Does a DMM calculate RMS at any frequency? - Page 1
Forum section: electronics / beginners

author: JustAnotherGuy999
author_rank: Contributor
timestamp: March 09, 2016, 08:23:11 pm
content: or there is a certain range that the DMM cant go beyond? if it can't, how do we know the RMS of a very high frequency signal?(im talking about true RMS multimeters)the same thing for peak to peak voltage.to make the question more concrete, can a fluke 87 measure RMS of any signal at any frequency, if it can't what should we do?

author: dadler
author_rank: Unknown
timestamp: March 09, 2016, 08:26:22 pm
content: Use an oscilloscope.Edit:The Fluke 87 (and every other RMS multimeter) is only specified for RMS voltage accuracy to a particular max frequency.Here is the Fluke 87 service manual. Check the specs in section 1-3:http://assets.fluke.com/manuals/83_85_87smeng0500.pdf

author: JustAnotherGuy999
author_rank: Contributor
timestamp: March 09, 2016, 08:30:29 pm
content: Quote from: dadler on March 09, 2016, 08:26:22 pmoscilloscopes are more expensive than DMM, because they have many and more important features than just measuring RMS, for example imagine you want to measure the RMS of a 2GHz signal, such an oscilloscope ain't cheap.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 09, 2016, 08:32:20 pm
content: There is a maximum bandwidth involved. I don't know if the fluke 87 has analog or digital true-rms conversion. I'd say digital, but I would have to review the teardown to confirm it.Fluke says the 87 does:AC bandwidth 20 kHz with low pass filter; 3 db @ 1 kHzAccording to the freely downloadable spec on fluke's website.Note that fluke does not say this for current.

author: dadler
author_rank: Unknown
timestamp: March 09, 2016, 08:36:08 pm
content: Quote from: JustAnotherGuy999 on March 09, 2016, 08:30:29 pmAnd, in general, they are required to measure accurate RMS voltage at high frequency. They may be more expensive, but that is neither here nor there. A DSO will almost always have better RMS accuracy at high frequency than a given DVM. All physical devices are bandwidth limited, DVMs and DSOs included.

author: JustAnotherGuy999
author_rank: Contributor
timestamp: March 09, 2016, 08:36:47 pm
content: Quote from: Jeroen3 on March 09, 2016, 08:32:20 pmmmm so DMM are used to measure RMS of very low frequency signals only, thanks jeroen3

author: Wytnucls
author_rank: Super Contributor
timestamp: March 09, 2016, 08:40:59 pm
content: Few portable multimeters go beyond 100kHz.Vpp is seldom accurate for repetitive transients below 250uS.Fluke 87V 20kHz and 250uS.Buy a more sensitive meter or use an oscilloscope.

author: retiredcaps
author_rank: Super Contributor
timestamp: March 09, 2016, 10:03:52 pm
content: I'm not sure if I'm interpreting the question correctly, but the Fluke 8060A specs are below.

author: exe
author_rank: Unknown
timestamp: March 09, 2016, 11:43:57 pm
content: Quote from: JustAnotherGuy999 on March 09, 2016, 08:30:29 pmSuch scope would cost a fortune, but... I don't think there are such multimeters that would do the job. I would say in most cases "true RMS DMM" means "it works for 50/60/120Hz". Dealing with >100MHz requires special equipment, probes, etc.

author: rfeecs
author_rank: Frequent Contributor
timestamp: March 09, 2016, 11:59:01 pm
content: For 2GHz and above, you would be using a power meter, or a spectrum analyzer.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 10, 2016, 07:08:08 am
content: What is the maximum bandwidth of a banana plug?

author: tggzzz
author_rank: Super Contributor
timestamp: March 10, 2016, 07:52:41 am
content: Quote from: JustAnotherGuy999 on March 09, 2016, 08:23:11 pmRTFM, or use google.For THz frequencies you need instruments like bolometersDon't use an oscilloscope, except in special circumstances. Their amplitude accuracy and range is limited.Define what you need to know, including range, accuracy, and repeatability. That requires statingwhyyou are measuring something. If you don't do that you will get correct answers that don't help you.Here's ageneral guidethat may help you.

author: newbrain
author_rank: Super Contributor
timestamp: March 10, 2016, 12:36:58 pm
content: Quote from: Jeroen3 on March 10, 2016, 07:08:08 amI defiiteley do not know for the plug, but you could use avery longsocket as a waveguide for some tens of GHz