Fast And Gate, 1 Ghz
in [Design]
|
Prev: Protecting 3x16 bits with 16 bits ? (crc8 + 8 bit hamming code seems perfect)
Next: A novel way to measure magnetic fields, and DC current withouta shunt?
From: Joerg on 17 Feb 2010 13:24 Phil Hobbs wrote: > On 2/16/2010 2:36 PM, Joerg wrote: >> John Larkin wrote: >>> On Tue, 16 Feb 2010 09:30:27 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> John Larkin wrote: >>>>> On Sun, 14 Feb 2010 10:00:39 -0800 (PST), dagmargoodboat(a)yahoo.com >>>>> wrote: >>>>> >>>>>> On Feb 14, 11:19 am, o...(a)uakron.edu wrote: >>>>>>> Can anyone suggest a fast AND gate at 1-2 Ghz. A friend of mine >>>>>>> needs >>>>>>> to measure a laser pulse duration on almost no budget, perhaps >>>>>>> 200-300$. The Pulse ranges from 4 to 20 Nanoseconds, and his current >>>>>>> idea is use the photodiode current to gate open a source of 1 ghz >>>>>>> pulses and integrate,. The 2 Ghz source is the easy part< I have >>>>>>> that.. The question is how to gate it. Sadly this is a single shot >>>>>>> event at 1-4 hz and thus a sampling scope is out. I have no >>>>>>> problem getting a photodiode with 47 picosecond rise time.. >>>>>>> >>>>>>> Suggestions, other then finding friends with higher salaries? My >>>>>>> friend is a independent field service engineer on medical lasers. He >>>>>>> has a idea for a replacement product to get him off the road, the >>>>>>> cost of travel is slaughtering his once profitable business. He >>>>>>> wants >>>>>>> to build the prototype, and see if his idea works compared to a >>>>>>> known >>>>>>> working pulsed laser source. >>>>>>> >>>>>>> Some modern form of ECL? >>>>>>> >>>>>>> Steve >>>>>> Seems like a job for a single-slope integrating A/D. Integrate a >>>>>> current onto a cap while the pulse flies, then de-integrate at your >>>>>> leisure in-between pulses. >>>>> That's the classic pulse stretcher, used to get picosecond resolution >>>>> edge measurements. That suggests an interesting and cheap circuit to >>>>> solve the OP's problem. But my camera broke so I can't post it now. >>>>> >>>> But I found it hard to convince folks to do it that way. Nowadays >>>> many seem almost hell-bent on doing it digitally. Maybe because the >>>> analog world is too close to voodoo for younger engineers ;-) >>> >>> It's tough to do picosecond time measurements all-digitally, although >>> it has been done. >>> >>> How about a semi-logarithmic pulse width detector: >>> >>> Input comparator steers a current blip into a capacitor. The cap has a >>> resistor to ground and a comparator working against some small >>> positive DC bias. An input pulse charges the cap, voltage proportional >>> to pulse width. The comparator tells you that you had a pulse, and the >>> comparator pulse width is a (nonlinear) reflection of the much faster >>> input pulse width; measure that with a reasonable counter. >>> >>> That's pretty simple and doesn't need a pretrigger or reset or >>> separate pulse-start detector. >>> >>> You could do the same thing with a constant-current discharge and a >>> negative-swing catch diode and get a linear stretch, but the catcher >>> should be temp compensated for best results. >>> >>> >>> ---- >>> | | in >>> | | >>> -------------- ------------------------------------ >>> >>> ________ >>> / \__________ >>> / \___________ >>> / \_________ >>> / >>> / fast charge slow discharge >>> / >>> ------------- >>> >>> ___________________________________________ >>> | | >>> | comp output | >>> _____________| |__ >>> >>> >> >> Yes, a "time stretcher" like this could get it into the realms of >> regular speed digital counters. FPGA and such. >> >> However, since Steve wrote about 1-4Hz PRF I don't see anything wrong >> with the classic integrate and hold approach. You've got tons of time to >> read it out, issue a text message that a pulse has come, turn on the >> coffeemaker ... :-) >> >> I'd do this analog -> ADC. >> > > One problem is that (AIUI) it needs to separate the pulse height from > the duration. Gated integrators don't do that--they integrate, so they > get the area under the pulse. > > One possibility would be a PECL or LVDS line receiver with a > constant-fraction discriminator trigger to drive pulse stretcher A, plus > a linear amp driving pulse stretcher B. That way, you can get the > average height and the duration separately. The constant-fraction > discriminator