- #1
nitin.jain
- 21
- 0
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! :)
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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