
- A passive SMA power splitter
- a couple short SMA cables
- an assortment of SMA M to SMA F adaptors (elbows work nice)
- A variable phase shifter from an HP 8405A (You can manage without it!)
 
Feed a good oscillator into the passive power splitter.
Connect the two outputs to the inputs of the DMTD unit.
Observe the count from your counter:
-If it's near full scale switch the start and stop cables
at the counter and or switch the RF input cables.
 
Once you have a count that is in the low end of the count
insert a very short cable or SMA connector into one or the other
RF input channel of the dual mixer.
Inserting the short delay into one of the channels will cause
the count to drop.
Continue adding small delays to that channel until the count
underflows to the full scale region.
Then remove the last delay added.
Then make sure all SMA junctions are tight!!!!
Now go ahead and take data to get your noise floor.
Take data for an hour and avoid drafts and movement around the
system.

If you have not read the DMTD tutorial post I'd recommend doing so.
 
Cheers,
 
Corby

[Mrt12]

Hi together

OK today I solved my problem at least partially!
Initially, I thought I will use ADE-1+ mixers. However I saw that the ADE-2+ is very similar but has a wider frequency range, so I bought that as well.
As I had my DMTD board not yet ready, I made some initial tests with the ADE-1+.  We can connect this mixer indeed as intended, i.e. use the input and output transformers in a differential fashion.
I assumed (but never checked!) that this is also the case for the ADE-2+. Now let's have a look at the mixers from below!



On the left there is the ADE-1+ and on the right the ADE-2+. The very top left and right pins are LO, whereas the very bottom left and right pins are RF. As we can see, the ADE-1+ indeed has only exactly one connection at these pins which goes directly into the LO and RF transformers, so this is fine. However, the ADE-2+ on the right has an additional green wire at the bottom left pin which goes to the center left pin - which is ground! so in my circuit, I have partially grounded my RF inputs. I noticed this problem when I replaced the coupling capacitors to the mixers with 0R resistors - the ECL gates and also the mixers became quite warm after a short while, and the mixers didn't work at all.
I now use ADE-1+ as intended initially, and indeed, this had some influence on the noise floor.



Also the phase difference looks much better now, the mysterious spurs I had previously are gone now!



The noise floor is still not very good, yet. But has improved much! And I think the remaining fluctuation on the phase may be caused by my coax cables and some temperature drift - when I came into my lab this morning, it was quite cool, and switching on the signal generators (with the jet engine fans, you know...) produces a lot of heat which causes the room temperature to increase and therefore the cables to change.
Currently my DMTD is improved much and I almost consider it usable, but getting rid of the remaining phase drift would be interestng!

The phase drift could still be due to a number of reasons; for instance my HP 5335A counter does not have a very good time base. It uses HP 10544A oven which is quite dated and it took months for it to settle and it has perhaps still not completely settled - maybe I should replace this oven someday.

Will do some further tests today or tomorrow, for instance with the FTS1000 which is the most stable and quiet source I have at the moment!

[tkamiya]

Can you not lock HP5335 to a lab standard, or another OCXO or something else that is known stable?

[Mrt12]

Hi Taka
I have only one LPRO-101 that is known stable, but I am unsure whether this is the right reference because I think the LPROs are known for being a bit noisy?
also I don't want to waste precious Rb lamp life, so I try to save it as much as I can. But maybe you are right and I have to use it now!

Maybe the noise floor I have is indeed caused by the jitter of the ECL gates?

[jpb]

I'm wondering if another alternative to the MC1650 might be the LTC6957-1 which is specifically designed as a low noise frequency reference buffer though it is not designed to drive mixers.
https://www.analog.com/media/en/technical-documentation/data-sheets/6957fb.pdf
It is quite pricey though.

[tkamiya]

You don't have any other OCXO?  I have all of my rubidium turned on 24x7.  Ha... 

[thinkfat]

Hi Taka
I have only one LPRO-101 that is known stable, but I am unsure whether this is the right reference because I think the LPROs are known for being a bit noisy?
also I don't want to waste precious Rb lamp life, so I try to save it as much as I can. But maybe you are right and I have to use it now!

