Re: More fun with avalanche pulsers


 

I bought ten BFR505 transistors which are supposed to be faster than 2N2369's. Unfortunately they come in SOT-23 packages (i.e. smaller than a mouse turd). I had a fun time installing one into my pulser. Dropped the first two (but found them later by lying on the floor for a really close look), the third sprang out of the forceps and I heard it "tick" somewhere across the room. Finally got the fourth mounted (emitter soldered directly to the SMA output connector).

I moved some components for the shortest possible lead lengths, but it still has a divot in the top of the pulse that I can't tune out. Could it be the open BNC hanging from the end of the charge line? Still likes 33 ohm better than 50 ohms of emitter load, too. Amplitude is decreased since the BFR505 avalanches at 30-32 volts instead of 50+ like the '2369.

The good news is that the risetime is indeed quite a bit faster. As best as I could measure, 10-90% is 400 ps with a 6.5 volt top :) This quite noticeable when using the 350 ps S-1 head, so I went back to the 75 ps S-2. Pics added to "Sampling with 3S2" album. (If I could figure out how to link individual pics in a post, I would!)

But the bad news (which others have noted) is that the jitter and noise is also increased. It's not really bad especially while smoothing, but I guess that's a consequence of a small, very fast (Ft=9 GHz) transistor?

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