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-----Original Message-----
From: Craig Sawyers [mailto:c.sawyers@...]
Sent: Tuesday, March 02, 2004 2:29 PM
To: TekScopes Yahoo Group
Subject: RE: [TekScopes] 7S/7T Sampling System

While on the sampling gear subject, could someone tell me more about S4. I
have been told that construction of S4 is quite terrible, so
sampling pulses
blow by on the bridge is significant and low level signals can be drown in
that noise. I gentleman who told me about that, said that he
almost scrapped
amplifier that he was building, but power meter showed no noise,
so he went
out and found S6, which confirmed that amplifier was all right.

The S4 has marginally higher bandwidth, but because of that rumor I stayed
away from it and opted for S6.

Well, there are some reports on the Picosecond Pulse labs website that they
produced during the 80's and early 90's with tests on sampling systems.
They found that there was a problem with the S4:

"The old Tek S4 sampler had the worst settling time performance. It showed
a gradual rise in 4ns to a max overshoot of 5.5% followed by an exponential
decay back to the 100% level requiring an additional 25ns."

"We found that the 200ps wide strobe caused a unique problem for the S4.
When a tunnel diode pulser was mounted directly on the input connector of
the S4, the leakage of the 200ps strobe was enough to cause false triggering
of the TD. The falsely triggered TD pulse would then enter the S4's diode
bridge during the 200ps on time. The resulting CRT waveform was quite
unstable, with sometimes a negative risetime display. The simple cure for
this is to introduce a delay line between the pulse generator and the
sampler. The delay must be greater than the strobe pulse duration. Thus
for the S4 a 500ps, 7mm (diameter) air line was used."

"Close inspection revealed that the S4 response had a very fast rise time,
with flat response for 200ps. At 200ps it has an abrupt +7% step. Then the
waveform continues to rise up to the 105.5% level in about 4ns. It then
slowly recovered back to the 100% level in 25ns. The Tek spec is <=10% and
some units used by the author in the past have been as bad as 10%."

By contrast, the S6 gets a much better write up - a mere single paragraph
that says it it much better than the S4. The other head that gets a really
good write up by PPL is the SD24. The benchmark for their tests was the
Hypres superconducting Josephson Junction sampler, which boasted a pulse
response of 5ps (70GHz).

Craig





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