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EEVBlog 121GW Discussion thread
dcac:
@Keyview
There might be a typo in the 121GW manual - the footnote (3) for 50mV DC probably also should apply to 50mV AC but here it refers to footnote (2)? which is not to be found.
Keyview:
--- Quote from: dcac on March 19, 2021, 05:52:23 pm ---There might be a typo in the 121GW manual - the footnote (3) for 50mV DC probably also should apply to 50mV AC but here it refers to footnote (2)? which is not to be found.
--- End quote ---
Thank you for sharing your observation on the potential typo.
I now checked with the Fluke 289 manual: The requirement for rel mode and zeroing for 50 mV DC is mentioned there, too, but no requirement for rel mode for 50 mV AC.
So, the footnote 2 on page 17 of the 121GW manual is maybe not to be read as footnote 3.
I consider to purchase the 121GW for low V AC measurements because it has a better accuracy than the Fluke 289 in this low area.
For example, measuring 1 mV AC with the Fluke 289 has an accuracy of 0.42 dB (65 Hz - 10 kHz). Whereas for the 121 GW it is 0.23 dB (45-400 Hz) and 0.34 dB (400 Hz - 5 kHz).
But maybe I should wait with the purchase until the footnote for the 50 mV AC range is clarified.
J-R:
Accuracy specifications can be a poor metric in my opinion. You have some equipment that just barely squeaks by and other equipment that exceeds the spec by a factor of 10 in some cases.
Got a bit carried away and tested a pile of my DMMs with a 3312A signal generator. 100.0000mV sine wave at 1KHz fed through a Gertsch RT-5 ratio transformer for a final output of 1mV. (I picked those values because I have recent calibration data for the 34401A which states 100.0000mV is 99.9932mV at 1KHz.)
34401A -- 0.994mV
Fluke 287 -- 1.002mV
121GW #1 -- 0.989mV
121GW #2 -- 0.985mV
Fluke 87V -- 0.99mV
Fluke 88V -- 0.86mV
BM869s -- 0.99mV
BM789 -- 0.84mV
Agilent U1461A -- 1.010mV
I did not use relative mode on any of these. All devices were powered up for a couple hours at 67F and allowed to settle before taking a reading.
After some further tinkering, I did notice the Fluke 287 drops to 0 around 0.150mV and the 34401A drops to 0.100mV at 0.300mV and to 0 around 0.200mV.
Both 121GW units had no trouble showing something meaningful down that low, so 0.100mV was 0.085mV and 0.050mV was 0.033mV for example...
I also ran the same tests at 200Hz and found the results to be virtually identical.
If my calculations are correct, the Fluke 287 was 0.02dB high, and the worst 121GW was 0.13dB low.
dcac:
--- Quote from: Keyview on March 19, 2021, 11:53:27 pm ---
--- Quote from: dcac on March 19, 2021, 05:52:23 pm ---There might be a typo in the 121GW manual - the footnote (3) for 50mV DC probably also should apply to 50mV AC but here it refers to footnote (2)? which is not to be found.
--- End quote ---
Thank you for sharing your observation on the potential typo.
I now checked with the Fluke 289 manual: The requirement for rel mode and zeroing for 50 mV DC is mentioned there, too, but no requirement for rel mode for 50 mV AC.
So, the footnote 2 on page 17 of the 121GW manual is maybe not to be read as footnote 3.
--- End quote ---
Yes perhaps - the signal path for AC mV is quite different from DC mV and so is any possible offset that needs to be canceled out. I wonder what footnote (2) is about though. I found an older manual from 2017 that is different and with no footnote for AC mV.
Keyview:
--- Quote from: J-R on March 20, 2021, 05:42:15 am ---Got a bit carried away and tested a pile of my DMMs [...]
--- End quote ---
Thank you very much for your excellent analysis!
This convices me to order the 121GW.
If this performance is typical for a 121GW then it might be worth to explicitly add in the user manual that the specifications are valid down to 0% of range - for V AC and the other measurements.
Otherwise one might wonder whether stating a lower bound for the specifications has been forgotten.
Thank you for your and dcac's swift and competent feedback!
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