# [time-nuts] Hanging bridge question

Bob Camp lists at rtty.us
Wed Mar 26 17:47:36 EDT 2014

```Hi

HP = Hewlett Packard

Bob

On Mar 26, 2014, at 10:31 AM, Matthew Martin <dr_grid at yahoo.com> wrote:

> 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
>
> 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
> 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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