Maybe the noise floor I have is indeed caused by the jitter of the ECL gates?

I would not worry about the Rb lamp life too much. Whether the LPRO is a good reference - well, it's not best-in-class but it's a solid performer with a much cleaner output than e.g. a FE5680A (source: http://www.ke5fx.com/rb.htm). You wouldn't be able to pitch it against a good GPSDO for small tau but for larger tau it can easily beat a GPSDO down to maybe 20000 seconds.

[5065AGuru]

Mrt12,

A 10544A (if operating normally) is just fine for the counter timebase!

I'd try a good OCXO as your offset oscillator and see it the noise floor gets better.

Cheers,

Corby

[Mrt12]

Hi together
I just testes these things. Currently I have locked all my equipment to a common HP 10811A. Its frequency may not be accurate, but it should at least be stable. I have no improvement of my noise floor so far!
The ECL gates have a jitter of around 0.7ps. I think with that I should hit the noise floor presumably somewhere at 7e-13. But currently, my noise floor is still around 1.5e-12. Phase is drifting around like crazy! I replaced the input coupling capacitors with C0G. No effect!
I removed the 5V DC/DC converter and use an external power supply (HP E3630A, this should be a really quiet one!). No effect.
I switched off all the lights (I do have LED lights in my apartment). No effect.
Switched off the cellphone. No effect (no big surprise, though).
Replaced one of the coupling capacitors to the mixers. No effect!
I'm going crazy. There is a bug somewhere, but I don't see it right now.
At least I solved the issue with the spikes I had, but that is only a cold comfort.
Currently I hit the 1e-13 noise floor at 100sec. That's too bad I guess!
I'm starting to believe in the transformer things.

[5065AGuru]

Mrt12,

You say "Currently I have locked all my equipment to a common HP 10811A. Its frequency may not be accurate, but it should at least be stable"

Does this mean your offset oscillator is still the 8663A that is driven by your 10811A?

Unless you can measure the AD of your 8663A output you can't really trust it!

In the DMTD tutorial my last post shows the noise floor degradation caused by an offset oscillator that is not at it's best.

You want better than 1.0X10-12th at 1 Sec AD for your offset oscillator.

Last weekend I had locked my HP 3325A to my good OCXO so I could do some tests at a 10Hz offset.
Noise floor was terrible! I then locked my Fluke 6160A to the OCXO and I got an excellent noise floor.

That's why I suggest that you use a good OCXO at 1Hz offset and see if your noise floor improves.

Cheers,

Corby

[KE5FX]

Mrt12,

You say "Currently I have locked all my equipment to a common HP 10811A. Its frequency may not be accurate, but it should at least be stable"

Does this mean your offset oscillator is still the 8663A that is driven by your 10811A?

Unless you can measure the AD of your 8663A output you can't really trust it!

8662A/8663A generators take an unusually long time to warm up because they use narrowband crystal filters in their VHF reference chain.  They eventually achieve decent residual phase stability, but you really need to run them for a couple of hours at a minimum before using them for anything that requires good phase stability.  That would definitely include acting as an offset oscillator for a DMTD.  There will be a lot of drifting and jumping in the first 30-60 minutes.

[Mrt12]

Mrt12,

You say "Currently I have locked all my equipment to a common HP 10811A. Its frequency may not be accurate, but it should at least be stable"

Does this mean your offset oscillator is still the 8663A that is driven by your 10811A?

Unless you can measure the AD of your 8663A output you can't really trust it!

In the DMTD tutorial my last post shows the noise floor degradation caused by an offset oscillator that is not at it's best.

You want better than 1.0X10-12th at 1 Sec AD for your offset oscillator.

Last weekend I had locked my HP 3325A to my good OCXO so I could do some tests at a 10Hz offset.
Noise floor was terrible! I then locked my Fluke 6160A to the OCXO and I got an excellent noise floor.

That's why I suggest that you use a good OCXO at 1Hz offset and see if your noise floor improves.

Cheers,

Corby

Hi Corby

yes I still do use my 8663A as offset oscillator. Why does the offset oscillator influence the noise floor? I thought as long as the zero crossings are closely aligned the offset osc should cancel out.
I'll try again today with some OCXOs!

