Avalance / JW charge line pulser for step waveforms ~45V out

[Insatman]

Hello, I've been building various versions of this kind of pulser for a long time.   In my working life we called them Fiducial pulsers or FIDs for short.  Often they were used with very short chargelines and added to waveforms on older analog oscilloscopes to provide a timing mark and thereby correlate multiple scope waveforms.  We were using film in those days.   

This version uses a 2n2222 and produces a risetime under 600ps.   The output is ~45V and can be rep-rated up to around a KHz or more.  Although the output voltage droops after several hundred Hz.
I'm sure the risetime could be further tweeked but it's good enough for my purposes.  There are much faster units available but they are only a volt or so output.   https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/

For those of you unfamiliar with this type of pulser, the pulse length is twice the electrical length of the chargeline used.

Photos, waveforms and schematic attached.

[Leo Bodnar]

Very nice, you can't have too many pulsers!

Did you have to match the charge line interface or select the line for best performance or random 50R line just happens to work?

45V amplitude and 600ps is 60kV/µs, great slew rate.

Leo

[Insatman]

Very nice, you can't have too many pulsers!

Did you have to match the charge line interface or select the line for best performance or random 50R line just happens to work?

45V amplitude and 600ps is 60kV/µs, great slew rate.

Leo

Just a standard 50ohm chargeline works fine.  To optimize the risetime, it's nice to have the first few cm of line a lower impedance and I intend to do this on my next PCB layout if possible.  For this iteration I simply used peaking capacitors.  About 4pf for this particular unit.  The amount varies depending on the individual transistor, layout, charge voltage and trigger coupling.

In the past I also used the 2N5551 which yields about a 90V output.  Risetime is ~1.1ns, around 80kV/us.

[Insatman]

Very nice.

So do you find that the particular manufacturing batch / reel or manufacturer of the 2n2222 used makes much difference in the performance or have you had good consistency of performance with all variants of 2n2222 which you have tried?

Do you find that the life time of the transistor at consistent performance levels is compromised at such voltages and pulse lengths and rep rates?

I've had pretty good luck with variants of 2n2222.  This particular one came in one of those cheap Chinese transistor assortment packs.  Occasionally you will find one that has poor risetime or self-fires at low voltage, but 95% seem to work well.  For best performance tweaking of the charge voltage to just under self-fire is recommended if you desire triggered operation.  If you just want the fastest risetime possible then up the voltage to self-fire level.  In self-fire oscillation you get slightly better risetime due to lack of trigger feedthru adding a "foot" onto the base of the pulse.   I always quote 10-90 Trise, although the 20-80 spec seems to becoming more prevalent these days.

As for lifetime, so long as the chargeline is fairly short..say under 5 meters, and the load is 50ohms you get good lifetime... I've seldom seen one of these fail in that service.   If you use long chargelines, say  over 100ns or drive low impedances, then the lifetime is shortened.   I once built a pulser for an expermental setup that used a 2N5551 with 10 parallel 50ohm/20ns chargelines driving a 5 ohm load at 100Hz.   The transistor would usually last at least an hour in this service, which was fine for the experiment.   The risetime was ~5-7ns...I don't remember exactly.