[time-nuts] Real-time Timestamp with Richard's PICTIC?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 26 19:44:38 UTC 2008


Tom

The design needs some rework to achieve useful performance.
Apart from the interpolator problems, you will need to use a different
microprocessor (suitable PIC and AVR micros are available).
A microprocessor which allows the value of an internal counter to be
sampled on the transition of an external signal is required.
Since there is no way to share the counter without introducing
significant complications at least 2 such internal counters are required.
An external synchroniser is required despite the one built into the chip
as the external interpolator has to measure the delay of the synchroniser.
The GPS PPS signal should be used to sample one counter whilst the event
of interest samples the other.
TThe count resolution is extended in software and combined with sawtooth
correction data and the time data from the GPS a timestamp noise level
of a few nanosec should be achievable.
However the events to be time stamped will need to be be several
microsec apart.
If you need lower deadtime you will have to use something like one of
the ACAM TDCs.

Bruce

Tom Bales wrote:
> At 07:00 AM 12/26/2008, time-nuts-request at febo.com wrote:
>   
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>> Today's Topics:
>>
>>    1. Re: Happy Holidays (Bruce Griffiths)
>>    2. PPS from Samsung GCRU (Mark Stephens)
>>    3. Re: Happy Holidays (Bruce Griffiths)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Fri, 26 Dec 2008 22:21:49 +1300
>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>> Subject: Re: [time-nuts] Happy Holidays
>> To: mccorkle at ptialaska.net,     Discussion of precise time and frequency
>>         measurement <time-nuts at febo.com>
>> Message-ID: <4954A22D.10802 at xtra.co.nz>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>>     
> Ho, Ho, Ho!  Thanks to Richard from the Cold North for such a nice 
> Christmas present!
>
> I've been working on a crackpot scheme to measure the time of arrival 
> of cosmic-ray events.  It's a "crackpot scheme" because I want to 
> record the times in a (no-doubt futile) attempt to see if any of the 
> events are correlated in time--sent from a common source in a uniform 
> pulse train, perhaps.
>
> So for, I've assembled some simple detectors, an HP53131 interval 
> counter, a Lucent RFTGm GPS/XO/Rb set for frequency reference, and a 
> bunch of cables with the result that I can record the time interval 
> between events to 0.1 nanoseconds, but not real arrival time.  I 
> figured that I would just add up all the interval times to get a sort 
> of real time, but after lots and lots of events, the cumulative error 
> probably defeats the purpose (and missing events hurt your cause a 
> lot).  So, I've been looking for a very precise, high-resolution 
> real-time event timestamper.
>
> This all started when I was listening to Tom van Baak talk about ADEV 
> of pendulum clocks at the BHI anniversary symposium last summer, and 
> how you could pick up the signature of astronomical events, such as 
> lunar tides.  At the time I was reading a report about some 
> physicists at Brookhaven discovering strange semi-annual rate 
> variations in the radioactive decay of nuclei, and I was toting 
> around a book about cosmic rays.  Funny how the brain works.  Oh, 
> yes--and the LHC was starting up.  So, imagine some spaceman a long 
> time ago in a galaxy far, far away firing up his proton accelerator, 
> sending off pulses of protons into space, and a few of them hitting 
> us here on earth every now and then.  It doesn't seem that anyone has 
> ever looked for such a signal, so why not look for one?  (There are 
> lots of complications, such as the motion of the earth, to deal 
> with--but you get the idea.)
>
> I haven't found a simple way to record a file of real-time time 
> stamps, at least without lashing together a bunch of clocks, interval 
> counters, GPIB controller, and so forth, but Richard's gift seems to 
> have everything needed to do the job, except for a GPS time receiver 
> to form the low-resolution part of the timestamp, and a software 
> means of concatenating the data for date, hour, minute, and second to 
> the high-resolution data from the PICTIC.  Can anyone offer suggestions?
>
> Tom Bales
> Coral Gables, FL
>
>
>
>   
>> Richard H McCorkle wrote:
>>     
>>> Ho Ho Ho
>>>
>>> Merry Christmas from the far north to all the
>>> Time-Nuts. As a gift for the holidays I have
>>> created a free version of the PICTIC for you
>>> and Didier has kindly posted the files at:
>>>
>>> http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:pictic
