# On Max Bandwidth of an Analog O'scope

Discussion in 'Electronic Equipment' started by alpha_uma, Sep 2, 2004.

1. ### alpha_umaGuest

I need some clarification on the concept. Is there any relationship at all
between a 'scope's max sweep rate and its rated max bandwidth in its specs
(shown, for example, on a metal model tag on the front of the 'scope)? If
the max sweep time/div is 0.2 microsec, is it incorrect/simplistic to think
that 1/(0.2 microsec) = 5MHz is the max bandwidth (because of the
mathematics of things like Nyquist Criteria)?

(For example, the front of the following V-212 o'scope says 20MHz, but it
has a max sweep rate of 0.2 microsec:

Thank you.
Al-U

2. ### Robert BaerGuest

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

3. ### Jim YanikGuest

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.

4. ### Terran MelconianGuest

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.

5. ### alpha_umaGuest

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?

2. 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

6. ### ddGuest

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 .

7. ### Winfield HillGuest

dd wrote...
12 bits maybe, but not 16. And that's rarely the reason that analog
scopes can be superior.

8. ### Keyser SozeGuest

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.

9. ### Terran MelconianGuest

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.