[time-nuts] Time interval counters

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Sep 26 01:54:20 EDT 2006


Most of the discussion on Time interval counters is about averaging many 
measurements to achieve a subnanosecond resolution.
It is possible to achieve subnanosecond single shot measurement 
resolution with a time interval counter if one uses a time to digital 
converter (TDC) chip.
Suitable TDC chips are available from http://www.acam.de/ 
<http://www.acam.de/>
In particular the TDC-GP1 has a resolution of better than 250picosec 
with a range of 200 millisec, which should be more than adequate for 
measuring the time delay between GPS and local standard derived PPS 
pulses. All it needs is a microprocessor to process the data and 
transmit it to a computer for logging.
The TDC-GP2 has a range of 4 millisec and a singlr shot resolution of 
about 65 picosec.
Although these chips are a little pricey at around the 100 euro mark 
they are a lot cheaper than an Agilent/HP 53131 or its equivalent.

It is possible to build a USB device that time stamps a PPS pulse with a 
65 picosecond resolution and a measuring range of centuries.
A PIC18F4550, a TDC-GP2 and a 74AC164. The 74AC164 synchronises the PPS 
pulse to the local high stability clock, the TDC-GP2 measures the 
synchroniser delay and the synchroniser output transition samples the 
PIC's internal timers. The PIC timer is clocked by the high stability 
clock and software extends the effective counter length to 64 bits or 
whatever length is desired.

Bruce




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