Re: Difference between a 475 and a 475A

Harvey White

There's a subtle "gotcha" in here.  It may be best thought of as the problem when designing a filter (lowpass, highpass, bandpass, doesn't matter, but something designed to cut off a bunch of frequencies - or allow them to go though)

In general, as I remember it, the sharper the response (the better the filter is at rejecting things really close to the edge of what it thinks is ok), the more ripple in the filter passband.

To state it another way, the filters that have the flattest response across the frequencies that are "OK" are the ones with the most shallow slope on the cutoff.  That is, they're the ones that aren't sharp.  The sharper the filter (the better it is for rejecting the "out of band" frequencies), the worse it is for flatness.  You can fix that, but only at the expense of adding more parts and other compromises.  Certain filter designs are optimized for flatness, certain for edge response.

Oscilloscope wise (and I can be wrong here), you optimize an oscilloscope response (because it has a LOT of filters for adjusting response and compensating for it, so think of the entire vertical amplifier as a big filter with gain) for either flatness (good for bandwidth measurements) OR you optimize it for pulse response (which means that the gain across the bandwidth is not even).

You get one or the other, depending on what you are measuring. Since the two adjustments (pulse fidelity and frequency fidelity (so to say) are complementary, you don't get both.

Typically, pulse response accentuates the high end response of the scope (and makes the pulse edges look good) at the expense of the frequency response (which may make the flat part of the pulse look bad, depending on frequency).

Hope that I explained it well enough, and I got it right.


On 11/19/2020 8:58 PM, Jeff Dutky wrote:
John Gord wrote:
the 475A was basically the same as the 475, but adjusted for higher bandwidth but perhaps slightly inferior pulse response
I'm not sure I understand what you are saying here: wouldn't "inferior pulse response" mean a longer rise time? Wouldn't a longer rise time necessarily mean a lower bandwidth?

I don't really understand all the math, but my impression, from evaluating my father's 475 and probes a couple months ago, was that you calculated bandwidth as the inverse of rise time multiplied by a constant (0.3-something?). Am I misunderstanding what is meant by "pulse response"?t

-- Jeff Dutky

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