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| Keithley 2015 THD with FFT and Harmonic graphs |
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| G0HZU:
--- Quote ---After getting my Keithley 2015 THD for some small amplifier projects I was shocked that nobody had put together some tools to automate the beast. --- End quote --- I bought mine a few years ago and created some automated apps for it within a few days of it arriving :) https://www.eevblog.com/forum/testgear/keithley-2015thd-initial-tests/ All the apps are fairly crude but they work well. I did the THD UI first, followed by a fairly decent 'Toneohm' app where the pitch of the audio from the PC speakers depends on the resistance of the device being measured. It worked quite well as the GPIB interface is quite quick. I also did a quick and dirty logger program to measure PSU stability. Not exactly modern or pretty but it works. --- Quote ---I haven't seen anyone share FFT spectrum plots from the device, so I'm hear to share! --- End quote --- Thanks for this. I missed the FFT plot command so I may have a play with this at some point :) How quickly can it create and refresh the spectrum plots using this command? |
| 2bluesc:
--- Quote from: G0HZU on May 31, 2017, 08:57:10 pm ---Thanks for this. I missed the FFT plot command so I may have a play with this at some point :) How quickly can it create and refresh the spectrum plots using this command? --- End quote --- I stumbled on your app, but saw that it was Windows based and realized that wouldn't work for me on Linux. :-// Did you ever release it to the public? As for the spectrum plots, I'm using the RS232 interface, and the data moves quite slow at 19.2kb/s which seems to be the highest speed (unless a new firmware improves this or there are undocumented features). I need to explore changing the ':FORMat:DATA` command to transfer the numbers as binary IEE754 floating point which should happen several times faster (think ~14 bytes as ASCII vs 8 bytes for a double). It takes maybe 5 seconds to just receive the FFT data over serial. I think the actual conversion time for the internal DSP is the considerably faster, and should be the same as you experience with your app. |
| 2bluesc:
After I get done exploring with my THD tests, I'm curious if I can make something like your DMM app. I'd like to see if it's possible to write a driver for sigrok and pulseview and if the sigrok UI tools like pulseview are useable to justify the effort vs a standalone app that suits my needs. |
| 2bluesc:
Played around a bit more with the FFT binning. It appears the `DIST:FREQ` command does modify the width of the bins on my 2015 as described for the 2015-P. If I set the 'DIST:FREQ' (apparently bin width in special cases) to 20 Hz and read all 1023 samples I get the attached plot. The original plots were with the frequency set to auto acquire and only reading the first 500 samples (at 60Hz bin width). The THD and THD+N numbers appear to be garbage, and this led me to believe it wasn't working earlier as it sets the 0 magnitude reference to whatever noise it reads at 20 Hz. After some normalization of the FFT plot values I get the attached graph with 20 Hz bandwidth per bin. I assume that I can easily sum the harmonics and derive my own THD and THD+N values with higher resolution for lower frequencies (there's no data past 20kHz for harmonic calculations). I will need to test the 20Hz vs automatic 60 Hz bin width and see if they agree for test values of 1kHz. Seems odd that Keithley wouldn't calculate intelligent THD, THD+N, SINAD values after adding a new SCPI bin width command that doesn't overlap the DIST:FREQ command. The Harmonics graph is a mess still, please ignore. :) |
| 1audio:
I'm almost done with a VB app for the Keithley that I'll be sharing. If you have the code for the harmonics it would be helpful. That's the next challenge. I'm using GPIB and its pretty quick. Supposedly it can make a lot of measurements very quickly if the internal processor is used to do the sequencing. One limitation is the internal source. I can't find a schematic to come up with ideas on improving it. I'm sure its the 12 bit DAC but its not a simple DAC upgrade if there's only 12 bits to drive it. |
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