Fast And Gate, 1 Ghz
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From: osr on 16 Feb 2010 18:17 The laser is 4 millijoules. give or take. The NB4N316M looks good, I think we may start there... Steve
From: Jim Thompson on 16 Feb 2010 18:22 On Tue, 16 Feb 2010 15:06:23 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Jim Thompson wrote: >> On Tue, 16 Feb 2010 14:40:00 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> osr(a)uakron.edu wrote: >>>> Since this is for my friend's field diagnostics, the pretrigger is the >>>> box held in his right hand while he hits the doctor's trigger pedal >>>> with his right foot. >>>> >>>> I've been told to look at LVDS receivers for the compare portion, any >>>> suggestions for a fast switch for the current steering, if we go that >>>> way? I'm looking at some 2N918 in the scrap box, but with a 2 nsec >>>> rise and fall that is probably too slow... >>>> >>> That would be like showing up at the Daytona race track in an AMC >>> Gremlin :-) >>> >>> You'd probably be looking at low capacitance RF transistors like the BFP620: >>> http://www.infineon.com/dgdl/bfp620.pdf?folderId=db3a30431400ef68011425b291f205c5&fileId=db3a30431400ef680114274deb27072a >>> >>> Although, depending on your precision requirements even the old BFS17 >>> might suffice. It can do a little under a nanosecond. When using really >>> hot RF transistors be careful, they can be oscillating without you even >>> knowing they do. >> >> A PECL NOR is an AND for low-true... that's quite fast. >> > >Yeah, but you can't get them for a buck :-) > >If the laser pulse comes in with gusto I'd be tempted to just amplify it >as needed (maybe with a limiter) and run it point blank into the base of >the RF transistor. I probably have some 48 year old MECL in my junk box ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Joerg on 16 Feb 2010 19:25 osr(a)uakron.edu wrote: > The laser is 4 millijoules. give or take. The NB4N316M looks good, I > think we may start there... > Got to overcome that 25mV hysteresis though. For this app a hysteresis might not be all that great because it'll skew your timing. Maybe Lasse knows a version sans hysteresis? Then there's the question on how to tap into the PD. If you can get >>25mV across 50-100ohms there, fine. Otherwise you'd have to roll a TIA but that's not a really big deal. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 16 Feb 2010 21:07 On Tue, 16 Feb 2010 14:08:17 -0800 (PST), osr(a)uakron.edu wrote: >Since this is for my friend's field diagnostics, the pretrigger is the >box held in his right hand while he hits the doctor's trigger pedal >with his right foot. > >I've been told to look at LVDS receivers for the compare portion, any >suggestions for a fast switch for the current steering, if we go that >way? I'm looking at some 2N918 in the scrap box, but with a 2 nsec >rise and fall that is probably too slow... > >Steve I'd go for one of the faster Analog Devices PECL-output comparators, ADCMP567 maybe, driving a pair of fast PNP transistors as the current steering thing. I've done it with MMBR521s with 33 ohm base resistors from the PECL drive. Some relatively slow closed-loop positive current source can drive the emitters of the pair, through a ferrite bead, or even just a resistor to a decently positive voltage. 5 or 10 mA is about right. The LVDS receivers make nice comparators for some applications, but are relatively slow and have a lot of built-in-on-purpose offset. John
From: Phil Hobbs on 16 Feb 2010 22:07
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 -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |