Page 145 - The Art and Science of Analog Circuit Design

Who Wakes the Bugler?

counterpart. The bottom picture shows the response when the coils (L384
and its mate) were disabled. (All three terminals of each coil were
shorted together.) This reveals that, without the coils, the response looks
very much like a single-time-constant response. The middle picture illus-
trates the progression of tuning after the shorts are removed. The pow-
dered iron slugs in the coil forms are adjusted to optimize the response,
The top picture shows the best response. The 10-to-90% risetime of the
beginning waveform is 75 nanoseconds, and in the final waveform it
drops to 28 nanoseconds. This is a ratio of risetimes of 2,6—near the
theoretical bandwidth improvement factor of 2.74. The final waveform
has peak-to-peak aberrations of 2%.
The total capacitance at the deflector node includes the deflection
plates, the wires to the plates, the beam power tube plate capacitance, the
wiring and coil body capacitance, the plug-in connector capacitance, the
mounting point capacitances, the chassis feedthrough capacitance, the
resistor capacitance, and possibly virtual capacitance looking back into
the tube. We can solve for the equivalent net capacitance per side by
working back from the 75nsec risetime and the 1.5k load resistance. This
yields about 23pF per side. Although each coil is one solenoidal winding,
it actually performs as two coils. The coil end connected to the tube plate
works as a series peaking coil, and the remainder as the actual T-coil.
L344, which is also a T-coil, appears upstream in the 3A6 schematic
fragment. Notice that the plate feeds the center tap of this coil. This is an
application of reciprocity (Look in your old circuit textbook!). If the
driving device output capacitance is significantly greater than the load
capacitance, it may be appropriate to use this connection.

Distributed Amplifiers in Oscilloscopes
The idea of a distributed amplifier goes back to a British "Patent Specification'' by
W.S. Percival in 1936. In August 1948, Ginzton, Hewlett, Jasberg, and Noe pub-
lished a classic paper on distributed amplifiers in the "Proceedings of IRE." At about
the same time, Bill Hewlett (yes, of HP) and Logan Belleville (of Tektronix) met at
Yaws Restaurant in Portland. Bill Hewlett described the new distributed amplifier
concepts (yes, he "penciled out" the idea on a napkin!). In 1948, from August
through October, Howard Vollum and Richard Rhiger built a distributed amplifier
under a government contract. This amplifier was intended for use in a high-resolu-
tion ground radar. It had about a 6nsec risetime and a hefty output swing. In order
to measure the new amplifier's performance, Vollum and Rhiger had outboarded it
on the side of an early 511 'scope, directly feeding the deflectors.
It soon became clear that what the government and industry really needed
was a very fast oscilloscope. I am not sure of the details or sequence of events,
but Tektronix—Howard Vollum's two-year-old company—was making history.
Vollum, Belleville, and Rhiger developed the 50MHz 517 oscilloscope, an oscillo-
scope with a distributed amplifier in the vertical deflection path. Vollum and
Belleville had successfully refined the distributed amplifier enough to satisfy this
oscilloscope vertical amplifier application. The product was successful and order

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