[time-nuts] TDC-GPX2 TDC chip

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Mar 15 23:07:20 EDT 2017

The datasheet is less than convincing.
The change in resolution/noise with time delay isnt well documented.
Performance may differ when a 1MHz reference clock is used as opposed to the 10MHz clock they appear to have used for some of the plots on the datasheet.
The customer support person appears to know little useful about tdcs.
Have been debating how to establish the actual performance.

A) use random source(s) to produce data for the fill the buckets histogram test.
For statistically useful results an extremely large number of random events is required.
e.g. for a 10MHz reference clock and 10ps bits need around 10,000 hits per bin for 1% rms
statistical variation in bin event totals. i.e. about 1E8 total events.
With a 1MHz reference clock need at least 1E9 events.

B) Use a pair of 10MHz oscillators with a small frequency offset between them one of which is used as the reference clock whilst the other is divided down to produce a suitable event rate. The leading edge of each successive event is swept slowly at a uniform rate across the reference clock cycle in steps of say 10fs or so per event. This requires that the sources have very low spur levels at least 120dB below the carrier and preferably more.
The reference 10MHz can be divided down to 5MHz, 2MHz or 1MHZ for testing with lower reference clock frequencies.

c) As above but use a single 10MHz source that acts both as reference clock for the TDC and as the input source for an extremely low spur offset generator, the output of which is divided down to create a suitable event rate.

D) Compare the TDC measurements with those obtained with a significantly lower noise TDC.
Still (a few of us at least) working on this. Initial crude and somewhat noisy setup suggested better than 10ps rms (limited somewhat by jitter of divider chain used to produce trigger signal). A later version achieved around 3ps rms. However comprehensive testing is still to be done.


>     On 16 March 2017 at 11:28 Christopher Hoover <ch at murgatroid.com> wrote:
>     http://ams.com/eng/Products/Precision-Time-Measurement/Time-to-Digital-Converters/TDC-GPX2
>     Claims to have single-shot accuracy of 10 ps.
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