[time-nuts] fast edge, rise time.

Bob Camp lists at rtty.us
Thu Dec 26 12:03:11 EST 2013


Hi

If you go the Krytron route you probably will need some fairly fancy transformers as well….

Bob

On Dec 26, 2013, at 11:19 AM, Jim Lux <jimlux at earthlink.net> wrote:

> 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)
> 
> 
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