DMTD board

[tkamiya]

Today's experiment:

Knowing something is not handling wrapped data well, I worked to eliminate the wrap in the first place.  That meant for measurement period, 1 hour, it must NOT wrap!

Using oscilloscope, I aligned DUT and reference PRECISELY.  Currently, they are drifting with respect to each other in order of 1x10^5 second per second.  At this level, adjustment of HP105B fine adjustment is such that it's not much of a turn but a very very light touch.  At this level, visual observation is reaching its limit.  I switched over to HP5335 and tried to null it completely.  Unfortunately, it wasn't possible as resolution of the knob is limited.  (wire wound multi-turn pot)

Per TimeLab phase difference graph, it moves 1 x 10^9 second per every 300 minutes.  It is drifting up then down, so i'd imagine this is a temperature shift in the room.  Both DUT, and the transfer oscillators are in individual large tupper ware box with covers on them.  I've let them sit overnight to reach thermal equilibrium.

My HP10811 board uses 723 as a pre-regulator to feed the oscillator, then 78L05 to further drop down the voltage.  It is fed to 25 turn pot.  I'm afraid tempco isn't good enough at this fine level.  But this is a circuit HP uses on its on gear.  So I assume what I will get is a typical of this oscillator.  I may have to use a hyper accurate voltage reference and super low tempco multiturn pot next.  They are very expensive!

Here's an Adev and Original phase difference at 15 minutes mark.  Cooling cycle of my A/C just started, so room temp will shift by 1 degree F shortly.

I'm going to commit this and will update at 30 minutes mark.

[tkamiya]

At 24 minutes mark, I am definitely seeing the effects of temperature change in the room.  A/C is still cooling at this moment.  Phase has shifted in plus direction by 1.5 x 10^-9 so far.  At 26 minutes mark, I hear A/C is ramping down.  Trajectory of phase graph has reversed.  Now it is on up and down, but overall unchanged. 

At 28 minutes point, A/C has ramped down further.  Phase is definitely on downward trend.  Moving rather fast.  -0.5 x 10^9 per minute.

Here's phase and Adev graph at this point.

Will be back shortly.

[tkamiya]

At 33 minute mark, phase has shifted down by 3 units in negative direction.  (3 x 10^-9)  Still no wrapping!  Adev is pretty much steady at 2.5x10^-12.  I'm impressed!  Now A/C has stopped.  Overall, that whole event resulted in reduction of Adev by less than one unit at 10^-12.  Looks like a small hill....   

At 37 minute mark, worse Adev point is evident at 3.55x10^-12.  Likely, aging might be the biggest factor at this point.  39 minutes.  Phase difference is rapidly declining, and it has now passed starting figure in opposite direction.  Phase difference is now essentially zero.  (at the start of this measurement, it was +2.5x10^-9)

At 42 minute mark.  There was a HUGE disturbance.  Frequency difference was well within 1 x 10^-11 before.  The jump measures in BOTH direction 4 x 10^10.  I have nothing to explain this change.  On Phase difference, I see a staircase effect.  This is a wrap.  Looking at a counter, it is showing 99 x 10^3.  It may have briefly touched a wrapping point and went back and forth few times. 

Here is a graph from 45 minutes mark.
 

[tkamiya]

After the jump previously mentioned (wrapping event), whole Adev curb went up by 0.5x10^-12.  Not a big deal but it did have an impact.  WHOLE graph went up.

Counter's physical display now reads 98.xxxxx 10^-3 and declining.  So close to wrapping point, there is a risk for another wrap.  At 55 minutes mark, there is no significant event.  Wrapping resulted in phase difference increasing by 1 x 10-7 and it is staying there.

Here's a set of graph at one hour mark.

This run is actually set for 3 hours.  I will report back then and upload a TIM file as well.

[FriedLogic]

I'm kind of wondering about DMTD.

