[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/
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.
More information about the time-nuts