# [time-nuts] Hanging bridge question

Bob Camp lists at rtty.us
Wed Mar 26 07:45:00 EDT 2014

```Hi

Take a look at the PIC-TIC stuff. They have the HP circuit in the middle of it. Bob Stewart posted a circuit with a pair of tri-state gates in it within the last month or so.

They all pretty much:

1) Measure the “coarse time” with a counter Today that’s just about always a counter in an MCU.
2) Based on the clock to the counter (say 25 ns), you have a roundoff / truncation error.  (say 0 to 25 ns)
3) You use a gate or two and your capture flip flop to convert the truncation to a pulse. (normally 25 to 50 ns)
4) You pick an R/C time constant to be “useful” (say 50 ns, could be less).
5) You charge the RC with the pulse
6) After the pulse is done, you open circuit the R/C so charge / discharge stops.
7) When you get around to it, you measure the voltage on the cap with an ADC

Starting from the 50 ns example, an 8 bit converter likely gives you 500 ps resolution. 10 bits gets you to 250 ps and 12 bits to 125 ps. More bits or a faster clock would do even better.

Since the R/C charge voltage vs time is pretty well known, you can do the first part of the math fairly easily.

You have a clock and flip flops are pretty cheap. If you want to shoot cal pulses at it, send it a 25 and 50 ns wide pules. The delta between the two should be pretty good. If you have the range, go to 75 ns and get 3 points to fit.

The basic R/C is about 5 cents. The one tri-state gate you need is about 16 cents. A quad nand is about the same these days. You already need a pair of flip flops to capture the pps edge (two to a package …). If you want to do the whole calibration thing, one of Bert’s \$2 CPLD’s has way more parts in it than you will ever need.

The ADC can be what you get with your MCU. In that case 12 bits may be stretching it. There are very nice 12 bit parts from TI that run about \$3 or so. 16 bits is still under \$10.

Bob

On Mar 25, 2014, at 11:08 PM, Jim Miller <jim at jtmiller.com> wrote:

> Bob
>
> I'm not sure who you're responding to but I have a couple of questions:
>
> TDC = Time Delay Correlator?
>
> Could you point me to one of these 50 cent threads? I've read a ton of this
> list from 2007 forward but must have missed that.
>
> Thanks
>
> jim ab3cv (much to learn)
>
> Hi
>
> There have been multiple posts about analog TDC's of various designs
> that get you into the sub 100 ps range without costing very much
> money. I believe the cheapest posted so far adds < 50 cents to a basic
> PIC based design.
>
> Bob
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