On Max Bandwidth of an Analog O'scope

There is a crude "relationship"; one spec cannot be used to determine the other.

"alpha_uma" wrote in news:tdxZc.297903$J06.296479@pd7tw2no:

No,there's no relation.Bandwidth is a spec for the vertical amp and CRT upper limit,and sweep rates determine how much horizontal resolution you can see on a signal,like part of one cycle or period,or just 2 or 3 cycles/periods.Horizontal sweep rates are tougher because you not only need speed,you need better linearity.(1 div measures the same anywhere on the sweep)

Then there's sweep magnifiers;essentially increasing the gain of the horizontal output amp,and only displaying a small portion(1/10th for an X10 mag) of the full sweep.

The bandwidth - usually given at the 3dB point - indicates the performance of the vertical amplifier. There is no relation between this and the sweep generator except that some human was hopefully involved at some point of the process and hopefully made a vaguely sane decision.

Suppose you had a 20 MHz oscilloscope with a maximum sweep rate of 1 microsecond per division (an exaggerated case). If you put a 20 MHz signal into it, you'd see 20 cycles in one division - it would just be a glowing rectangle on your screen. If you did the same thing on an oscilloscope with a lower bandwidth and same sweep speed, you wouldn't get the rectangle - you'd just get a line, because the vertical amplifier couldn't reproduce the 20 MHz signal.

So, why do oscilloscopes tend to have an analog bandwidth beyond what you seem to think is necessary based on the sweep speed? It's so that you can see more complex waveforms than sine waves at the maximum sweep speed. Say you have your 0.2 us/dev scope, and you want to see a 5 MHz square wave. You'll need to be able to reproduce several harmonics of 5 MHz in order for your wave to look square instead of sinusoidal.

Very helpful explanation. Thanks

Let me see if I get this arithmetic correct. With a 20MHz input sine wave (say, 2V peak-to-peak), if I increase the sweep speed from 1 microsec per div to 0.2 microsec per div, the display on the V-212 o'scope will then (theoretically) change from showing a "fuzzy" rectangle to showing 4 cycles per division instead. But 4 cycles per division is still relatively "crowded" in the limited physical screen space of a division. And since the V-212 only has max sweep speed of 0.2 microsec/div, it will be impossible to see any finer details of this input sine wave on the V-212. Am I getting it correct so far?

Is the following thinking correct?

1. As the bandwidth of the o'scope is a function of the "sophistication" of the vertical amplifier, would it be correct to say that the bandwidth is a function of the "resolution" of the output signal from the vertical amplifier?

1. Having faster sweep speeds is just as important as having enough bandwidth. Faster sweep speeds allow us to see more details. Switching to a faster sweep speed allows us to "zoom-in" closer to see finer details.

Thanks

In message , alpha_uma writes

A hobby horse of mine: Analog scopes (CRT without A/D converter) are often better than digital because dynamic range is much greater than that given by 12 to 16 bits .

Lets just say that you can see some really strange things on a digital scope that do not appear on an analog scope of the same bandwidth.

Remember that any digital scope uses quantization and is an approximation of a continuous system.

An analog scope will render a more faithful representation and degrade less strangely near its bandwidth limit.

There is no substitute for knowing how your tools work.

That sounds right.

I wouldn't really put it that way. Resolution is a more suitable concept for talking about digital, and sophistication isn't a precisely defined term except maybe in marketing.

Sometimes seeing a vertical line is useful, it's just that looking at sine waves isn't one of those times. For example, to see a square wave, you'll need a higher bandwidth than to see a sine wave of the same frequency, but you won't need a faster sweep speed.