Bandwidth of probe vs scope?

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Hi - when a scope specifies a, say, 1GHz bandwidth, my understanding is that that means the -3db point on the analog front of the scope is at 1GHz. Is that correct? Similarly, when a probe specifies a 1GHz bandwidth, I again understand that to mean that the probe's -3db point is at 1GHz. So that would suggest a 1GHz probe with a 1GHz scope has -6db attenuation at 1GHz. Is that true? Or am I missing something?

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Not necessarily so.

Depends on test conditions .

Typically a scope bandwidth is specified within a 50 ohm measurement environment. Scope input is terminated in a 50 Ohm feed through termination and then connected to a 50 ohm levelled source whose frequency is scanned to check for displayed amplitude reduction of 0.707 the low frequency reference amplitude.

Probe termination specification needs to be understood and applied in order to ensure the specified bandwidth is obtained.

When the probe is terminated in the correct fashion then and only then can we discuss the effect of co-located poles the oscilloscope and probe transfer function.

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I have forgotten the details, but there is a formula that combines the bandwidths of the probe and scope by using the rise times of probe and scope in calculations.

However, that is a good point, is the bandwidth specified at 50ohm input or 1Mohm input, but my old scope does not even have 50 ohm input.

And also good point that it depends on the test case, as there are many different ways to approximate the rise time from the bandwidth.

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Jepael wrote:
... there is a formula that combines the bandwidths of the probe and scope by using the rise times of probe and scope in calculations.
Root the squared rise-times sum, bandwidth = approx. 0.33/rise-time, rise-time is from amplitude 10% to 90%.
Ref. Probing High-Speed Digital Designs by Dr. Howard Johnson.
Jepael wrote:
... but my old scope does not even have 50 ohm input.
Probing pointers, Take 2 by Jack Ganssle.
"I used an Agilent MSO-X-3054A scope with selectable input impedance, set to 50 ohms. This is critical for the shop-made probe; the normal 1 MΩ simply will not work. If your scope doesn't have a 50-Ω mode, use a series attenuator such as the 120082 from Test Products International (this part doesn't seem to be on their web page, but Digikey resells them). Agilent's N5442A is a more expensive but better-quality alternative."
Jepael wrote:
... as there are many different ways to approximate the rise time from the bandwidth.
Yea, usually rely on the maker's specs. Better is to measure using a sampling oscilloscope (not the usual real-time scope) and a fast rise/fall signal source (strong ECL clock, diode-pulsed (?) source).
Edit: corrected rise-time statement.

"Dare to be naïve." - Buckminster Fuller

Last Edited: Fri. Jun 8, 2012 - 06:38 PM
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I'm glad I'm just an old bit-pusher and only have to think of the world in terms of ones and zeros. ;)

Interestingly, the discussion on bandwidth of 'scopes in the Wikipedia article doesn't mention probes at all:
http://en.wikipedia.org/wiki/Osc...

Agilent has a 10-page app note:
http://cp.literature.agilent.com...
No mention of probe there either.

This little article just has a mention to consider the probe also: :)
http://gpete-neil.blogspot.com/2...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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"1 GHz probe" is a bit ambiguous because it does not say what the measurement conditions are. One possibility is that when it is on a scope with a bandwidth much higher than 1GHz, you get an observed bandwidth of 1GHz. It might mean that with some specific 1GHz scope, you can add the probe and still get 1GHz.

Rise times or bandwidths combine as the square root of the sum of the squares.

Passive probes have some special "problems" because of the transmission line characteristic of the cable. Historically, the solution has been to make the center conductor resistive to damp reflections. But, this limits bandwidth. A more recent alternative is to make the whole system "properly terminated" but thats not without problems at low frequencies. Not sure what is done for general purpose probes, these days.

Jim

 

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