8662A/8663A generators take an unusually long time to warm up because they use narrowband crystal filters in their VHF reference chain.  They eventually achieve decent residual phase stability, but you really need to run them for a couple of hours at a minimum before using them for anything that requires good phase stability.  That would definitely include acting as an offset oscillator for a DMTD.  There will be a lot of drifting and jumping in the first 30-60 minutes.

aha, this is one interesting result! thanks for that. I didn't consider this. I only know the 8663A has a good phase noise performance.
I definitely didn't let the 8663A warm up for long enough. Just power on and connect and then measure.

[tkamiya]

Corby,

A while back, you've mentioned you are working on a new TIC design.  How is that going?  I'm thinking of starting an 8 channel DMTD (basically 4 of your boards in one box) so I'll have to get 4 of such things.  I have enough HP counters but they are loud!

[notfaded1]

So ideally for my offset for 5Mhz with 1Hz offset with more digits on my Frequency counter would be 4.99999900000.  Used with DUT and REF showing 5.00000000000 or as close as possible.  I've been working on trimming some of the 1050A's I recently acquired.  With the rotary trimmer wheels it's pretty easy to change frequency.  The parts are in 4x10^-12.  My 5065 is a little off from 10MHz so I've been debating what to do about that?  I've been trimming the 1050A's to my 5061B with pretty good luck although after changing the frequency wheels it seems to take a little while to settle on a stable frequency.  I make a change then run timelab vs. the 5061B for a few hours and watch the frequency difference graph in timelab.  It's nice to make a run then start a new one after changing the wheels then start a new run in a different color on the graph to see how it changed.  You can see the frequency change for little while until it mostly stabilizes finally after a few hours.  The bipolar voltages for the board has also been a learning experience for me creating stable +-5 and +-12VDC with a linear power supply.  I'm getting there though.  It helps hearing about what some of you did first.

Bill

[5065AGuru]

Taka,

The counter is looking good.

I've been using the prototype for all my Simple Dual Mixer testing and also with a couple of my other DMTD systems and it works great.

I'm not sure when it will be released, I'll get some feedback on that.

It uses a Parallax Propeller Mini 32150 that you can get direct from Parallax for $24.99 plus shipping.

All you need to add is a couple BNC input jacks, a reset switch, and two resistors.

To interface with USB you will also need to add an Adafruit FTDI Friend @ $14.95.

I think you could also interface via an opto-isolator to an RS232 port.

DigiKey and others also sell them for a couple dollars more.

I'll update when I can.

Cheers,

Corby

[5065AGuru]

Mrt12,

Yes if closely aligned the offset oscillator noise mostly cancels out.
However look at the Nov 28 entry in the DMTD tutorial post.
The oscillators into the dual mixer are an excellent FTS 1200 and a very nice HP 10811.
All plots were run close to coincidence.
The offset oscillator had 6 noise levels added to it from no noise to 2.26X10-11th, and AD plots were made for each noise level as shown.
As you can see for the level of coincidence I was at the noise level makes a big difference.
That's why I'd recommend a noise floor run with a good OCXO as the offset oscillator first, and then you can see if other offset sources will work for you.

Cheers,

Corby

[tkamiya]

Corby,

That's good to hear.  I'll be the backseat child...
Are we there, yet??   

[notfaded1]

I decided to go the other way with the offset oscillator.  This is what I'm looking for correct?  In testing my 1050A's I found one that seemed to possibly be happier higher than 5MHz on the straight from LO 5MHz.  The 1MHz and 10MHz on the 1050A are derived while the 5MHz is directly from the 1000B LO.  Another thing I've noticed is when changing the voltage control on these they seem to drop a little from where they are set after a few hours so it takes a little time (and multiple retries) to get them pretty close to spot on and stable.  One of the 3 1050A I have is really stable at 5MHz.  In timelab after 8 hours the frequency stability measurement still shows in 10^13.  I think this will be my 5MHz REF for short term stability.

Bill

[tkamiya]

Aging OCXOs....    I wonder if 60 days is enough to start comparing??  Some appears to be new while some were well used and possibly sat in warehouse for years. 

I have 8 more boards to make.