Bob, Thanks. That was too obvious, but having not looked at HP's similar circuits I ruled it out. Many thanks. Still lots to learn here…..
Matt -------------------------------------------- On Wed, 3/26/14, Bob Camp <li...@rtty.us> wrote: Subject: Re: [time-nuts] Hanging bridge question To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Date: Wednesday, March 26, 2014, 2:47 PM Hi HP = Hewlett Packard Bob On Mar 26, 2014, at 10:31 AM, Matthew Martin <dr_g...@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 <li...@rtty.us> wrote: > > Subject: Re: [time-nuts] Hanging bridge question > To: "Discussion of precise time and frequency measurement" <time-nuts@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 <j...@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 >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.