[time-nuts] DMTD using TBolts

WarrenS warrensjmail-one at yahoo.com
Sun Oct 23 10:16:48 UTC 2011

Attached is an ADEV graph showing the noise floor of a Dual DMTD TBolt which 
is using the GPS just as the transfer Osc.
This DMTD TBolt tester setup shows a noise floor of:
2e-11 at 1 sec, 5e-12 at 10 seconds, 1.6e-12 at 100 seconds and  5e-13 at 
1000 seconds.
Not Great but useful for many things, especially considering the only H/W 
needed is two Tbolts and whatever you want to use for the 10 MHz ref osc.
The noise floor was measured with the same Osc going to both Tbolts and a 
common GPS signal.
Ideally that way neither signal contribute to the noise floor reading which 
is now the difference of the two Tbolts.

Also shown is the results using a single TBolt-Tic which is measuring it's 
own internal Osc with TC set to 500 sec.
In this case the noise level past 100 sec is the TBolt Osc noise, below 100 
second the value is the noise floor of the test system.
The advantages of the Single TBolt-Tic is that it does not need a second 
TBolt and No reference Osc is needed, the GPS provides that function.
I compared the results to the actual 10MHz output using an external Adev 
test setup and found that a TBolt can measure it's own ADEV noise at taus 
greater than a couple hundred seconds using the PPT data.
For Taus past 10K seconds I found that using the Tbolts Phase data instead 
of the PPT data, gives better results in both the Single and Dual Tic 

The plots show that TBolt-Tics are pretty usable for measuring ADEV down to 
1 second, and start becoming useful for low noise nut measurements at Taus > 
100 seconds.
This is in the same neighbor as an HP 5370A/B.
For low noise ADEVs at taus below 1 to 10 seconds still need something much 
better than this such as a TPLL which has about 3 decades less noise at low 

At least one of the Tbolts needs to be setup to accept an external 10MHz Osc 
signal. (RF gnd isolation transformer preferred)
These results where achieved without the Tbolts talking to each other.
By setting them both to the exact same antenna setting they stayed on the 
same birds and switched within about 15 sec of each other.
This delay difference does add some very noticeable noise spikes but that 
did not seem to effect the ADEV reading past 100 seconds.
Still need some simple stand alone text S/W to calculate the raw difference 
between the two Tbolt data streams, so this becomes easy to use.


Magnus Danielson Wrote:
I like this idea. I would need a couple of thunderbolts to try it.

A side-effect is that the transfer oscillator also provides a UTC source.

Care should be taken such that the same birds is selected. I think a
normal mode T-bolt should be used as normal reference and bird-master,
and the selection be forced onto the slave measurement channels. Would
not require very much of a software.

Only works for 10 MHz clocks I guess.

The software could produce a socket for timelab or it could provide data
on a timelab compatible file, since timelab is able to get data from an
updating file.

The overall processing and parsing should be fairly simple to produce
the raw-data. Sounds like a fun project.

Anyone with a few spare thunderbolts that I could get to try this out?
Bad oscillator isn't a problem.



On 10/21/2011 12:05 AM, WarrenS wrote:
> Tbolt-Tic is what I call a Tbolt when it is NOT used as a GPSDO
> controller but instead used as a high resolution time interval counter.
> (really a time difference logger).
> How do you make a single TBolt-Tic much better?
> Simple, By making a dual TBolt-Tic.
>  From an offshoot of my Common view TBolt experiments,
> I connected a common GPS antenna to two Tbolts that both have been
> modified to use an external osc.
> Results:
> This makes a simple, high performance, DMTD tester with low ps
> resolution using the 'noisy' GPS signal as the common "offset" Osc.
> DMTD and common view both work on the same basic principle.
> The end results is that the noise of the common signal source (GPS in
> this case) tends to cancel when the difference between the two Tbolt
> data streams is use.
> The data difference then gives the freq and the phase difference between
> the two 10 MHz Oscillators that are being compared. (neither of which
> needs to be disciplined or have an EFC input).
>  From the phase difference and the PPT Freq difference data, standard
> low noise floor ADEV plots can be made with tau zero = 1 sec.
> No software yet to make it all easy and automatic, but hopefully that
> will happen in a future version of LadyHeather and/or TimeLab.
> Maybe, the existing Plotter S/W can already take the raw data from two
> Tbolts, find the differences by subtracting both the Phase and PPT
> columns and plot the results.
> If not, any volunteer to come up with a SIMPLE routine to do the above 
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