Circuit for producing a single-shot cycle

1. Jan 27, 2010

nitin.jain

Hi all,
I wish to build a triggerable circuit that gives out a single-cycle, such as the one given in the pic attached (the middle one, with magenta and blue colours). While square/trapeziod/ramp cycles are good, even an exponential fall in the beginning and rise in the end would be fine (the bottom-most graph). The symmetry in the 'low' and 'high' lobes is also not a requirement. The most important thing is a very fast ascend from the low to high value.

What I thought and have also shown in this pic (graphs 1 and 2) was making use of an opamp-based differentiator driven by a normal function generator (such as Agilent's 80 MHz one) - a triggered pulse from it, with some specific tRise and tFall (considering the 10% to 90% or vice-versa definition).
I need 2*B = 3.5V for my application. Taking tRise = tFall = 2.5 ns and tinkering with the amplitude A and the RC values, I could most likely achieve this.
However, the problem that I see already is as follows (please let me know if the following analysis is correct):
As told above, for my application, the critical part is the switching time from low to high (illustrated in the figure in blue colour). In other words, the differentiator output needs to switch from at least -0.8*B to +0.8*B in less than 200 ps, which translates to a minimum slew rate of around 15000 V/microS. The best opamp I've found so far turns shy even of 10000 V/microS, so it's a long way to go!
So my questions are:
1) If I instead switch between smaller peak levels (let's say from -0.4*B to 0.4*B, so as to meet the slew rate constraint comfortably), can I then cascade a (broadband?) amplifier to reach the requisite voltage levels, without compromising on the fast switching time?
2) Or, are you aware of an opamp that does have a fast enough slew rate. If yes, please let me know as well.

Last but not the least, any other not-too-complicated designs will also be appreciated! :)

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Last edited: Jan 27, 2010
2. Jan 27, 2010

f95toli

What about using a step recovery diode?
This is used to achieve very fast rise/fall times with some pulse generators, including the SRS DG535 I use in my lab. That said, it is not a very flexible solution and I don't even know if one could use this trick your case.

There is a link in to an app-note in the links section of the wiki for the diodes, it might contain some useful info.

However, anything faster than 1ns tends to be seriously tricky to design and at 280ps you are well into high-speed electronics territory.

3. Jan 27, 2010

nitin.jain

Thanks for your reply! I've actually just discovered the wonderful and mysterious world of Step Recovery Diodes too!
And I believe you allude to the HP appnote http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/an918.pdf" [Broken], is that right?
It does seem nicely descriptive (like most old documents are)! I shall go through it tomorrow and hopefully, it shall provide me some implementable idea.

Last edited by a moderator: May 4, 2017