Craig, thanks!
+11 Informative..
Your post encapsulates about a dozen questions I've had re refurb of the pure unobtainium stuff.
Printed!
It's Very nice to know that, with some precise fiddling - many of these can be resurrected.
(And thanks to Stan, repeatedly - for supplying the unobtainium Information!)
GaAs back with a vengeance, eh? 150 Ghz might get us 5000 Channels - with nothing watchable..
Two ears and a tail - {static-free, of course}
Ashton
Craig Sawyers wrote:
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I have read postings about using some fast Schotky
diodes in place of some of Tek's GaAs diodes but I have no personal
experience with this . . .
I did this with a defunct S3a 1GHz sampling probe. The brige was toast
(well, some of the diodes were leaky in reverse bias). Stan helped out by
looking up the spec of the original bridge diodes for me.
I found that the basic spec, most critically reverse leakage and capacitance
were matched rather well with Agilent microwave Schottkys, HSMS8202. These
are diode pairs in a single SOT23 surface mount package. By flipping one of
them over, and soldering the pair of pins one edge together you end up with
a bridge. Costs about 3 bucks.
Replacing the defunct bridge with this one worked perfectly - specification
of the resulting probe is exactly as it ought, full 1GHz bandwidth and
aberrations well within spec. I suspect that this approach would work well
with all the earlier samplers up to around 4GHz, including the S1 and S2.
The most obvious difference between GaAs, silicon junction and Schottky is
the foward voltage drop. GaAs is well over a volt, Si is about 0.7V, and
Schottky is about 0.3V. As I said I had no trouble, but potential problems
might arise with the reverse bias voltage and the magnitude of the strobe
signal. Usually these have a preset pot associated with them, and so can be
adjusted somewhat.
With the 1S1, the early GaAs bridge diodes were replaced later with Si ones
so it is certainly possible to do this on the 1S1.
The reason for GaAs was it was the only way to get speed in the early days
of samplers. Then silicon processing caught up, and GaAs production went
into a bit of a decline until LEDs came along, which are based on metallic
alloys of III/V semiconductors like GaAs. And now, GaAs is used in the very
fastest transistor technologies, the so called HEMT (high electron mobility
transistor) - these beasts still have useful gain at 150GHz plus.
Later (and much faster) sampling heads like the S4 and S6 used Schottky
diodes from day one. In fact to get the speed these are impressively tiny
to get the capacitance and lead inductance down - the junction is only 3um x
3um, and ridiculously easy to vapourise with static discharge, and event
that would ruin your day. The thick film, charge trap heads simply don't
lend themselves to being fixed on these fast samplers; if you zap them they
are scrap.
Other things that sampler fixers might like to bear in mind is that "back
diodes" (tunnel diodes with a very low peak current of about 100uA)are what
the microwave fraternity now confusingly calls tunnel diodes. They are
essentially used as zero bias detectors, and I think that close equivalents
to the BD4 could be found. Likewise the snap-off diodes that are used in
the strobe circuits are still available from the microwave component
suppliers, now called step-recovery diodes. Anyone wanting a browse through
this sort of arcana could look at
The only tricky thing is true tunnel diodes, for which there is no chance of
a commercially available equivalent.
Craig
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