Oscilloscope Probe Bandwidth Confusion - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: January 08, 2019, 05:13:50 am
content: My first scope was/is a Tektronix 2215. It was a gift, perfect for a noob, but it came without any probes.I researched probes, before buying a set and learned that the probe's bandwidth rating should match the scopes bandwidth rating. And, that you did not want to buy probes with greater bandwidth capability than that of the scope. This was confirmed through multiple sources. The reasoning and sources are long forgotten.Now, I have watched this video:In it, the Rhode & Schwarz engineers plainly state, repeatedly, that the probe's bandwidth should be a minimum of 1.5 x Scope Bandwidth; @ 6:34 and other times.So, which is it?And, are probes marketed with their bandwidth being stated as appropriate for X scope bandwidth? In other words, a probe marketed as a 60MHz compatible probe actually has 90MHz of bandwidth capability? If not, I bought the wrong probes... Given my first research, I bought 60MHz probes, as the 2215 has 60MHz bandwidth capability.My new Siglent SDS1104X-E scope, having 100MHz bandwidth, comes with PP510 probes, having x10 bandwidth of 100MHz.Thank you.

author: ataradov
author_rank: Super Contributor
timestamp: January 08, 2019, 05:26:06 am
content: Probe bandwidth must be at least the scope bandwidth, that is obvious. Having higher bandwidth helps because the typical definition of bandwidth is the frequency at which the signal attenuation is 3 dB. Now if you have scope with the same bandwidth your total attenuation at the end of the spectrum is going to be -6 dB. So having wider probe bandwidth helps.And I see no way it can hurt anything.But in reality your typical $6 probes from eBay actually perform much better than what is stated on the label.And in a grand scheme of things it does not matter unless you are doing some really precision measurements.

author: tautech
author_rank: Super Contributor
timestamp: January 08, 2019, 07:31:54 am
content: Quote from: t1d on January 08, 2019, 05:13:50 amMatching a probe to a scope is a bit of an art and not at all straightforward to be a good match for the front end stages.There's some good info on this matter here:https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg1434665/#msg1434665

author: dzseki
author_rank: Frequent Contributor
timestamp: January 08, 2019, 08:05:08 am
content: I think the different manufacturers have different meaning on scope and probe bandwidth. For example the famous Linear technology AN-47 note includes a Tektronix probe guide, where they state the probe bandwidth is meant for the whole system, eg. a 100MHz Tek probe with a 100MHz Tek scope will give 100MHz -3dB bandwidth at probe tip. Actually if you do a bit of testing you may find that your Tek 2215 also has bandwidth well beyond 60MHz.Also, I have a Tek P6106 250MHz probe and a Tek P6105 100MHz probe, both with 2m long cable. These old heads are kind of modular so you can change the cable and the tip and also the compensation box. I’ve played quiet extensively with them back in the day and confirmed that in fact the cable and head parts are the same for the two heads, only the compensation box is different. That said the overall bandwidth isn’t that different, only that the P6105 probe has simplier compensation network and will have more peaking at higher frequencies, while with the P6106 it is possible to set the response perfectly flat.

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2019, 11:11:24 am
content: A higher bandwidth probe works fine on a lower bandwidth oscilloscope. The probe bandwidth specification is more about the probe being suitable for use up to its bandwidth specification than anything like the -3dB point. Using a slow 100MHz probe on a 300MHz oscilloscope with a suitably fast signal will reveal problems with transient response and poor passband response.At higher bandwidths, say 300MHz and higher, probes may be tailored to correct the response of the oscilloscope. In this case, using a 500MHz probe with a 500MHz oscilloscope may result in a lower bandwidth and using the wrong 500MHz probe can result in poor transient response. This is why Tektronix sometimes issued A and B versions of their probes when newer oscilloscope models came out. The A and B versions had the same bandwidth as the original but were intended to be used with a specific model oscilloscope to produce the correct response.Another subtle distinction is what is actually being displayed. Some manufacturers calibrate the system to display the signal including the effect of the probe loading and some calibrate the system to show what the signal presumably looks like without the probe. I think HP did the later but I do not know about now.

author: t1d
author_rank: Super Contributor
timestamp: January 09, 2019, 06:36:57 am
content: All very excellent information; thank you , guys.