[time-nuts] On some pitfalls of the dual mixer time differencemethod of horology

Ulrich Bangert df6jb at ulrich-bangert.de
Sun Oct 1 06:49:53 EDT 2006


Hello Paul-Henning,

> I would find two of the best Audio ADCs I could lay my
> hands on.  Some of these are incredibly good by any standard. 
> Sampling the mixer outputs at approximately 96kHz with 16 bit 
> effective resolution is well within the possible.
> 
> All the repeated steps of amplification/limiting to find the 
> zero crossing can be almost entirely replaced by a a single FFT.
> 
> You are not limited by the 96kHz sampling frequency in the 
> timing of the resulting zero crossing, because you can 
> interpolate between the sample points.
> 
> On a Loran-C signal, I have been able to do zerocrossing 
> measurements with 1nsec resolution, despite the fact my 
> sampling clock was only 10MHz.

Yes, i am aware of this and have done so with 24 Bit ADCs on a sound
card which's sampling rate was phase locked to my best standard.
However, dsp technology seems to be not that common and you will find
what i have described in 99% of all publications, so the people that do
it the conventional way must have faced the described problems.

> I wouldn't at all be surprised if this is how TSC does it.

They say that they have two mixers and two "low noise zero crossing
detectors" but as i believe: Some people don't tell us because they do
not know while others do net tell us because there is a commercial
interest in keeping details secret. 

> All the repeated steps of amplification/limiting to find the 
> zero crossing can be almost entirely replaced by a a single FFT.

If dsp methods are a choice one can even do better (at least for simple
sines) as described by Mr. Greenhall in

www.tmo.jpl.nasa.gov/progress_report/42-121/121G.pdf

The clue here is that not only the zero crossings and its surroundings
are used for interpolation but ALL samples are used for a comlete signal
reconstruction and noise removement. For a simple sine this is of course
easier done as for the more complex Loran-C signal. 

> You can calibrate out almost all the asymetries
> by simply flipping the input signals and pretty much all of 
> them if you can also do A/B switching on which mixer feeds which ADC.

Perhaps this is really the main clue. TSC has a switch matrix between
the mixers and their TWO DDS circuits. They say the can use their device
in "2 DDS mode" when the two input signals are far apart or in "Single
DDS" mode when the signals are not more than 2 Hz apart. Perhaps they
can use this switch matrix also in the sense of A/B switching that you
are thinking about. Any one a good idea what a RAY-3 23 dBm mixer may
produce a isolation without a LO signal?

> Obviously, damn good electronics is still necessary to 
> control cross-talk etc.

After my experiments i shall underwrite this a number of times!

Best regards
Ulrich Bangert


> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Poul-Henning Kamp
> Gesendet: Sonntag, 1. Oktober 2006 12:08
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] On some pitfalls of the dual mixer 
> time differencemethod of horology
> 
> 
> 
> I would find two of the best Audio ADCs I could lay my
> hands on.  Some of these are incredibly good by any standard. 
> Sampling the mixer outputs at approximately 96kHz with 16 bit 
> effective resolution is well within the possible.
> 
> All the repeated steps of amplification/limiting to find the 
> zero crossing can be almost entirely replaced by a a single FFT.
> 
> You are not limited by the 96kHz sampling frequency in the 
> timing of the resulting zero crossing, because you can 
> interpolate between the sample points.
> 
> On a Loran-C signal, I have been able to do zerocrossing 
> measurements with 1nsec resolution, despite the fact my 
> sampling clock was only 10MHz.
> 
> You can calibrate out almost all the asymetries
> by simply flipping the input signals and pretty much all of 
> them if you can also do A/B switching on which mixer feeds which ADC.
> 
> I wouldn't at all be surprised if this is how TSC does it.
> 
> Obviously, damn good electronics is still necessary to 
> control cross-talk etc.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by 
> incompetence.
> 
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