makes sure that the pulse height variation doesn't change > the measured pulse length very much. > Yes, either that or just limit the signal. If the laser pulse is small the simplest thing would be an amp with enough gain to get you past 500mV for the lowest expected amplitude and then cap that with a Schottky. Once when I did that (non-optics app) it was met with disgust in the design review, branded as a sledge hammer method. But it sure worked and was cheap :-) If $10-20 extra are ok and power consumption isn't a concern a nice PECL solution should work. However, a hysteresis is not so cool here and many line receivers have some Schmitt behavior. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 17 Feb 2010 13:49 On Tue, 16 Feb 2010 22:07:52 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 2/16/2010 2:36 PM, Joerg wrote: >> John Larkin wrote: >>> On Tue, 16 Feb 2010 09:30:27 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> John Larkin wrote: >>>>> On Sun, 14 Feb 2010 10:00:39 -0800 (PST), dagmargoodboat(a)yahoo.com >>>>> wrote: >>>>> >>>>>> On Feb 14, 11:19 am, o...(a)uakron.edu wrote: >>>>>>> Can anyone suggest a fast AND gate at 1-2 Ghz. A friend of mine needs >>>>>>> to measure a laser pulse duration on almost no budget, perhaps >>>>>>> 200-300$. The Pulse ranges from 4 to 20 Nanoseconds, and his current >>>>>>> idea is use the photodiode current to gate open a source of 1 ghz >>>>>>> pulses and integrate,. The 2 Ghz source is the easy part< I have >>>>>>> that.. The question is how to gate it. Sadly this is a single shot >>>>>>> event at 1-4 hz and thus a sampling scope is out. I have no >>>>>>> problem getting a photodiode with 47 picosecond rise time.. >>>>>>> >>>>>>> Suggestions, other then finding friends with higher salaries? My >>>>>>> friend is a independent field service engineer on medical lasers. He >>>>>>> has a idea for a replacement product to get him off the road, the >>>>>>> cost of travel is slaughtering his once profitable business. He wants >>>>>>> to build the prototype, and see if his idea works compared to a known >>>>>>> working pulsed laser source. >>>>>>> >>>>>>> Some modern form of ECL? >>>>>>> >>>>>>> Steve >>>>>> Seems like a job for a single-slope integrating A/D. Integrate a >>>>>> current onto a cap while the pulse flies, then de-integrate at your >>>>>> leisure in-between pulses. >>>>> That's the classic pulse stretcher, used to get picosecond resolution >>>>> edge measurements. That suggests an interesting and cheap circuit to >>>>> solve the OP's problem. But my camera broke so I can't post it now. >>>>> >>>> But I found it hard to convince folks to do it that way. Nowadays >>>> many seem almost hell-bent on doing it digitally. Maybe because the >>>> analog world is too close to voodoo for younger engineers ;-) >>> >>> It's tough to do picosecond time measurements all-digitally, although >>> it has been done. >>> >>> How about a semi-logarithmic pulse width detector: >>> >>> Input comparator steers a current blip into a capacitor. The cap has a >>> resistor to ground and a comparator working against some small >>> positive DC bias. An input pulse charges the cap, voltage proportional >>> to pulse width. The comparator tells you that you had a pulse, and the >>> comparator pulse width is a (nonlinear) reflection of the much faster >>> input pulse width; measure that with a reasonable counter. >>> >>> That's pretty simple and doesn't need a pretrigger or reset or >>> separate pulse-start detector. >>> >>> You could do the same thing with a constant-current discharge and a >>> negative-swing catch diode and get a linear stretch, but the catcher >>> should be temp compensated for best results. >>> >>> >>> ---- >>> | | in >>> | | >>> -------------- ------------------------------------ >>> >>> ________ >>> / \__________ >>> / \___________ >>> / \_________ >>> / >>> / fast charge slow discharge >>> / >>> ------------- >>> >>> ___________________________________________ >>> | | >>> | comp output | >>> _____________| |__ >>> >>> >> >> Yes, a "time stretcher" like this could get it into the realms of >> regular speed digital counters. FPGA and such. >> >> However, since Steve wrote about 1-4Hz PRF I don't see anything wrong >> with the classic integrate and hold approach. You've got tons of time to >> read it out, issue a text message that a pulse has