Products > Test Equipment
Measuring Distortions with the Scope:What you see is not what you really have..
rf-loop:
--- Quote from: Martin72 on December 31, 2022, 05:40:19 pm ---
--- Quote from: rf-loop on December 30, 2022, 07:12:20 pm ---Martin, you can try one small thing with SDG1000X
Load 50ohm.
1kHz
Set next SDG displayed output levels exactly
These are only some examples.
111.8 mVrms* look what THD you get.
Voltage level band change here (you can hear relay)
111.9 mVrms* look THD
353.4 mVrms* look THD
Voltage level band change here (you can hear relay)
353.5 mVrms* look THD
1.118 Vrms* look THD
Voltage level band change here (you can hear relay)
1.119 Vrms*
up to 3.536Vrms*
* Listen relay! for make sure V band border. I do not have any idea why it change some times example 353.4 or .5 and 353.5 or .6 and so on (in other V band change same, some times just tiny amount different)
--- End quote ---
Setup: SDG1062X with 50 ohm load termination connected to the A1.
Values with * = relay switch point
111.7mVrms 0.044%
111.8mVrms* 0.117%
.
.
353.4mVrms 0.043%
353.5mVrms* 0.116%
.
.
1.116Vrms 0.042%
1.118Vrms* 0.114%
.
.
1.2Vrms 0.110%
1.4Vrms 0.100%
1.6Vrms 0.091%
1.8Vrms 0.083%
.
.
.
2.9Vrms 0.050%
3.0Vrms 0.048%
3.056Vrms* 0.047%
3.535Vrms 0.041%
On every "relais switch point" THD increases, then decreasing until the next switch point with one exception: The last switch point won´t be affected(or it is a different relais from a different circuit).
Interesting things, thank you rf-loop :-+
--- End quote ---
Are these THD (Not THD+N) values plain THD without filtering and how many harmonics included in counting. (looks bit high values if plain THD and No filters.)
Martin72:
Hi,
No filters were set (forget, there is a high-pass filter from 400Hz and a band-pass filter that lets through everything from 22Hz to 22Khz), the A1 has only THD+N (IMHO), bandwidth up to 130Khz.
The data sheet of the SDG1000X series gives a THD of 0.075% (10Hz...20Khz), the measured values are significantly lower.
rf-loop:
Here my some older measurements and also some today data (perhaps these need do again, data partially from very old paper notebook.)
Revised, supplemented and updated table.
Plain THD (Not THD+N) %
SFILT: NO
Harmonics up to 7th.
Signal 1kHz.
Measured only using 50 ohm load.
Meter is Keithley 2015 THD
Edit: image changed. New revised image:
markone:
--- Quote from: rf-loop on December 31, 2022, 07:07:20 pm ---Here my some older measurements and also some today data (perhaps these need do again, data partially from very old paper notebook.)
Plain THD (Not THD+N) %
SFILT: NO
Harmonics up to 7th.
Signal 1kHz.
Measured only using 50 ohm load.
Meter is Keithley 2015 THD
--- End quote ---
I would say that those numbers are great for a generator that goes up to tens of megahertz @ few hundreds of euros, i personally own a SDG2042x "upgraded" and i never expected a spectral purity at low frequencies on par with fancy audio 24 bits DACs for the simple reason that it's not its job.
At the same time i do not expect that a 12 bits DSO is able to provide an actual dynamic range over 73 dB, regardless of any mathematical process in place, at this frequencies a good soundcard does a much better job.
Martin72:
--- Quote from: rf-loop on December 31, 2022, 07:07:20 pm ---Here my some older measurements and also some today data (perhaps these need do again, data partially from very old paper notebook.)
Plain THD (Not THD+N) %
SFILT: NO
Harmonics up to 7th.
Signal 1kHz.
Measured only using 50 ohm load.
Meter is Keithley 2015 THD
--- End quote ---
Hi,
The THD specification of the A1 says nothing about the number of measured harmonics, but the measuring range is given, 20hz to 40Khz.
Whether this also means that at a fundamental frequency of 1Khz it also measures up to the 40th harmonic, I don't know.
I will ask Neutrik.
"Tomorrow" I repeat the measurements, then with the 20Khz filter, possibly the values decrease.
Either way, both measurement series are far below the specifications mentioned - a very good result for such a "cheap" device.
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