Dang, I was just about to suggest that one. Another shower idea.
I don't suppose that banging a tuned circuit has any inherent benefits.
I was thinking about a scope probe using a dual-gate mesfet (you can still buy them) as a source follower, with G2 connected to the source. Cin should be pretty low. You could make a scope probe with a tiny plug-in pcb with the fet on it, easily replacable every time you blow up the fet.
John
I used to use MRF966s for that--dual-gate GaAs FETs in in the old Motorola Macro-X package, so the G1 lead made a decent probe tip, especially if you put on a bit of epoxy to prevent it from bending where it came out of the case. I'd solder them straight to the end of a piece of RG174/U. I usually used them common-source, so I could supply the bias on the same coax--the load was 50 ohms to 5 volts, iirc. (That worked fine since I was usually looking at
I could lay out a small PCB-only probe, vaguely sort of like this...
ftp://jjlarkin.lmi.net/PogoStyx.JPG
It could have an SMB or MCX coax connector, or just a coax pigtail going to a BNC or SMA. Maybe a Pomona box on the other end, with coax termination, wall-wart connector, an official scope probe.
1 GHz or so, 0.5 pF, should be feasible. It would be dirt cheap to make.John
before
Are those pogo pin probes?
Hey, I have some dirt. I'd like to have one.
John
before
That could be pretty useful for a lot of things. Can I have input protection for 1 pF?
Cheers
Phil Hobbs
gig
going
before
No, that's what throw-away fets are for.
A couple of 0.2 pF schottkies would improve ESD protection a little, but wouldn't help for hard voltage overloads unless we added an R-C current limiter, which would sure de-elegantize the front end.
John
gig
going
before
Yeah, I have a box full of these little boards in the pic, maybe 100, and another box full of pogos, maybe 1000. These little boards are fun for tuning the values of surf-mount parts, live in-circuit.
If you want a few, send me a self-addressed envelope.
JL
18 Otis St San Francisco CA 94103
That's 0.1"-spaced graph paper, so those boards are ~3"x0.4", right? Pogo pins are ~1"? (That's actually kinda long for a pogo pin, it seems...)
I got a couple of pounds of pogos on ebay, cheap. They were probably left over from a bed-of-nails fixture or something.
John
John, Phil, what dual-gate MESFETs are still available?
For the foxhunt sniffers that VK3YNG makes, the front-end has an amazing dynamic range combined with low noise. He has limited stock of the FETs that make that work, so the unit may go out of production within a year. He said: "all of these devices have been obsolete for quite some time. The NEC parts in particular are available on the surplus market but more than 80% of what we have procured to date have been fake. Usually just lead frame and plastic." Although there are some surplus NE25139's about, they're SOT143 which is much too big for the existing design.
Last time I asked you mentioned these parts, which are all obsolete, unobtainable, or too big: Toshiba 3sk240, Renesas 3sk239A, Nec 3sk299, Infineon cf739, and the enhancement-mode 3sk292.
Is there anything small and available still?
Clifford Heath.
In the mail tomorrow. I misspelled San Fransisco on the envelope, but not intentionally.
You are very generous. Thank you.
John
Just checked, and NE25139 and NE25339 are now obsolete. Pity.
NEC has killed most of their discrete gaasfet parts.
John
gig
going
before
Well, if there are any suitable FETs left, I'd love to have some of those. The list of discontinued ones is quite depressing.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
...
I see, your application is different.
the BW requirement is much larger, I think we had something in the order of
10 MHz. It was mandated by the signal risetime, defined by the probe geometries and the ion drift speed. These were simulated/established using SIMION.thanks, Jure Z.
I'd be a lot more comfortable at 1 MHz, but we'll see what we can manage. If there's a lot of ionic current, we're probably dead, but there may well be some way of moving the goal posts to make it work.
This client has come up with a couple of interesting problems so far, and hopefully there will be lots more in due course.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
Presumably your VDS is small enough that the substrate diode is thoroughly off?
Chris
Yup. Tens of millivolts. MOSFETs don't play nice the way bipolars do.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
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