Keithley 2015 THD - wildly varying measurements at 1Vrms on auto - Page 1
Forum section: electronics / testgear

author: jpb
author_rank: Super Contributor
timestamp: January 05, 2015, 10:02:53 pm
content: Whilst using my 2015 to measure THD on my new Agilent 33522A at different frequencies I was puzzled by wildly differing readings at 20Hz for a 1Vrms signal.I set the voltage range to auto (as advised in the manual) and found that when I measured the 1Vrms signal after measuring a lower amplitude signal then theTHD was measured at 0.017%.On the other hand if I went up to 2Vrms and then back to 1Vrms the THD figure was 10 times as great (approximately 0.18%) !I realised that this came down to range setting. On the 10V range the figure is 0.18% whilst on the 1V range it is 0.017%.The 0.017% figure is the more accurate one (full range and agrees with/better than the spec of 0.04% typical) but it is slightly disturbing to get a reading 10 times too bigon an auto setting.

author: electr_peter
author_rank: Unknown
timestamp: January 13, 2015, 10:21:13 pm
content: This is the reason why you would always want to use lowest possible range for a signal to get a good accuracy.If DMM has accuracy of 1%+10 digits, thenRangeSignalReadingTolerance, +/-VError, +/-%2.000V1.2341.2340.022341.820.00V1.2341.230.11239.1200.0V1.2341.21.01282For THD measurement same thing applies.

author: G0HZU
author_rank: Super Contributor
timestamp: January 13, 2015, 11:19:40 pm
content: I tried to replicate this problem on my 2015 THD and found that it behaved fine. However, I control the 2015 remotely when measuring stuff like THD.I wrote this THD program for it over a year ago but I'm pretty sure I added the ability to make sure the meter always selected the best range. I tried stepping the signal back and forth in small steps or in one big chunk from 1V to 2V and back again and it gave consistent readings.See below for an old screenshot of the SW running on the 2015. Ignore the blue arrow as this just shows the marker feature I added to it. To the user it appears to run in real time so gives a reasonably fluid display.Maybe your meter has an older version of firmware? Mine is V B17.Note: I rarely use this meter manually (horrible UI and display) and I prefer to use it as a logger via GPIB for various tasks. I don't think I'd want to wade through all the front panel menus to set up a THD measurement by hand. I think I tried it a couple of times and then wrote the above program to do it all for me.

author: G0HZU
author_rank: Super Contributor
timestamp: January 13, 2015, 11:30:51 pm
content: Update:I just loaded the SW program in VB and altered my program to let the meter manage the ranges by itself and I finally managed to get the same symptoms as you did. i.e. there seems to be a significant issue here if you let the meter use its AUTO range.That's less than impressive...

author: G0HZU
author_rank: Super Contributor
timestamp: January 13, 2015, 11:47:15 pm
content: The other thing is that I don't have a lab grade (low distortion) signal source here to allow critical measurements at low AF frequencies. So I don't think I'm measuring the limits of the meter here when set on the optimum range.See below for a typical measurement at 20Hz. I also need to update the SW to improve the frequency readout as it shows 20Hz as 2E-02kHz.I generally don't bother looking at the THD figure as I'm much more interested in looking at the relative levels of all the harmonics with this meter.But If I up the signal level to 2V and back down again to 1V the harmonic distortion jumps up quite alarmingly if the meter is allowed to optimise the range by itself