>>>
>>>
>>> And to all a good night,
>>>
>>> Richard
>>>
>>>
>>>
>>>       
>> Richard
>>
>> Your analog interpolator needs a little work.
>> The switching speed of the 2N3906's is so slow because the 2N3906's that
>> switch the current to ground or into the 470pF cap are saturated.
>> They will switch much faster if used as current mode switches.
>> In fact you can just replace the 2N3906 whose collector is connected to
>> the 470pF cap with a fast switching diode and the current source will be
>> switched much faster.
>> The 2N7000's have a relatively high non linear output capacitance
>> replacing them with something like 1/6X 74HC05 would be better.
>> The 470pF silver mica caps will have relatively large dielectric absorption.
>> NP0/C0G caps would be a better choice, particularly if higher input
>> pulse rates were to be used.
>>
>> Have you actually measured the linearity of the interpolator?
>>
>>
>> Bruce
>>
>>
>>
>>
>> ------------------------------
>>
>> Message: 2
>> Date: Fri, 26 Dec 2008 21:04:24 +1100
>> From: "Mark Stephens" <marks at twotoe.com>
>> Subject: [time-nuts] PPS from Samsung GCRU
>> To: <time-nuts at febo.com>
>> Message-ID: <C1483046D5364BD59D25D2E12D545016 at twotoe.com>
>> Content-Type: text/plain;       charset="iso-8859-1"
>>
>>
>> Just wanted to bounce some thoughts off the group:
>>
>> I have a Samsung GCRU and I need to permanently tap the 1PPS signal.
>>
>> Can the PPS output of the GPS RX be used without interfering with 
>> the accuracy of the unit?
>>
>> Is there is another PPS output, preferably buffered and in phase 
>>     
> >from the GCRU?
>   
>> If I am forced to use the GPS RX PPS output, at this stage I would 
>> use either a RS232 line driver, or a 74HC04 to buffer the output.
>>
>> What sort of problems should I look out for?
>>
>>
>> Happy holidays,
>> Mark
>>
>>
>>
>>
>> ------------------------------
>>
>> Message: 3
>> Date: Fri, 26 Dec 2008 23:28:51 +1300
>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>> Subject: Re: [time-nuts] Happy Holidays
>> To: Discussion of precise time and frequency measurement
>>         <time-nuts at febo.com>
>> Message-ID: <4954B1E3.3040004 at xtra.co.nz>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> Bruce Griffiths wrote:
>>     
>>> Richard H McCorkle wrote:
>>>
>>>       
>>>> Ho Ho Ho
>>>>
>>>> Merry Christmas from the far north to all the
>>>> Time-Nuts. As a gift for the holidays I have
>>>> created a free version of the PICTIC for you
>>>> and Didier has kindly posted the files at:
>>>>
>>>> http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:pictic
>>>>
>>>>
>>>> And to all a good night,
>>>>
>>>> Richard
>>>>
>>>>
>>>>
>>>>
>>>>         
>>> Richard
>>>
>>> Your analog interpolator needs a little work.
>>> The switching speed of the 2N3906's is so slow because the 2N3906's that
>>> switch the current to ground or into the 470pF cap are saturated.
>>> They will switch much faster if used as current mode switches.
>>> In fact you can just replace the 2N3906 whose collector is connected to
>>> the 470pF cap with a fast switching diode and the current source will be
>>> switched much faster.
>>> The 2N7000's have a relatively high non linear output capacitance
>>> replacing them with something like 1/6X 74HC05 would be better.
>>> The 470pF silver mica caps will have relatively large dielectric 
>>>       
>> absorption.
>>     
>>> NP0/C0G caps would be a better choice, particularly if higher input
>>> pulse rates were to be used.
>>>
>>> Have you actually measured the linearity of the interpolator?
>>>
>>>
>>> Bruce
>>>
>>>
>>>
>>>       
>> The quickest fix is to drop the differential drive to the 2N3906's and
>> bias the base of the 2N3906 whose collector is connected to the 470pF
>> cap at +2.5V.
>> This will ensure the 2N3906's switch much faster.
>> The maximum voltage on the 470pF cap should be about 1V to avoid
>> saturating the transistor whose collector is connected to it.
>> The opamp gain network should then be adjusted to produce the required
>> output swing.
>> If the maximum charging time of the capacitor is 400ns then the charging
>> current should be about 4.7E-10/4E-7 or about 1.2mA.
>> The 2N3906's will switch a bit faster if the current is increased to 5
>> or 10mA in which case the 470pF cap should be replaced by a ~2nF cap
>> (with 5mA charging current).
>> Use an NP0/C0G for low dielectric absorption.
>>
>> Bruce
>>
>>
>>
>> ------------------------------
>>
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>> End of time-nuts Digest, Vol 53, Issue 101
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>>     
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