[time-nuts] Experience with THS788 from TI?

[David]

On Wed, 21 Mar 2012 11:51:53 -0400, Ben Gamari <bgamari at gmail.com>
wrote:

>On Wed, 21 Mar 2012 09:44:14 -0500, David <davidwhess at gmail.com> wrote:
>> I am surprised it is not more accurate and precise.  Even old discrete
>> designs can get down to 10ps or better.  I wonder what market it is
>> for where space is at that much of a premium.
>> 
>Out of curiosity, would you happen to have an example of discrete TDC
>design? Recently I've been exploring the TDC design space as these
>devices are a critical part of our experiments (I do spectroscopy of
>biological molecules). I'm currently (slowly) working on a FPGA TDC
>design (based on the PandaDAQ[1] and CERN's Spartan 6 TDC design) but it
>seems it will be non-trivial to get down to the 12 ns the commercial
>offerings provide (although at great cost). What would a discrete TDC
>design look like?  Are there any designs in the open?

I read through the THS788 datasheet.  TI DLL multiplies the 200
external clock to 1.2GHz for the 833ps basic cycle.  That is further
divided into 64 13ps time intervals which are latched to generate the
6 least significant bits of the time stamp.  I suspect the time
intervals are implemented as a locally generated 64 phase 1.2GHz
clock.

Check out the design for the Tektronix 7T11/7T11A sampling sweep unit
or their earlier sampling sweep plug-ins.  The service manuals with
theory and schematics are available online.

You might also want to look at the clock delay timer design in the
Tektronix 2230/2232 and the 2440 oscilloscopes which is used to align
equivalent time acquisitions with the waveform record.

An all digital design like the THS788 does have a much higher
throughput but the analog designs I mention above did not have
throughput as a requirement.