Page 149 - The Art and Science of Analog Circuit Design

Carl Battjes

Distributed Deflector for a Cathode Ray Tube

In 1961, Cliff Moulton's IGHz 519 'scope led the bandwidth race. This
instrument had no vertical amplifier. The input was connected to a
125-ohm transmission line which directly fed a single-ended distributed
deflection system. Schematics in Figures 10-8 and 10-9 show somewhat
pictorially what a distributed deflector looks like. The 519 deflector is not
shown. Within the CRT envelope was a meander line distributed deflec-
tion plate. Tuning capacitors were located at the sharp bends of the mean-
der line. The line was first tuned as a mechanical assembly and later
incorporated into the CRT envelope.
Terminated distributed deflector structures create a resistive driving-
point impedance in place of one lumped capacitance. They also synchro-
nize the signal travel along the deflection plate to the velocity of the
electron beam speeding through the deflection plate length. If a distrib-
uted deflector is not used, deflection sensitivity is lost at high frequency
due to transit time. Relative sensitivity is

-
JL where f is frequency and f te is an inverse transit time function.
/«*
This is usually significant at 100MHz and above, and therefore dis-
tributed deflectors show up in 'scopes with bandwidths of 100MHz or
higher. Various ingenious structures have been used to implement distrib-
uted deflectors. All could be modeled as assemblies of T-coils. The effec-
tive electron beam deflection response is a function of all of the T-coil tap
voltages properly delayed and weighted.

Theoretical and Pragmatic Coil Proportions

1
The basis for the earliest T-coil designs was m-derived filter theory. The
delay lines and the distributed amplifier seemed to work best when the
2
coils were proportioned—as per the classic Jasberg-Hewlett paper —at
m = 1.27 (coupling coefficient = 0.234). This corresponds to a coil length
slightly longer than the diameter. In the design phase, there was an in-
telligent juggling of coil proportions based on the preshoot-overshoot
behavior of the amplifier or delay line. The trial addition of bridging
capacitance invariably led to increased step response aberrations.

m-derived filters were outcomes of image-parameter filter theory of the past. The parameter "m"
determined the shape of the amplitude and phase response. "m"=1.27 approximated flat delay
response. Filters could not be exactly designed, using this theory, because the required termina-
tion was not realizable.
This classic paper described both the m-derived T-coil section and, very briefly, the constant-
resistance T-coil section. The use of these sections in distributed amplifiers was the main issue
and nothing was mentioned of other uses.

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