Re: Difference between a 475 and a 475A

Tom Lee

Yes, that's basically it. An impulse and a step both have spectra that extend to infinity, so both will exercise the channel over an infinite bandwidth. The requirements for a perfect time-domain response are a flat gain to all frequencies, and also a phase response that varies linearly with frequency. That latter criterion is just a high-falutin' way of saying that you want all Fourier components to experience the same time delay.

Real amplifiers roll off and shift phase nonlinearly, of course, so you want to approximate the above two criteria to the best possible extent. If you focus on only one, the other tends to suffer. Scope designers face a tough challenge, and Tek's engineers mastered the art of designing amplifier chains with remarkably well-behaved response over a broader band than others thought possible with the technologies of the day. The tricks (oops, I mean methods) they came up with remain relevant to this day.

-- Cheers,

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/19/2020 18:55, Jeff Dutky wrote:
Dr. Lee, and Harvey White,

Thanks for the informative responses, that does make more sense, especially in light of an impulse input, which I understand implies a broad spectrum of underlying frequencies including very high frequencies (in order to reconstruct the impulse from the elements of the Fourier decomposition). Those high frequencies, which are required to get a good steep rise time, will also show up as high frequency ringing after the rise and fall, which messes up the fidelity to the overall shape of the pulse.

This also makes sense to me from a mechanical analog to the electrical signal: if you have a damped spring system you can either get fast changes in the displacement, but have lots of wiggling afterwards, or you can have very little wiggling but the displacement is slowed down a lot. A loose piston attached to the spring lets the spring change length very quickly, but doesn't do much to damp overshoot and undershoot or to dissipate the energy in the spring after the change in length, but a stiff piston which will rapidly stop the under/overshoot will also make it much harder and slower the change the length of the spring.

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