Post by thread:[PIC:] Measuring nanoseconds

Does anybody know of a way to make a PIC measure time intervals shorter than it's instruction cycle? I need to measure the time between a transmitted and a reflected radio pulse in the 3 ns range, and would like to do it with a PIC. Doing it directly is pretty much out of the question due to the slow instruction cycles (200ns) , but maybe there's a trick that would make it possible? The overall code execution speed can take as long as needed, but the nanosecond resolution must be reasonably accurate (+/- 1ns). Ideas? Has anybody faced a similar problem? I'd love a point in the right direction. It seems this must have been solved for radar applications or other wave-propagation measurements, but searching on Google doesn't turn up much. The application (so far) is just a thought experiment regarding medium-distance ranging accurate to within a meter (about 1ns for speed-of-light radio reflections), but it's something I'm pretty fascinated with and if I can come up with a viable system I'd love to build it as an educational project. Thanks in advance! -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

On Wed, 19 May 2004, Robert B. wrote: > Does anybody know of a way to make a PIC measure time intervals shorter > than it's instruction cycle? I need to measure the time between a > transmitted and a reflected radio pulse in the 3 ns range, and would > like to do it with a PIC. You might consider producing a well defined (voltage and current) pulse of the length you wish to measure, use that to squirt a bit of charge onto a capacitor, and then measure the voltage off the capacitor later. By choosing appropriate signal levels and RC values you can adjust this idea for the range of pulse lengths that you are interested in. Long ago someone experimented with the idea of driving pulses into a "flash" AtoD converter. Some of these have a uniform RC ladder network inside and that posed some interesting possibilities. But I don't know whether that was later patented or not, I was gone before that was done. Check carefully if that matters. There were several enhancements made in the implementation of this idea. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

Don your idea sounds like a pretty good plan, so I sketched this up to see if we're on the same page. Presuming I could trigger a square wave high at the time of transmission and low at the time of reception for the input pulse (maybe using a high speed flip flop or something?), then a well defined current would flow through R1 charging the capacitor for X amount of time, the time being available by solving V = Ve*[1-*e^(-t/(R1*C1))] for t. V is supplied by reading the Pic's ADC, and R1*C1 is a constant. For reset another transistor could provide a path to ground as needed. [Input Pulse] _|_ R1 R2 +5 ____/ Q1\___/\/\/\_ C E | | C1 ---||---[Gnd] | | ___[PIC I/O Pin] | _|_ |_______/ Q2\____[Gnd] | C E | [Pic ADC] Is this what you had in mind? I suppose it would need some calibrations due to the capacitance of Q2 and the other circuit elements, but in the end that would be constant too, and scaling C1 in the equation should compensate for that? I'm still learning a lot about electronics, so please please please point out any flaws in this thought process... Thanks for the idea! :D {Original Message removed}

In general for the short time intervals involved, this is better done by having a constant current source charging a capacitor through a diode. You pull the constant current to ground with a switch transistor - the diode thus disconnectds the capacitor. You discharge the capacitor to ground with another switch transistor. This is the starting point. To start timing, you release the short on the constant current output. The cap charges linearly through the diode until your timing interval ends, at which time you short the constant current to ground again. The diode disconnects the capacitor again, which now has a voltage on it which is proportional to the length of time the constant current switch was open (current flowing into capacitor). With a good current source and fast switch transistors this can be used to measure very short time intervals. Roy J. Gromlich -- Original Message -- From: "Robert B." <piclistKILLspamNERDULATOR.NET> To: .....PICLISTKILLspam.....MITVMA.MIT.EDU Send: 2004-05-19 Subject: Re: [PIC:] Measuring nanoseconds Don your idea sounds like a pretty good plan, so I sketched this up to see if we're on the same page. Presuming I could trigger a square wave high at the time of transmission and low at the time of reception for the input pulse (maybe using a high speed flip flop or something?), then a well defined current would flow through R1 charging the capacitor for X amount of time, the time being available by solving V = Ve*[1-*e^(-t/(R1*C1))] for t. V is supplied by reading the Pic's ADC, and R1*C1 is a constant. For reset another transistor could provide a path to ground as needed. [Input Pulse] _|_ R1 R2 +5 ____/ Q1\___/\/\/\_ C E | | C1 ---||---[Gnd] | | ___[PIC I/O Pin] | _|_ |_______/ Q2\____[Gnd] | C E | [Pic ADC] Is this what you had in mind? I suppose it would need some calibrations due to the capacitance of Q2 and the other circuit elements, but in the end that would be constant too, and scaling C1 in the equation should compensate for that? I'm still learning a lot about electronics, so please please please point out any flaws in this thought process... Thanks for the idea! :D {Original Message removed}

