[time-nuts] fast edge, rise time.

[Jim Lux]

On 12/26/13 8:07 AM, ct1dmk wrote:
> Hello,
>
> I'm willing to generate a pulse (of some few hundred volts)
> by discharging a capacitor into a pulse transformer
> I'm solely interested is the active edge (call it either rise or fall
> depending on the
> wiring of the output of the transformer).
>

ns risetime pulses sounds like fairly straight forward radar stuff.

the inductance of the transformer is going to be your challenge, 
depending on the energy level.  What about a non-transformer 
alternative?  Can you just charge your cap up to the few hundred volts 
and have a switch that can take the voltage?

How much energy do you need?  You said a few hundred volts, but is that 
microjoules, joules, or kilojoules?

A small triggered spark gap would be one way.  There's also the ever 
popular krytron, which has very good timing accuracy.

YOu might look for circuits used for exploding bridge wires (EBW)


> The target is 4ns, while ideas seemed to be clear at some point, now I'm
> having doubts if better to use a MOSFET or a bipolar transistor
> as the switch element. Experiments with MOSFETs presented me some
> difficulties charging the gate capacitance having some trouble to
> achieve something in the 4ns region. Well 4ns seems hard whatever device
> anyway.

You want some sort of RF transistor here.  What about one of the new 
LDMOS FETs: some have fairly impressive voltage handling, and if they 
work at 1 GHz for radar applications, they will work for you.

What about stacking a bunch of MMIC RF amplifiers (e.g. like the ERA or 
GAL from minicircuits)


Other traditional approaches to fast pulse generation are avalanche 
transistors.


There's also a variety of interesting pulse forming networks that can 
generate fast rise time high voltage pulses.  Blumlein arrangements are 
one.  Your 100ns pulse is fairly long for a transmission line scheme, 
though (20-30 m of coax)