DMTD board

[Gerhard_dk4xp]

> I'm making a PCB design for HP10811.  Should be handy
> for offset or reference oscillator.  I'll post here when it's done.

I have made one for 10811A, Morion MV89, MTI-260 and
some less known, up to 100 MHz. It has a coolrunner CPLD
to generate a local 1pps. There is also a post amplifier
to increase power output to ~20dBm, with the option
to use it as a push-pull frequency doubler, with 2 crystal
notches against sub/harmonics. 10 MHz Morions have
an internal doubler and can use that.
There is a 2FF phase comparator in the $3 CPLD and the
low pass filter on the board to lock the oven as a clean-up
oscillator to an incoming reference frequency.
The hardware could also lock to an incoming 1pps, for
GPS etc. This has not yet been completely implemented in
the CPLD. Frequency locking works.
Vtune direction and some common frequencies are strapping
options and can be changed without touching the VHDL
of the CPLD.

Everything is optional. Think of it as a toolbox.
Someone in the US had boards produced locally from the
Gerber set, just for mechanical mounting.

Cheers, Gerhard

[tkamiya]

Gerald,

While yours in a Swiss Army knife, mine is a Ginsu knife.  (American joke, sorry)  I wanted to keep all that separate.  I have an external low noise amplifier board to go with it.  I also didn't use SMD for ease of assembly.

My initial goal was to make a burn-in tool.  But then I decided to make it more general interface board. 

One of the painful part was that commonly available socket for 10811 comes in two different pitch between rows.  I'll see if a fab house can actually do this.

[5065AGuru]

Mrt12,

I don't know the jitter of the 1650/1651 but the noise floor of the simple dual mixer is close to 3.5X10-13th at 1 Sec.

Cheers,

Corby

[tkamiya]

I had finally done a 24 hour noise floor test successfully.  The graph goes straight down to 20,000s and ends with 1E-17 somewhere.  I'd expected to start turning up way before then.  I wonder if this is too good to be true? 

Input is an output from 10MHz OCXO tied right at input of DMTD. 
Reference is 1Hz difference using 108663.  Purchased new but well aged for about a year or so.
Counter is TAPR TICC.
Recorded the data first and imported it into TimeLab, scale factor 10E-7

[awallin]

The graph goes straight down to 20,000s and ends with 1E-17 somewhere.  I'd expected to start turning up way before then.  I wonder if this is too good to be true? 

looks ok, the slope is slightly less than tau^(-1). if you press 't' you get TDEV which should show a slightly rising slope?

[tkamiya]

Tdev shows going as low as 5E-14 at 100s and starts to go up.  I am not familiar with tdev.  Will you give a quick education on this for me?

[Mrt12]

Will you give a quick education on this for me?

indeed this would be interesting - what does a TDEV of 5e-14 at 1sec mean?

[tkamiya]

I've read the definition of it but I am not understanding the difference between Adev and Tdev in practical sense.  In particular, what does it mean by Adev showing continuously declining graph and Tdev shows more less what I expect to see on a typical OCXO. 

[Mrt12]

I've read the definition of it but I am not understanding the difference between Adev and Tdev in practical sense.  In particular, what does it mean by Adev showing continuously declining graph and Tdev shows more less what I expect to see on a typical OCXO. 

I think I do have a simplified explanation of what ADEV is.
Suppose we operate a clock timer with an oscillator. Since the oscillator is not perfectly stable, the time this clock shows us wobbles a bit, i.e. sometimes it is a bit early and sometimes it is a bit late.
In my understanding, if I look at this clock every second, the amount how much too early or too late it is is the ADEV at tau=1sec. Further, if I look at the clock every 10 seconds, the ADEV at tau=10s tells me how much it will be too early or too late. And so on. No?
I know this is not the definition, but a more practical approach. I may be totally wrong.

On the other hand I cannot imagine what TDEV could be then 

[5065AGuru]

Taka,

Nice work!

Your noise floor looks great!

So now you can compare 5065A, test almost any Quartz, test any telecom type Rubidium, test any Cesium.

Of course you could also compare almost the complete range of two active Masers and the complete range of two passive Masers!

Now all you need to do is search through your oscillators to find the best one to use as the reference.

HooRay

Cheers,

Corby

[tkamiya]

Thanks!

I know I have a couple of Masers in my junk drawers somewhere....

[Mrt12]

Thanks!

I know I have a couple of Masers in my junk drawers somewhere....

whaaaaat! everybody seems to have fancy Masers and BVAs and stuff except me 

[tkamiya]

Today's task is going to be including a difference oscillator in the DMTD box.  I am finding I get most use out of 1Hz difference.  So that will be my default.  Then when I need something else, I can feed something else.

Question:
TAPR TICC has a need for 10MHz clock.  Does it make sense to use a good one or feed the same signal that will be used as a reference?  (not the 10MHz - 1Hz, but 10MHz itself)  I'm trying to reduce the cable clutter and fluctuations that will bring.  I want each piece to be self-contained as possible.  The usual lab clock (GPSDO) is useless for this task as short time stability is not that great being GPS driven)

[thinkfat]

Today's task is going to be including a difference oscillator in the DMTD box.  I am finding I get most use out of 1Hz difference.  So that will be my default.  Then when I need something else, I can feed something else.