Robert, More like 6 or 7 ns for a radar meter. c is ballpark of 30cm/ns and it has to go both ways for radio ranging applications. Dave Robert B. wrote: >The application (so far) is just a thought experiment regarding >medium-distance ranging accurate to within a meter (about 1ns for >speed-of-light radio reflections), but it's something I'm pretty fascinated >with and if I can come up with a viable system I'd love to build it as an >educational project. > -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

Here is an idea: You come up with some fast analog circuitry that generates the pulse, starts charging a capacitor, then stops charging the capacitor when the pulse reflection is sensed, and running that signal to a sample-and-hold op-amp. The analog stuff would have to be really fast to avoid adding signifigant delays to the measurement. Then the PIC can simply measure the voltage on the capacitor and process it at it's leisure. -- Lawrence Lile Senior Project Engineer Toastmaster, Inc. Division of Salton, Inc. 573-446-5661 voice 573-446-5676 fax "Robert B." <piclistspam_OUTNERDULATOR.NET> Sent by: pic microcontroller discussion list <@spam@PICLISTKILLspamMITVMA.MIT.EDU> 05/19/2004 08:52 PM Please respond to pic microcontroller discussion list To: KILLspamPICLISTKILLspamMITVMA.MIT.EDU cc: Subject: [PIC:] Measuring nanoseconds Does anybody know of a way to make a PIC measure time intervals shorter than it's instruction cycle? I need to measure the time between a transmitted and a reflected radio pulse in the 3 ns range, and would like to do it with a PIC. Doing it directly is pretty much out of the question due to the slow instruction cycles (200ns) , but maybe there's a trick that would make it possible? The overall code execution speed can take as long as needed, but the nanosecond resolution must be reasonably accurate (+/- 1ns). Ideas? Has anybody faced a similar problem? I'd love a point in the right direction. It seems this must have been solved for radar applications or other wave-propagation measurements, but searching on Google doesn't turn up much. The application (so far) is just a thought experiment regarding medium-distance ranging accurate to within a meter (about 1ns for speed-of-light radio reflections), but it's something I'm pretty fascinated with and if I can come up with a viable system I'd love to build it as an educational project. Thanks in advance! -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

You might want to look at 'radio altimeter' methods. A swept RF frequency produces a beat frequency proportional to distance of the reflecting source. With an additional heterodyne down to a low IF frequency you could probably use the PIC timers directly in accumulate mode. The old Hewlett Packard survey units used an RF modulated LED to determine range by measuring the phase shift in the detected retroreflected signal. Both of these methods avoid the use of 'measuring nanosecods.' Robert David Minkler wrote: {Quote hidden}> > Robert, > > More like 6 or 7 ns for a radar meter. c is ballpark of 30cm/ns and it > has to go both ways for radio ranging applications. > > Dave > > Robert B. wrote: > > >The application (so far) is just a thought experiment regarding > >medium-distance ranging accurate to within a meter (about 1ns for > >speed-of-light radio reflections), but it's something I'm pretty fascinated > >with and if I can come up with a viable system I'd love to build it as an > >educational project. -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu