bandwidth ? is 100MHz scope really 100 MHz ?

Most scopes I've seen specify 'usable' bandwidth based on the response of the vertical amps. If its a 100 MHz scope, the respose of the amp will be down 3 dB or less at 100 MHz. In other words, you'll be able to see a signal, but the accuracy (vert) is poor.


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when an oscope is advertised as 100 MHz scope what exactly is the useable bandwidth ?

my 100Mhz scope only has Horizontal divisions down to .05 usec which means 5 x 100 MHz periods squashed into a division, yes ? (since 100 MHz = .01 usec period) is that considered a useful display of 100MHz signal ?

probably i am missing something or maybe 100 MHz just means you can see it but really only usefull up to 50 Mhz ?

do they intend you can detect and see 100 Mhz wave i would anticipate that 100 MHz scope means the best you can view is one

100 MHz period across the scope screen.

so in my case 10 divisions so ( .01 usec / 10 div) = .001 usec per division which i do not seem to have

what am i missing ? thanks for any help, robb

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Robb, You might also be able to get more detail if your scope has a 5X or so expansion of the horizontal sweep... Nelson

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Nelson Gietz

This is a complex issue. When a car is advertised as 35mpg, what is the useful mpg? Scope vendors have marketing people too.

Assuming they're being truthful, it normally means that the signal will be attenuated no more than 3dB at 100 MHz. But depending on how they peaked it to get that bandwidth, performance beyond 100MHz. might not be what you'd expect.

Assume you stick in a 100 MHz. sine wave. You know it's a sine wave, so you don't even have to look at the display. It's only interesting if you have a signal that is not a sine wave. And to make it interesting takes additional frequencies that are higher than 100MHz. which can't get thru the amplifier anyway.

You need a vertical bandwidth that's higher than the highest frequency components you wish to view. Don't buy a 100MHz scope to look at 100 MHz. square waves...cause you're gonna see sinewaves...sort-of.

A quality scope will have a horizontal magnifier that will let you see more horizontal resolution...but it won't really give you any more information.

And if it's a digital scope, the plot thickens even more. mike

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