bandwidth ? is 100MHz scope really 100 MHz ?

[robb]

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

 

[Steve]

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.

Steve

"robb" <some@where.on.net> wrote in message
news:13iefdd1h8p3l78@corp.supernews.com...

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

 

[Nelson Gietz]

"Steve" <zarco@sonic.net> wrote in message
news:472741dd$0$79887$742ec2ed@news.sonic.net...

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.

Steve

"robb" <some@where.on.net> wrote in message
news:13iefdd1h8p3l78@corp.supernews.com...
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

Robb,

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

 

[mike]

robb wrote:

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


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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