come, turn on the >> coffeemaker ... :-) >> >> I'd do this analog -> ADC. >> > >One problem is that (AIUI) it needs to separate the pulse height from >the duration. Gated integrators don't do that--they integrate, so they >get the area under the pulse. > >One possibility would be a PECL or LVDS line receiver with a >constant-fraction discriminator trigger to drive pulse stretcher A, plus >a linear amp driving pulse stretcher B. That way, you can get the >average height and the duration separately. The constant-fraction >discriminator makes sure that the pulse height variation doesn't change >the measured pulse length very much. > >Cheers > >Phil Hobbs Most CFDs discriminate the leading edge of a pulse, and only do that well for some fixed width. I've managed to make a CFD that's pretty good over a 3:1 or so width range and a healthy range of pulse heights (for readout of an MCP/2d-delay-line-detector imaging thing) but that won't do width. If one did a fast peak holder, the main pulse could be run through a delay line until the peak detector settled. Then compare the delayed pulse to 1/2 of the detected peak amplitude. It's getting complicated. John
From: Joerg on 17 Feb 2010 13:56 John Larkin wrote: > On Tue, 16 Feb 2010 22:07:52 -0500, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> On 2/16/2010 2:36 PM, Joerg wrote: >>> John Larkin wrote: >>>> On Tue, 16 Feb 2010 09:30:27 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >>>>> John Larkin wrote: >>>>>> On Sun, 14 Feb 2010 10:00:39 -0800 (PST), dagmargoodboat(a)yahoo.com >>>>>> wrote: >>>>>> >>>>>>> On Feb 14, 11:19 am, o...(a)uakron.edu wrote: >>>>>>>> Can anyone suggest a fast AND gate at 1-2 Ghz. A friend of mine needs >>>>>>>> to measure a laser pulse duration on almost no budget, perhaps >>>>>>>> 200-300$. The Pulse ranges from 4 to 20 Nanoseconds, and his current >>>>>>>> idea is use the photodiode current to gate open a source of 1 ghz >>>>>>>> pulses and integrate,. The 2 Ghz source is the easy part< I have >>>>>>>> that.. The question is how to gate it. Sadly this is a single shot >>>>>>>> event at 1-4 hz and thus a sampling scope is out. I have no >>>>>>>> problem getting a photodiode with 47 picosecond rise time.. >>>>>>>> >>>>>>>> Suggestions, other then finding friends with higher salaries? My >>>>>>>> friend is a independent field service engineer on medical lasers. He >>>>>>>> has a idea for a replacement product to get him off the road, the >>>>>>>> cost of travel is slaughtering his once profitable business. He wants >>>>>>>> to build the prototype, and see if his idea works compared to a known >>>>>>>> working pulsed laser source. >>>>>>>> >>>>>>>> Some modern form of ECL? >>>>>>>> >>>>>>>> Steve >>>>>>> Seems like a job for a single-slope integrating A/D. Integrate a >>>>>>> current onto a cap while the pulse flies, then de-integrate at your >>>>>>> leisure in-between pulses. >>>>>> That's the classic pulse stretcher, used to get picosecond resolution >>>>>> edge measurements. That suggests an interesting and cheap circuit to >>>>>> solve the OP's problem. But my camera broke so I can't post it now. >>>>>> >>>>> But I found it hard to convince folks to do it that way. Nowadays >>>>> many seem almost hell-bent on doing it digitally. Maybe because the >>>>> analog world is too close to voodoo for younger engineers ;-) >>>> It's tough to do picosecond time measurements all-digitally, although >>>> it has been done. >>>> >>>> How about a semi-logarithmic pulse width detector: >>>> >>>> Input comparator steers a current blip into a capacitor. The cap has a >>>> resistor to ground and a comparator working against some small >>>> positive DC bias. An input pulse charges the cap, voltage proportional >>>> to pulse width. The comparator tells you that you had a pulse, and the >>>> comparator pulse width is a (nonlinear) reflection of the much faster >>>> input pulse width; measure that with a reasonable counter. >>>> >>>> That's pretty simple and doesn't need a pretrigger or reset or >>>> separate pulse-start detector. >>>> >>>> You could do the same thing with a constant-current discharge and a >>>> negative-swing catch diode and get a linear stretch, but the catcher >>>> should be temp compensated for best results. >>>> >>>> >>>> ---- >>>> | | in >>>> | | >>>> -------------- ------------------------------------ >>>> >>>> ________ >>>> / \__________ >>>> / \___________ >>>> / \_________ >>>> / >>>> / fast charge slow discharge >>>> / >>>> ------------- >>>> >>>> ___________________________________________ >>>> | | >>>> | comp output | >>>> _____________| |__ >>>> >>>> >>> Yes, a "time stretcher" like this could get it into the realms of >>> regular speed digital counters. FPGA and such. >>> >>> However, since Steve wrote about 1-4Hz PRF I don't see anything wrong >>> with the classic integrate and hold approach. You've got tons of time to >>> read it out, issue a text message that a pulse has come, turn on the >>> coffeemaker ... :-) >>> >>> I'd do this analog -> ADC. >>> >> One problem is that (AIUI) it needs to separate the pulse height from >> the duration. Gated integrators don't do that--they integrate, so they >> get the area under the pulse. >> >> One possibility would be a PECL or LVDS line receiver with a >> constant-fraction discriminator trigger to drive pulse stretcher A, plus >> a linear amp driving pulse stretcher B. That way, you can get the >> average height and the duration separately. The constant-fraction >> discriminator makes sure that the pulse height variation doesn't change >> the measured pulse length very much. >> >> Cheers >> >> Phil Hobbs > > Most CFDs discriminate the leading edge of a pulse, and only do that > well for some fixed width. I've managed to make a CFD that's pretty > good over a 3:1 or so width range and a healthy range of pulse heights > (for readout of an MCP/2d-delay-line-detector imaging thing) but that > won't do width. > > If one did a fast peak holder, the main pulse could be run through a > delay line until the peak detector settled. Then compare the delayed > pulse to 1/2 of the detected peak amplitude. > > It's getting complicated. > What if Steve would take one or two cheapo MMIC and railed the last one into a Schottky so the amplitude is always the same? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 17 Feb 2010 14:18 On Wed, 17 Feb 2010 10:56:13 -0800, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> On Tue, 16 Feb 2010 22:07:52 -0500, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> On 2/16/2010 2:36 PM, Joerg wrote: >>>> John Larkin wrote: >>>>> On Tue, 16 Feb 2010 09:30:27 -0800, Joerg <invalid(a)invalid.invalid> >>>>> wrote: >>>>> >>>>>> John Larkin wrote: >>>>>>> On Sun, 14 Feb 2010 10:00:39 -0800 (PST), dagmargoodboat(a)yahoo.com >>>>>>> wrote: >>>>>>> >>>>>>>> On Feb 14, 11:19 am, o...(a)uakron.edu wrote: >>>>>>>>> Can anyone suggest a fast AND gate at 1-2 Ghz. A friend of mine needs >>>>>>>>> to measure a laser pulse duration on almost no budget, perhaps >>>>>>>>> 200-300$. The Pulse ranges from 4 to 20 Nanoseconds, and his current >>>>>>>>> idea is use the photodiode current to gate open a source of 1 ghz >>>>>>>>> pulses and integrate,. The 2 Ghz source is the easy part< I have >>>>>>>>> that.. The question is how to gate it. Sadly this is a single shot >>>>>>>>> event at 1-4 hz and thus a sampling scope is out. I have no >>>>>>>>> problem getting a photodiode with 47 picosecond rise time.. >>>>>>>>> >>>>>>>>> Suggestions, other then finding friends with higher salaries? My >>>>>>>>> friend is a independent field service engineer on medical lasers. He >>>>>>>>> has a idea for a replacement product to get him off the road, the >>>>>>>>> cost of travel is slaughtering his once profitable business. He wants >>>>>>>>> to build the prototype, and see if his idea works compared to a known >>>>>>>>> working pulsed laser source. >>>>>>>>> >>>>>>>>> Some modern form of ECL? >>>>>>>>> >>>>>>>>> Steve >>>>>>>> Seems like a job for a single-slope integrating A/D. Integrate a >>>>>>>> current onto a cap while the pulse flies, then de-integrate at your >>>>>>>> leisure in-between pulses. >>>>>>> That's the classic pulse stretcher, used to get picosecond resolution >>>>>>> edge measurements. That suggests an interesting and cheap circuit to >>>>>>> solve the OP's problem. But my camera broke so I can't post it now. >>>>>>> >>>>>> But I found it hard to convince folks to do it that way. Nowadays >>>>>> many seem almost hell-bent on doing it digitally. Maybe because the >>>>>> analog world is too close to voodoo for younger engineers ;-) >>>>> It's tough to do picosecond time measurements all-digitally, although >>>>> it has been done. >>>>> >>>>> How about a