Now we have Corby's DMTD and some of us have it working, what can we do to move this forward?  Improve noise floor?  Make low pass filter adjustable?  What determines DMTD's performance, anyway?  I personally have a plan to make this a "one box" solution.  Package DMTD, TICC, and an adjustable hetrodyne local source.  All I need in addition is reference source and a computer.  For the local source, I am going to use 11801-6011 and have a switchable EFC control that are preset to 1Hz, 5Hz, 10Hz, and external.  For switching, mercury wetted relays will be switch controlled.  I am also wondering if I can pack a small PC and CRT into discarded oscilloscope case and make it truly a one box solution.  I have a few TINY version of Lenovo PC, and small LCD displays.

Ideas anyone?

Just a thought - but before you build it all into a one-box solution, think about whether you want to do three-cornered-hat measurements. You may want to build 3 DMT boards and a three way splitter for the offset oscillator into the same case - plus 3 TICCs. Obviously this puts the costs up considerably but it potentially allows you to get around the need for a very good reference oscillator or at least allows you to order oscillators in terms of ADEV.

  I was thinking about trying a microcontroller dev board as a cheap way of measuring multiple channels. Some have quite a few input capture pins that link to 32 bit timers, and use internal clocks in the 100MHz region. Add a stable 1PPS and clock drift could also be corrected for.

  The Propeller Mini 32150 that Corby mentioned might be able to that sort of thing too, but I'm not at all familiar with it.
...edit: I see that it can be made to something similar, and multple channels as well!
https://www.eevblog.com/forum/metrology/a-simple-time-interval-counter-for-dmtd-work/msg3396377/#msg3396377
http://leapsecond.com/prop/index.htm#protic

[tkamiya]

Here is my 3 hour results.

Wrapping is not periodical and its impact is not that much on Adev curb.  I've noticed when wrapping happens early in first go around, it's unrecoverable.  When it's later, eventually, it settles down to a reasonable amount.

[thinkfat]

There's still something odd going on around the wrap points. See how the noise level increases directly after the wrap? It eventually goes back to where it was, but effectively you can throw away this measurement. It's now showing what your oscillator is doing, instead contains a lot of stuff that comes out of the measurement system.

PS: if you look back at the curves I posted based on your TICC test run, there is no such effect after a wrap. The data is clean once you manage to import it properly. The data from your HP5335A isn't unwrapped properly, either, and it seems there's something really odd going on in a certain time interval range. It looks like a measurement error, loss of resolution or something similar happening for small time intervals. If I were you, I'd ditch the HP, go back to the TICC and spend the extra time when importing the data.

[tkamiya]

Since last night, I changed one testing parameter.  I've set DUT and REF very VERY close to each other in frequency.  My goal was to do a 30 minutes run without a wrap.  I almost made it.  At 26 minutes, it wrapped.  Then wrapping frequency increased.

What this low difference in frequency did was, when it does cross, a little noise can cause it to roll back, then another noise can roll it again.  The slope of change is very shallow.  The graph is showing such.  So what this means is, I need to eliminate the drift even more, start half way between the phase wrap point.  But that kind of defeats the purpose of measuring.

What I don't understand is this stair case effect after the wrap.  If I eliminate this graph (which is frequency difference), it does not form a contiguous line.  AND if you look little after, it goes back to the original Y axis and continues on.  It's like frequency shifting. But it only happens after wrapping for short duration and goes back.  I can't imagine what is causing this.  It's really hard to see but there IS an inflection point in phase graph indicating this "going back to the same Y" phenomenon.

Still there is SOMETHING going on. 

I WONDER....  if noise is rather high and frequency is changing rather highly, it's going back and forth between zero point BETWEEN the collection point?  So count increases and decreases but we won't see it.  Then when the noise level is far enough, it steady increases?

That means trigger point is too low.  No.  DMTD is a comparator system isn't it?  So it will swing logic 0 to logic 1.  Comparatorr gain is too high?  Dampening factor is too low?  Low pass cut off is odd?  Did I put a wrong part somewhere?

I'll have enough parts to make two more in few days.  I wonder if I should...

