Page 152 - The Art and Science of Analog Circuit Design

Who Wakes the Bugler?

Phase Compensation withT-Coils

The portable 453 needed a compact delay line for the vertical system that
didn't require tuning. Kobbe had designed and developed a balanced-
counterwound delay line for the 580 series of 'scopes. We made it still
smaller. This delay line worked well at 50MHz, and had reasonably low
loss at 150MHz. Unfortunately, the step response revealed a preshoot
problem. The explanation in the frequency domain is nonlinear phase
response. High-frequency delay was insufficient, and one could see it as
preshoot in the step response. Three sections of a constant-resistance-
balanced T-coil structure added enough high-frequency delay to clean up
the preshoot, and even speed the risetime by moving high frequencies
into their "proper time slot." T-coil sections can provide delay boost at
high frequencies if the T-coil section is proportioned differently from that
of the peaking application. A negative value for "k" is usually appropri-
ate and is realized by adding a separate inductor in the common leg.

Integrated Circuits
In the late '60s, when the 454A was being developed, George Wilson, head of
the new Tektronix Integrated Circuits Group at that time, wanted to promote the
design of an integrated circuit vertical amplifier. I rebuffed him, saying, "We can
never use ICs in vertical amplifiers because they have too much substrate capac-
itance, too much collector resistance, and too low an f t." I was correct at the time,
but dead wrong in the long run. In the 70s, Tektronix pushed 1C development in
parallel with the high-bandwidth 7000 series oscilloscopes.

Figure 10-10. 2 , 2 P 3 . 3 P 3 . 3
Correcting
Insufficient High-
Frequency Delay.

1 . 8 P
1.7-11 1

3 . 3 P 3 . 3 P

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