Fast rise time 2nS or 1nS signal generator - Page 1
Forum section: electronics / testgear

author: diydidi
author_rank: Contributor
timestamp: December 06, 2014, 10:12:21 am
content: HiAnybody know of a not too expensive fast rise time square generator capable of giving rise time of 1 or 2nS?I need it to calibrate some of my analog scopes.

author: nixfu
author_rank: Unknown
timestamp: December 06, 2014, 11:13:16 am
content: Maybe ring type oscillator could get something that small. That basically gets you down to the time it takes to travel down the logic gate ring and back to the beginning. They are simple to make using any odd number of gates.http://en.wikipedia.org/wiki/Ring_oscillatorOr if you want something REALLY fast you can check out the video Dave made about a pulse generator that can provide pulses measured in picoseconds that you can build yourself.

author: donmr
author_rank: Regular Contributor
timestamp: December 06, 2014, 05:27:29 pm
content: Look up Step Recovery Diode (SRD). If you are up to a little assembly you can build a circuit to run from any ordinary pulse source (even 555 timer chip) and speed up its edges.http://literature.cdn.keysight.com/litweb/pdf/5954-2056.pdf

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2014, 05:35:17 pm
content: Quote from: nixfu on December 06, 2014, 11:13:16 amThese aren't all that useful - they're too fast for their own good; the pulses that come out aren't just fast, they're messy. If you're trying to calibrate something, you want a fast rise, but you also want asmoothrise that settles to a flat top with minimal ringing.In fact, a lot of these analog scope calibration routines specify a rather higher rise time than you really need, and what's truly important is the shape of the pulse.

author: biot
author_rank: Regular Contributor
timestamp: December 06, 2014, 05:45:47 pm
content: w2aew made a really nice video about a Schmitt trigger oscillator which is essentially a square wave output with about 2ns rise time:

author: marshallh
author_rank: Unknown
timestamp: December 06, 2014, 06:03:39 pm
content: These days you could just use a 74 series chip, pick any one with an output. Its output will be plenty fast, maybe too fast

author: tggzzz
author_rank: Super Contributor
timestamp: December 06, 2014, 06:14:17 pm
content: Quote from: diydidi on December 06, 2014, 10:12:21 amECL logic gates and fast comparators can both have much faster risetimes than you specify, and (provided the normal RF precautions are taken) the transition can be well-defined. So, have an odd number of ECL logic inverters or fast comparators with each output feeding another's input - i.e. a ring oscillator.To reduce the frequency without changing the risetime, make one of the lines an impedance-controlled line which is about 6" long per ns increase in period (notenot nS, that's conductance!))The same effect can, of course, be achieved by having an RC filter in between two stages. But beware of slower transitions in the circuits "near" the RC filter - use extra gates to sharpen up the edges.

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2014, 06:27:51 pm
content: Instead of ring oscillators, which are going to give a rather high frequencyandrise time, why not just use a couple inverters to reshape arbitrary square waves? Feed in any frequency you want from a signal generator, get the same frequency with fast rise out the other end.

author: tggzzz
author_rank: Super Contributor
timestamp: December 06, 2014, 06:36:16 pm
content: Quote from: c4757p on December 06, 2014, 06:27:51 pmThat would work, of course, with either logic gates or comparators. Look for ECL outputs (of whatever variety) for short risetimes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2014, 12:18:51 am
content: High speed logic families 74LVC and up are about fast enough. The pin drivers in this family are a little bit under 50 ohms, so you should be able to drive a pulse right into 50 ohm coax without too much reflection; you may need an additional series resistor to get source termination just right.A schmitt trigger (74LVC14, etc.) or chaining a few gates (you'll have plenty to spare in a SO14/16 size -- though they're also available in 1G, 2G, etc. mini packages) will get you about as fast a rise time as you can get, from whatever your source happens to be. You can also use the extra gates as an oscillator or pulse generator, depending on if you want external sync or what.Tim

author: edavid
author_rank: Super Contributor
timestamp: December 07, 2014, 12:26:58 am
content: The LTC6905 is a nice 1 chip solution:http://www.pa4tim.nl/?p=2677

author: tggzzz
author_rank: Super Contributor
timestamp: December 07, 2014, 09:40:59 am
content: Quote from: T3sl4co1l on December 07, 2014, 12:18:51 amHow so? The data sheet says "<2ns".Other logic families have transition times much less than 1ns. The first example that comes to hand has a risetime of35psand minimum pulse width of 100ps. Seehttp://www.analog.com/en/special-linear-functions/comparators/adcmp582/products/product.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2014, 11:19:57 am
content: Oops, more specifically that should say "as fast as you can get [from the gate]". Since output risetime generally depends on input risetime -- not so much with buffered families, but the general idea is, you chain enough together and the rise time saturates to whatever you get when the input and output risetimes are equal. Sure it might be a little faster if you drove it with something screaming fast, but where are you going to get that, right?Even among CMOS, there are certainly faster families, and other devices (like comparators) that are even better examples than LVC, yes.I would shy away from ECL, because the voltages weird (DC bias) and small swing. Of course, that's fine if you don't need much voltage, and as long as all you're looking at is the step or a short pulse, you can remove the bias with a coupling capacitor.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: December 07, 2014, 11:51:39 am
content: Quote from: T3sl4co1l on December 07, 2014, 11:19:57 amThought you might have meant something like that!QuoteThe device I mentioned is a comparator, so it could be driven by virtually any generator with any reasonable voltages. That page references both ECL and PECL output variants, so the power supply could be convenient. DC bias on the output is unlikely to be relevant for the OP's purpose, and could be removed by the scope's AC coupling.But for an analogue scope, it is unlikely they need 35ps risetime

author: Lukas
author_rank: Frequent Contributor
timestamp: December 07, 2014, 06:32:20 pm
content: 74LVC1G series gates are able to generate rather clean looking pulses, I did some experiments a while back:http://www.mikrocontroller.net/topic/217945#2205244German, but the pics speak for themselves.

author: tggzzz
author_rank: Super Contributor
timestamp: December 07, 2014, 08:43:00 pm
content: Quote from: Lukas on December 07, 2014, 06:32:20 pmThat is indeed a very good risetime, and, if my very rusty German can be trusted, is probably limited by the scope.A decent measurement is worth any amount of conjecture; thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2014, 03:17:52 am
content: Yeah, from those pictures I would guess it's more like 600ps or less, of course there's going to be some more "crunch" if you view it with a proper say 3GHz+ scope. It'll be a little sharper, but you'll also see parasitics you didn't think were possible (offhand -- the grounding lead has maybe 2-5nH of inductance, and the whole circuit itself maybe 1pF to ground plane -- overall, a 1/4 wave stub ca. 2.2GHz). The gate probably doesn't go quite fast enough to excite that major resonance, but probably has enough junk up there to leave some squigglies.I'm not really sure what the ~500MHz supply resonance seems to be; maybe something with connector leads and matching. It's pretty low amplitude, so not a big deal, but there nonetheless. There's probably more components to it, that aren't seen due to bandwidth.The "<2ns" rating they give the logic family is limited by load capacitance and supply voltage; at 3-5V supply, the pin drivers are pretty darned stiff, and especially if the load is impedance matched rather than bulk probe capacitance, the actual rise time can be a bit better than that.Tim