[FriedLogic]

I'll have enough parts to make two more in few days.  I wonder if I should...

  If you have only made one and are having strange problems, making another one could be useful. If they're both the same, that's fine as it just means that you now have 2 of them to test the rest of your setup even faster.
  I find that when it comes to development, it's useful have 2 boards (or whatever) available as it can make it easier to find problems, as well as seeing the effects of any attempted improvements.

  I've got one made up which seems to be working well, but I am getting oscillations on the noise floor test at 50s-800s which I'm still working to eliminate.

[thinkfat]

Randomness is in the nature of noise. You will always find those oscillations, and you'll find them more frequently the closer your DUT and reference are together. There's nothing you can do about that. Even if you eliminate all measurement noise (which is impossible), there will of course still be the phase noise (or jitter) from the oscillators.

If your system setup or analysis software cannot deal with time interval wraps, tough luck. Anyway, IMHO a DMTD system is not meant to be run for days. There are better ways to analyze long term stability of clocks. Your 53132A using a Rb standard as a reference will do a better job at this.

[FriedLogic]

  I don't think it's random, although whether it's removable or not is another question. I've attached a couple of plots from one run.
  I've not used anything with this sort of single shot resolution before, so it's a new game for me to chase down the source of a picosecond or so of ripple.
  At the moment I'm just using a multi output linear supply, so improving the filtering there is an obvious thing with all the switching supplies built into everything these days.

  It does get more challenging to use them for long runs, but a 53132A is not inexpensive, and neither is a TICC - especially outside of the US. Something based on a few TDC7200s or maybe a GPX2 would work for multiple channels.

[FriedLogic]

  Is there any reason not to use an LT1012 here? I just ask because there seems to be a bit of a shortage of the LT1008 at the moment.

[5065AGuru]

Taka,

Is IMG_4824.jpg a phase plot?

Also which portion is at minimum count and which at maximum?

My guess is that it wrapped down from a low count into the max count region.

If that's the case the noise from the offset oscillator will increase and this would explain why "the noise level increases directly after the wrap"

I think I mentioned once before but I select the input jacks of the mixer so that as the DUT ages it causes the phase numbers to decrease.

Then I adjust the DUT until the counts are low and rising slowly.

Then take your data. Your phase plot will show a rising curve that is slowing down as aging progresses.

It will eventually turn around and start dropping to the wrap point.

This will trace a parabola as shown in one of my posts.

This also allows the maximum amount of time prior to wrapping.

It also has the advantage of usually staying in the lower count region longer.

Cheers,

Corby

[tkamiya]

Corby,

Yes, that is a phase plot.  Please see IMG_4797.jpg to see the relationship between phase and frequency, and in regards to timing of wrap and slope change.

You are exactly right.  The frequency had a downward trend, and wrapped backwards.  In other words, it went from MIN to MAX.  That was my misguided attempt to zero the slope.  I completely forgot it will wrap both ways....  I blame few drinks for that.  I completely understand your logic on why the change in noise level.  What I have problem with, is the fact that change are abrupt.  Of course phase change artifact is abrupt, but change in slope happens; then it traces the original slope.  We are talking about changes in 0.0001Hz, so it may have something to do with resolution.

I am not discounting the possibility of outside influence either. 

I'm starting to think wrapping to be a major issue in successful DMTD measurement.  Also random noise near MIN and MAX can cause wrap to happen multiple times in succession. 

Right now, I can't put my head around this but how about lowering resolution, so it  has wider range between MIN and MAX?  I'll sleep on this idea. 

[tkamiya]

FriedLogic,

I don't think it's random either.  I see this graph as of random noise modulated by a lower frequency sine wave.

[tkamiya]

Corby,

Having OCXO as a standard, definite long term drift exists as well as short and mid term.  Obviously, this setup cannot be used to measure Cesium for days.  In using DMTD method, what is a reasonable expectation?  I was quite happy to have 25 minutes run.  With practice, I can probably do a bit better.

