Yet another sub-nanosecond pulse generator thread
(cross-posting to time-nuts)
After paying only limited attention to this topic, I suddenly have a
need for a pulse generator that has <150 ps risetime and a pulse width
of at least 2 ns. I've looked at the classic Jim Williams avalanche
generator, but I don't want to have to deal with the (relatively) high
voltage source needed.
I've done microwave design using Gunn diodes, so I'm drawn to using a
step-recovery diode. The topology seems very straightforward, and I can
build it right onto a BNC connector, no PCB.
I'm thinking using an SMD835 diode, biased at ~1ma. The (sketchy)
datasheet claims a T of 20 nsecs and a Tr of 85 ps, Cj of 0.4 to 0.8 pf.
Questions:
The obvious, is it reasonable?
Is the bias current reasonable? I'm assuming the bias current is
actually dependent on the repetition rate, you need enough current to
replenish the charge within one pulse cycle. I suppose I could compute
it from the stated junction capacitance, but I'm not sure that's the
only factor
Will the stored charge actually give me the desired transition rate into
50 ohms? Hmm, again I should be able to compute this, but any other
factors ignoring the non-diode ones like cap inductance?
How should I compute the coupling cap from the diode to the load? Use
the impedance at the pulse rep rate? Seems reasonable. BTW, I don't care
about droop in the pulse, just the risetime. (measuring overshoot in
an HF amp). Again, just want to verify that the obvious answer is the
correct one. I clearly need to be very careful about the inductance.
Thanks, Bill
--
Bill Ezell
The day Microsoft makes something that doesn't suck
will be the day they make vacuum cleaners.
Or maybe Windows 10.
Maybe look at the SRD1 from Stanford Research Systems? See http://www.thinksrs.com/products/DG645.htm
If you need the whole generator, maybe the SG382 with the optional rear panel clock outputs http://www.thinksrs.com/products/SG380.htm
Best Regards,
Laurence Motteram
Calibration & Service Manager
Scientific Devices Australia
Ph: +61 (0)3 9569 1366
M: +61 (0)425 765 019
www.scientific-devices.com.au
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@febo.com] On Behalf Of BIll Ezell
Sent: Wednesday, 13 April 2016 6:27 AM
To: volt-nuts@febo.com
Subject: [volt-nuts] Yet another sub-nanosecond pulse generator thread
(cross-posting to time-nuts)
After paying only limited attention to this topic, I suddenly have a need for a pulse generator that has <150 ps risetime and a pulse width of at least 2 ns. I've looked at the classic Jim Williams avalanche generator, but I don't want to have to deal with the (relatively) high voltage source needed.
I've done microwave design using Gunn diodes, so I'm drawn to using a step-recovery diode. The topology seems very straightforward, and I can build it right onto a BNC connector, no PCB.
I'm thinking using an SMD835 diode, biased at ~1ma. The (sketchy) datasheet claims a T of 20 nsecs and a Tr of 85 ps, Cj of 0.4 to 0.8 pf.
Questions:
The obvious, is it reasonable?
Is the bias current reasonable? I'm assuming the bias current is actually dependent on the repetition rate, you need enough current to replenish the charge within one pulse cycle. I suppose I could compute it from the stated junction capacitance, but I'm not sure that's the only factor
Will the stored charge actually give me the desired transition rate into
50 ohms? Hmm, again I should be able to compute this, but any other factors ignoring the non-diode ones like cap inductance?
How should I compute the coupling cap from the diode to the load? Use the impedance at the pulse rep rate? Seems reasonable. BTW, I don't care about droop in the pulse, just the risetime. (measuring overshoot in an HF amp). Again, just want to verify that the obvious answer is the correct one. I clearly need to be very careful about the inductance.
Thanks, Bill
--
Bill Ezell
The day Microsoft makes something that doesn't suck will be the day they make vacuum cleaners.
Or maybe Windows 10.
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