semi-logarithmic pulse width detector: >>>>> >>>>> Input comparator steers a current blip into a capacitor. The cap has a >>>>> resistor to ground and a comparator working against some small >>>>> positive DC bias. An input pulse charges the cap, voltage proportional >>>>> to pulse width. The comparator tells you that you had a pulse, and the >>>>> comparator pulse width is a (nonlinear) reflection of the much faster >>>>> input pulse width; measure that with a reasonable counter. >>>>> >>>>> That's pretty simple and doesn't need a pretrigger or reset or >>>>> separate pulse-start detector. >>>>> >>>>> You could do the same thing with a constant-current discharge and a >>>>> negative-swing catch diode and get a linear stretch, but the catcher >>>>> should be temp compensated for best results. >>>>> >>>>> >>>>> ---- >>>>> | | in >>>>> | | >>>>> -------------- ------------------------------------ >>>>> >>>>> ________ >>>>> / \__________ >>>>> / \___________ >>>>> / \_________ >>>>> / >>>>> / fast charge slow discharge >>>>> / >>>>> ------------- >>>>> >>>>> ___________________________________________ >>>>> | | >>>>> | comp output | >>>>> _____________| |__ >>>>> >>>>> >>>> Yes, a "time stretcher" like this could get it into the realms of >>>> regular speed digital counters. FPGA and such. >>>> >>>> However, since Steve wrote about 1-4Hz PRF I don't see anything wrong >>>> with the classic integrate and hold approach. You've got tons of time to >>>> read it out, issue a text message that a pulse has come, turn on the >>>> coffeemaker ... :-) >>>> >>>> I'd do this analog -> ADC. >>>> >>> One problem is that (AIUI) it needs to separate the pulse height from >>> the duration. Gated integrators don't do that--they integrate, so they >>> get the area under the pulse. >>> >>> One possibility would be a PECL or LVDS line receiver with a >>> constant-fraction discriminator trigger to drive pulse stretcher A, plus >>> a linear amp driving pulse stretcher B. That way, you can get the >>> average height and the duration separately. The constant-fraction >>> discriminator makes sure that the pulse height variation doesn't change >>> the measured pulse length very much. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Most CFDs discriminate the leading edge of a pulse, and only do that >> well for some fixed width. I've managed to make a CFD that's pretty >> good over a 3:1 or so width range and a healthy range of pulse heights >> (for readout of an MCP/2d-delay-line-detector imaging thing) but that >> won't do width. >> >> If one did a fast peak holder, the main pulse could be run through a >> delay line until the peak detector settled. Then compare the delayed >> pulse to 1/2 of the detected peak amplitude. >> >> It's getting complicated. >> > >What if Steve would take one or two cheapo MMIC and railed the last one >into a Schottky so the amplitude is always the same? Then amplitude variations would change the width. John
From: Joerg on 17 Feb 2010 14:35
John Larkin wrote: > On Wed, 17 Feb 2010 10:56:13 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Tue, 16 Feb 2010 22:07:52 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>> On 2/16/2010 2:36 PM, Joerg wrote: >>>>> John Larkin wrote: >>>>>> On Tue, 16 Feb 2010 09:30:27 -0800, Joerg <invalid(a)invalid.invalid> >>>>>> wrote: >>>>>> >>>>>>> John Larkin wrote: >>>>>>>> On Sun, 14 Feb 2010 10:00:39 -0800 (PST), dagmargoodboat(a)yahoo.com >>>>>>>> wrote: >>>>>>>> >>>>>>>>> On Feb 14, 11:19 am, o...(a)uakron.edu wrote: >>>>>>>>>> Can anyone suggest a fast AND gate at 1-2 Ghz. A friend of mine needs >>>>>>>>>> to measure a laser pulse duration on almost no budget, perhaps >>>>>>>>>> 200-300$. The Pulse ranges from 4 to 20 Nanoseconds, and his current >>>>>>>>>> idea is use the photodiode current to gate open a source of 1 ghz >>>>>>>>>> pulses and integrate,. The 2 Ghz source is the easy part< I have >>>>>>>>>> that.. The question is how to gate it. Sadly this is a single shot >>>>>>>>>> event at 1-4 hz and thus a sampling scope is out. I have no >>>>>>>>>> problem getting a photodiode with 47 picosecond rise time.. >>>>>>>>>> >>>>>>>>>> Suggestions, other then finding friends with higher salaries? My >>>>>>>>>> friend is a independent field service engineer on medical lasers. He >>>>>>>>>> has a idea for a replacement product to get him off the road, the >>>>>>>>>> cost of travel is slaughtering his once profitable business. He wants >>>>>>>>>> to build the prototype, and see if his idea works compared to a known >>>>>>>>>> working pulsed laser source. >>>>>>>>>> >>>>>>>>>> Some modern form of ECL? >>>>>>>>>> >>>>>>>>>> Steve >>>>>>>>> Seems like a job for a single-slope integrating A/D. Integrate a >>>>>>>>> current onto a cap while the pulse flies, then de-integrate at your >>>>>>>>> leisure in-between pulses. >>>>>>>> That's the classic pulse stretcher, used to get picosecond resolution >>>>>>>> edge measurements. That suggests an interesting and cheap circuit to >>>>>>>> solve the OP's problem. But my camera broke so I can't post it now. >>>>>>>> >>>>>>> But I found it hard to convince folks to do it that way. Nowadays >>>>>>> many seem almost hell-bent on doing it digitally. Maybe because the >>>>>>> analog world is too close to voodoo for younger engineers ;-) >>>>>> It's tough to do picosecond time measurements all-digitally, although >>>>>> it has been done. >>>>>> >>>>>> How about a semi-logarithmic pulse width detector: >>>>>> >>>>>> Input comparator steers a current blip into a capacitor. The cap has a >>>>>> resistor to ground and a comparator working against some small >>>>>> positive DC bias. An input pulse charges the cap, voltage proportional >>>>>> to pulse width. The comparator tells you that you had a pulse, and the >>>>>> comparator pulse width is a (nonlinear) reflection of the much faster >>>>>> input pulse width; measure that with a reasonable counter. >>>>>> >>>>>> That's pretty simple and doesn't need a pretrigger or reset or >>>>>> separate pulse-start detector. >>>>>> >>>>>> You could do the same thing with a constant-current discharge and a >>>>>> negative-swing catch diode and get a linear stretch, but the catcher >>>>>> should be temp compensated for best results. >>>>>> >>>>>> >>>>>> ---- >>>>>> | | in >>>>>> | | >>>>>> -------------- ------------------------------------ >>>>>> >>>>>> ________ >>>>>> / \__________ >>>>>> / \___________ >>>>>> / \_________ >>>>>> / >>>>>> / fast charge slow discharge >>>>>> / >>>>>> ------------- >>>>>> >>>>>> ___________________________________________ >>>>>> | | >>>>>> | comp output | >>>>>> _____________| |__ >>>>>> >>>>>> >>>>> Yes, a "time stretcher" like this could get it into the realms of >>>>> regular speed digital counters. FPGA and such. >>>>> >>>>> However, since Steve wrote about 1-4Hz PRF I don't see anything wrong >>>>> with the classic integrate and hold approach. You've got tons of time to >>>>> read it out, issue a text message that a pulse has come, turn on the >>>>> coffeemaker ... :-) >>>>> >>>>> I'd do this analog -> ADC. >>>>> >>>> One problem is that (AIUI) it needs to separate the pulse height from >>>> the duration. Gated integrators don't do that--they integrate, so they >>>> get the area under the pulse. >>>> >>>> One possibility would be a PECL or LVDS line receiver with a >>>> constant-fraction discriminator trigger to drive pulse stretcher A, plus >>>> a linear amp driving pulse stretcher B. That way, you can get the >>>> average height and the duration separately. The constant-fraction >>>> discriminator makes sure that the pulse height variation doesn't change >>>> the measured pulse length very much. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> Most CFDs discriminate the leading edge of a pulse, and only do that >>> well for some fixed width. I've managed to make a CFD that's pretty >>> good over a 3:1 or so width range and a healthy range of pulse heights >>> (for readout of an MCP/2d-delay-line-detector imaging thing) but that >>> won't do width. >>> >>> If one did a fast peak holder, the main pulse could be run through a >>> delay line until the peak detector settled. Then compare the delayed >>> pulse to 1/2 of the detected peak amplitude. >>> >>> It's getting complicated. >>> >> What if Steve would take one or two cheapo MMIC and railed the last one >> into a Schottky so the amplitude is always the same? > > Then amplitude variations would change the width. > Ok, what if he'd take three MMIC for a whole lotta gain? :-) If you need super precision a comparator without hysteresis would be the ticket. Provided that the PD setup itself is faster than the MMIC. If not you might as well use MMIC. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |