HP 3456A VS 87V AC Volts Wrong or User Error? - Page 1
Forum section: electronics / beginners

author: denimdragon
author_rank: Regular Contributor
timestamp: September 09, 2017, 03:06:17 am
content: I purchased an HP 3456A off eBay a couple of weeks ago and the first thing I did was check the DC voltage (5V reference from voltagestandard.com) and resistance (Vishay S102C Metal Foil Resistor .01%) Both measurements were great. I never checked the AC voltage because I don't have anything as a reference.Today I needed to use the AC voltage function because I am trying to learn how to calculate capacitor ESR with a function generator and oscilloscope (I have an ESR meter, but I don't like using "crutches" during the learning process). I need to set the FG (BK 4011) @ 1.00 Vpp output with a 200KHz square wave so I was using my Fluke 87V to do so and thought that this was a good time to see how the 3456A measures AC voltage also. Well, I have two very different readings. The 87V reads 2.637V and the 3456A reads 3.6742VIt could be that I have something set up wrong on the 3456A, as I don't know much about measuring AC volts and I'm just learning more about the 3456A. I looked in the manual for help, but didn't really see any thing that would have given me a tip of what I've been doing wrong for the past 4 hoursIf anyone can give me some advice that would be great! I really want to know what's going on so I can continue my little experiment as well as knowing that my 3456A and my 87V are ok. I've include pictures below of the readings and also the settings I used for measurement on the 3456A.Any help would be greatly appreciated.

author: Damianos
author_rank: Frequent Contributor
timestamp: September 09, 2017, 05:27:28 am
content: Quote from: denimdragon on September 09, 2017, 03:06:17 amWhen we do tests of this type (comparison of instruments) "normally"/usually we start by an easy test by using a sinusoidal signal in the frequency range of 40 to 200 Hz. This will give to us a first indication for the "health" of our instruments. If this first test has good results then we continue with higher frequencies and other shapes of signals, else we start studying of what is going wrong...The signal that you tried probably is at the limits or, worse, outside of the instruments' specifications.Study the manuals of the instruments that you use. There are specifications of minimum and maximum frequency that they can "try" to measure signals. Don't forget that they are almost always referred to sinusoidal signals. Signals with different shapes contain harmonics of sinusoidal that extend to infinite frequency (theoretically). Note that in the manuals there is also a reference about "crest factor", study it.Always have in mint of what are the specifications, in the conditions that the instrument is called to operate: range of measurement, frequency, shape of signal (and temperature, for precise defined results).Regards,Damianos

author: denimdragon
author_rank: Regular Contributor
timestamp: September 09, 2017, 04:19:28 pm
content: Quote from: Damianos on September 09, 2017, 05:27:28 amThank you for this. I totally forgot about the odd number harmonics present in square waves. I'll have to go back and read up on the manual a little more, but I will try the technique you suggested first and also look into the crest idea. The Fluke is newer, and I know that it takes into account some things that maybe the 3456A doesn't if not set up correctly. I'm going to do a bit more research and come back with results.

author: alm
author_rank: Super Contributor
timestamp: September 09, 2017, 06:46:53 pm
content: I believe the Fluke 87V has a bandwidth of 20 kHz. The 3456A is higher (250 kHz), but accuracy is much worse than at lower frequencies. Even a 200 kHz sine wave would have been a stretch for both meters, but the 3456A would definitely be more accurate at that frequency. If you want to compare the meters, compare them with sine waves below 20 kHz or square waves below maybe 2 kHz. Also check the accuracy specifications for both meters, the claimed accuracy will often vary with frequency.

author: macboy
author_rank: Super Contributor
timestamp: September 11, 2017, 04:08:08 pm
content: 200 kHz square wave with a multimeter? Almost no chance. You'll need several MHz of bandwidth, and precious few have that regardless of cost.What did you expect to see as a reading on your RMS multimeter for a 1 Vpp Square wave input? If you can't answer that, then you will get nowhere. (Hint: definitely not anything resembling 1.000 VAC)Set the function gen amplitude using the oscilloscope that you are using for the impedance testing. A multimeter isn't a good choice in this case.

author: denimdragon
author_rank: Regular Contributor
timestamp: September 11, 2017, 06:23:21 pm
content: Quote from: macboy on September 11, 2017, 04:08:08 pmI had no expectations as I am currently not fluent enough in this discipline to be aware of WHAT to expect, hence the title of the thread, and the thread being in the "beginners" section of the forumI really appreciate your suggestion in regards to using the oscilloscope. The main thing that I want to learn now is how the AC waves are interpreted on both of the instruments. I've been reading the 3456A manual the last couple of nights. Now is a good time to learn.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 11, 2017, 07:20:51 pm
content: The meter reads the RMS of AC voltage within the designed frequency range. It does not read peak or peak to peak voltage, but you can visually observe the signal peaks on the oscilloscope and easily set the source peak to peak voltage up accordingly, which is why this was suggested. Maybe do a Google search on the difference between RMS, peak and peak to peak voltage as well as how to calculate RMS for sine waves and square waves. The shape of the waveform does affect the outcome so the formula will be different in each case. The research will help you understand why.

author: pelule
author_rank: Frequent Contributor
timestamp: September 11, 2017, 09:58:42 pm
content: This online calculator may help:http://daycounter.com/Calculators/RMS-Calculator.phtmlBR/PeLuLe