# [time-nuts] Hanging bridge question

Matthew Martin dr_grid at yahoo.com
Wed Mar 26 10:31:25 EDT 2014

```Hi,

Just a quick question from a novice.  Sometimes I see abbreviations here and don't know, but usually I
can make a good guess.  Your first paragraph, "HP" is perhaps high precision?  Just want to make sure
I am not missing some other meaning.

Thanks, learning a lot from reading this group!

Matt Martin

--------------------------------------------
On Wed, 3/26/14, Bob Camp <lists at rtty.us> wrote:

Subject: Re: [time-nuts] Hanging bridge question
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Date: Wednesday, March 26, 2014, 4:45 AM

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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