
Active signal processing in the time domain 111
the next sample. The main requirement is for a
very fast analog gate driven by a very narrow
sampling pulse. In order to achieve the widest
bandwidth and shortest rise time possible, the
sampling pulse is made very narrow; in particular
the trailing edge, where it switches from on to off,
is particularly fast. In a state-of-the-art instrument
such as the Tektronix 7S11 plug-in with sampling
head type S4 in a TOGO series mainframe, the rise
time is only 25 ps, corresponding to a bandwidth of
14 GHz. To achieve this, a sampling pulse 200 ps
wide but with a trailing edge aperture time of only
20