[time-nuts] Hanging bridge question

Matthew Martin dr_grid at yahoo.com
Wed Mar 26 18:07:50 EDT 2014


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 <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, 2:47 PM
 
 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
 > 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
 >> _______________________________________________
 >> time-nuts mailing list -- time-nuts at 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 at 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 at 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 at febo.com
 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
 and follow the instructions there.
 


More information about the time-nuts mailing list