[time-nuts] Of rubidium life and piggy-bank anemia....

[Ulrich Bangert]

Magne,

I had thrown in my claim over the TCH more to continue this funny
thread. But now that you take it seriously I must confess that I have
been thinking in the last weeks and months about a multi-input time
tagging event counter. With todays fpgas it should not be too difficult
to design a SOC (System On Chip) that includes

1) a number of inputs, say 8 or so that can accept 1pps signals or
frequencies between 1 HZ and say 50 MHz with programmable dividers to
make 1 pps signals out of the extrenally applied frequencies. Naturally
the inputs would need logic levels which are easily given with 1 pps.
For applying sinusoidal signals an external clock shaper as published by
Bruce would be needed. 

2) One input for the reference frequency of 10 MHz. Using the onchip dcm
availbale in the fpga today, an internal frequency of 200-250 Mhz phase
locked to the reference can be generated giving a coarse time resolution
of 4-5 ns. This clock is expected to have a jitter of 40-60 ps and is
applied to a 64 bit counter to make the "coarse" part of the time tag.

3) For each 1 pps signal there is a interpolator of its own. The
interpolator is a tapped delay line made out of fast logic elements. On
the positive slope of the 1pps the contents of the main reference
counter is latched into a 64 bit latch of its own. In addition the
interpolator is triggered to make a "photograph" of what the main clock
signal was like in terms of time delay against the triggering slope. I
intend to make the delay line that long that several periods of the main
clock are to be seen within the delay line. This will allow not only an
in situ calibration of the delay line (we need to know the average delay
per logic element) but also to use not only ONE positive slope of the
clock but a number, say 3 or 4. This, in conjunction with the dither
introduced by the clock jitter, should give a an overall averaged "fine"
result with an resolution better than 100 ps. 

4) Since some form of intelligence is needed to handle everything the
device features a 32 bit risc controller. This contoller receives an
interrupt whenever one of the 1 pps takes place. It will then read out
the coarse and the fine result for this pps, compute around a bit with
the values and spit out an result in MJD over a rs232 line. This
controller will also listen to control commands coming over the rs232
that set the divider ratio for the inputs, initialize the MJD setting
and perhaps some other things.

Everthing described above can be realized today in a chip that sells for
30.00 US $ in single quantity. 

Now comes the bad news: 

The ram for the built in risc controller must be realized externally.
That also applies to the controller's code which must be stored
non-volatile. The fpga needs an configuration chip. The fpga needs at
least 3 very clean supply voltages. That all makes it NOT a real SOC
system but to a system out of 4-5 chips partly in nasty 208 pin flat
packs. The whole thing cannot be built without having a precision pcb
for it. That makes the whole project much more complex and elaborate to
just handle it along the way.

For that reason I have decided to buy me this smart device and
experiment with it:

http://www.enterpoint.co.uk/moelbryn/darnaw1.html

This one does include everything that is needed and some more and will
allow to breadboard the rest of the circuit.

Will keep the group informed about any progress of that project.

Best regards
Ulrich Bangert  

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Magne Mahre
> Gesendet: Freitag, 30. November 2007 14:55
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Of rubidium life and piggy-bank anemia....
> 
> 
> Ulrich Bangert wrote:
> > Use the three-cornered-hat method to rank your clocks!
> 
> The literature seems to say that you need to do the 
> measurements simultanously to get good results from the TCH 
> method.  I  guess most of us have only one TIC at home, so I 
> wonder how the results will be affected by taking them one at a time ?
> 
> --Magne
> 
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