Question:
TAPR TICC has a need for 10MHz clock.  Does it make sense to use a good one or feed the same signal that will be used as a reference?  (not the 10MHz - 1Hz, but 10MHz itself)  I'm trying to reduce the cable clutter and fluctuations that will bring.  I want each piece to be self-contained as possible.  The usual lab clock (GPSDO) is useless for this task as short time stability is not that great being GPS driven)

The TICC is in the NF domain of the DMTD, so the stability requirements for its clock are not too strict. Long-term stability is probably more important when you're doing time tagging. I'd say using a GPSDO is fine. The counter clock is not going to determine the noise floor of the system. After all, this is what the whole DMTD design is about, right?

This makes the idea of using a Bodnar frequency standard even more attractive. I could get the "big" one, which has two programmable outputs, and use one as the offset oscillator and the other one as the reference clock for the TICC...

[tkamiya]

TICC is used as in interval counter.  For that, TICC counts pulses between start and stop.  If time base moves around too much, even if input signals are constant, it will read differently resulting in "noise".  True, it does not need to be as accurate as if I am counting 10MHz directly but still, too much fluctuation will result in increased inaccuracy.  This is why I was wondering about GPSDO.  It has significant short term "yank" compared to a naked OCXO. 

I've tried using HP8644A with low phase noise option as difference clock.  It didn't go very well.  4 orders of magnitude difference in readings.  I'm still not sure of the requirement of this source.  For now, I'm using UCT108663 OCXO for that.

[Mrt12]

Hi Taka,

TICC is used as in interval counter.  For that, TICC counts pulses between start and stop.  If time base moves around too much, even if input signals are constant, it will read differently resulting in "noise".  True, it does not need to be as accurate as if I am counting 10MHz directly but still, too much fluctuation will result in increased inaccuracy.  This is why I was wondering about GPSDO.  It has significant short term "yank" compared to a naked OCXO. 

I've tried using HP8644A with low phase noise option as difference clock.  It didn't go very well.  4 orders of magnitude difference in readings.  I'm still not sure of the requirement of this source.  For now, I'm using UCT108663 OCXO for that.

basically right, BUT: if you heterodyne your 10MHz down to 1Hz, the gain is 1e7. Therefore, if your TIC has, for instance, a resolution of 100ns, which would be 1e-7, your timing resolution at 10MHz is 1e-14. A very cheap XTAL which is not even ovenized has maybe 50ppm tolerance, this is so tiny that you don't even notice it at 10MHz.

[tkamiya]

I have some TCXO package....  that will be perfect and it doesn't even have to be kept on.

[thinkfat]

The transfer oscillator is a different story, though. As we learned, its noise contribution will not entirely cancel out even though it is equally applied to both channels.

[tkamiya]

"Transfer Oscillator" is the difference signal?

[awallin]

I've read the definition of it but I am not understanding the difference between Adev and Tdev in practical sense.  In particular, what does it mean by Adev showing continuously declining graph and Tdev shows more less what I expect to see on a typical OCXO.

I don't know if there's any deep insight wrt. TDEV, but for a time-interval (or time-difference) measurement system it is maybe easier to look at the TDEV-plot which is mostly flat with a small slope - compared to ADEV where you expect a steep -1 (or so) slope 'forever' (as shown above, although no microwave clocks really go into 1e-17 and below) if the measured time-interval stays bounded.

some playing around with a noise-generator (e.g. https://github.com/aewallin/colorednoise) and an ADEV-library (e.g. https://github.com/aewallin/allantools) is suggested for the adventurous and curious...

[awallin]

"Transfer Oscillator" is the difference signal?

In a DMTD this is the 'offset-oscillator' tuned e.g. to 10MHz-10Hz in order to get ca 10 Hz outputs for the two 10MHz DUTs being compared. It needs to preferably be of similar quality to the DUTs being tested, IIRC.

[thinkfat]

"Transfer Oscillator" is the difference signal?

In a DMTD this is the 'offset-oscillator' tuned e.g. to 10MHz-10Hz in order to get ca 10 Hz outputs for the two 10MHz DUTs being compared. It needs to preferably be of similar quality to the DUTs being tested, IIRC.

Looking back at the DDS the "Small DMTD" by W. Riley used as the TO, the AD9951 that was used has a data sheet phase noise of -132dBc/Hz @1kHz  (best case with refclock multiplier disabled). The Bodnar GPS reference clock is spec'd with -143dBc/Hz @1kHz. Seems they play in a similar league.

[5065AGuru]

Taka,

The reference for the TIC is NOT critical.
I have used a 14 pin DIP OCXO, a nice TXCO, and a cheap crystal and got good results with all of them!
I'd just use what is convenient for you!

Cheers,

Corby

[Mrt12]

Taka,

The reference for the TIC is NOT critical.
I have used a 14 pin DIP OCXO, a nice TXCO, and a cheap crystal and got good results with all of them!
I'd just use what is convenient for you!

Cheers,

Corby

Hey Corby,
that is indeed one interesting result and good news! Have you seen differences in the ADEV or TDEV when your TIC runs from an expensive reference vs. when your TIC uses a cheapo crystal?
I want to make my own TIC as well, but I want to use time tagging. I assume you have used a "normal" TIC. Do you know the quality of the reference clock makes a difference if time tagging or a normal TIC is used?

[notfaded1]

Does anyone know if having a 2.4/5GHz wireless router near any of the gear that we usually use for these kinds of projects involving TIC's, counters, 5 and 10MHz LO's and GPSDO's cause any major effect on frequency nearby?  Or has anyone compared and found that it makes a big difference?

Bill