[Bruce G]

Surely an LTC6957-1 could be used as a more readily available input device?
At least its Phase noise is known and is lower than that of the LTC695-3/4 variants at low offset frequencies.
It also allows some tuning of its input stage bandwidth to better suit the input signal amplitude and fequency.

[jpb]

Surely an LTC6957-1 could be used as a more readily available input device?
At least its Phase noise is known and is lower than that of the LTC695-3/4 variants at low offset frequencies.
It also allows some tuning of its input stage bandwidth to better suit the input signal amplitude and frequency.

I agree - in fact I suggested this back on page 13 of this thread though without going into the reasons you've covered. I suppose the main argument against is it is quite an expensive.
I use it in my experimentation as you can get it on a handy (if somewhat expensive) demo board.

[5065AGuru]

Taka,

I do all my runs at 5Mhz with a 1Hz offset.

This gives you twice as much time (200ns vs 100ns between wraps)

However you will still have wraps on longer plots.

Yes if the change is slow enough you can get several steps at the wrap.

This is not a problem as you can eliminate them also by the remove step function.

Say my long plot has 3 wraps and I want to use the entire plot.

I would use single step to remove one step at a time and then have the complete plot with no wraps.

I will post an example here soon of how to properly use the single step that way as it's not straightforward until you are familiar with it.

Cheers,

Corby

[5065AGuru]

Taka,

I posted the unwrapping stuff in the DMTD tutorial post.

Cheers,

Corby

[tkamiya]

Thank you!

[Bruce G]

The built in hysteresis of the MC1650/1 results in AM to PM conversion which degrades the PN performance compared to a hysteresis free comparator when AM noise is present.
Has the effect of AM noise been considered and/or measured?

[Bruce G]

I used the demo board to measure the PN at 10MHz down to an offset of 0.01Hz of the LTC6957-4 and found that it was a little lower than that plotted on the datasheet.

[5065AGuru]

Bruce,

As noted in the beginning posts:

 "This was only to scratch an itch of mine to see how the Efratom TS105 stages would work as a standalone Dual Mixer.
I was not trying to build "modern"!
The 1650 or 1651 were purchased on ebay for reasonable prices.
I also wanted something that could be easily assembled as Sometimes that scares people off."

So I did not consider AM noise effects.

I have tested it against 4 other dual mixers of different designs and MANY Quartz and Rubidium oscillators and other than a bit higher noise floor at the lowest Tau it has performed well.

Well enough that I consider the itch scratched!

Cheers,

Corby

[tkamiya]

I made a simple serial read only terminal using Arduino.  All it does is read from TTL out of Propeller board and display it on 16x2 LCD.  The code is ugly as I needed to avoid certain things to keep up with 115K baud output.  Use of strings slowed down processing so much it wasn't even keeping up with 9600 baud.  I've used 16x2 I2C LCD display and Arduino Nano board.  Connection is just SCL, SDA, VCC, GND, and serial RX.  It only uses one line.


// ------------------------------------------------------------
// Serial monitor
// ------------------------------------------------------------
//  serialMonitor v1.0
//
// reads from serial input and prints on LCD
// this code is speed optimized.  Do not use strings or or statement
// as it will not keep up with 115K serial input
//
#include <Wire.h>
#include <LiquidCrystal_I2C.h>                          // I2C LCD library
LiquidCrystal_I2C lcd = LiquidCrystal_I2C(0x27, 16, 2); // LCD type is 16x2

void setup()
{
  Wire.begin();
  Serial.begin(115200);
  lcd.init();
  lcd.backlight();
  lcd.clear();
  lcd.print("Starting DMTD...");
}

void loop()
{
  if (Serial.available())
  {
    lcd.clear();
    while (Serial.available() > 0)
    {
      char inputChar = Serial.read();
      if (inputChar != '\n')
      {
          if (inputChar != '\r')
          {
               lcd.write(inputChar);
          }
      }
    }
  }
}