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DIY Audio Analyzer with AK5397/AK5394A and AK4490
DIY Audio Analyzer with AK5397/AK5394A and AK4490
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Old 19th November 2018, 09:54 PM   #2481
JensH is offline JensH  Denmark
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Join Date: Jul 2009
DIY Audio Analyzer with AK5397/AK5394A and AK4490
I agree that the noise looks suspiciously low. It is difficult to read the exact value, but in the flat region it could be something like -177dBV/sqrt(Hz). This is equivalent to 1.41 nV/rtHz. The input noise is low, but not quite that low. I would expect something like 4 nV/rtHz.

@PaulBC
Can you provide some details of the test setup, like cable connections, terminations etc.?
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Old Yesterday, 01:37 PM   #2482
PaulBC is online now PaulBC  Taiwan
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@JensH, you are right. It was just a simple stupid mistake. I thought the unit of the input gain setting in ARTA is dB, therefore I set it as "20". However, it is a 20X gain, not a 20dB gain (i.e., 10X gain). That is why the noise floor is only half of the expected value. My apologies.

The key I posted this screenshot is to show there is no 60Hz harmonics under the -20dBV input configuration if the transformer is moved away, not the noise level. It is just a shorted-input measurement. However, even a shorted-input measurement, these 60Hz harmonics are also there under the -10dBV and -20dBV input configurations with the internal transformer. Therefore, moving the transformer away benefits not only single-ended but also low-level balanced measurements.

I attached three screenshots:
1. noise floor with a correct gain setting (-20dBV, shorted input).
2. single-ended measurement with 0dBV input and output.
3. single-ended measurement with -20dBV input and output.

For 2 and 3, I used zfe's configuration and the results are consistent with zfe's results except the odd harmonics of 60Hz. It shows that moving the transformer much further away just reduces the odd harmonics of 60Hz with a single-ended measurement. To suppress the even harmonics of 60Hz, some additional efforts have to be made (e.g., battery or SMPS).
Attached Images
File Type: png RTX6001_PSD_BAL_-20dBV.PNG (79.2 KB, 92 views)
File Type: png RTX6001_THD_SE_0dBV.PNG (82.3 KB, 89 views)
File Type: png RTX6001_THD_SE_-20dBV.PNG (82.0 KB, 89 views)
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Old Yesterday, 01:45 PM   #2483
zfe is offline zfe
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I would qualify a shorted input as balanced measurement.
For the unbalanced noise floor I would go for the setup of a unbalanced loop back measurement with no signal playing.
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Old Yesterday, 02:13 PM   #2484
PaulBC is online now PaulBC  Taiwan
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Join Date: Aug 2009
@zfe, as you wish.
Attached Images
File Type: png RTX6001_PSD_SE_0dBV.PNG (80.0 KB, 75 views)
File Type: png RTX6001_PSD_SE_-20dBV.PNG (80.3 KB, 73 views)
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Old Yesterday, 04:28 PM   #2485
zfe is offline zfe
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Join Date: Dec 2014
Thanks.
So yes, with the transformer far away it looks much cleaner.
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Old Yesterday, 04:48 PM   #2486
NicMac is offline NicMac  Italy
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DIY Audio Analyzer with AK5397/AK5394A and AK4490
I wonder how much better the R-core transformer from the original plans would be.
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Old Yesterday, 07:39 PM   #2487
zfe is offline zfe
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Join Date: Dec 2014
There is also a lot of pickup of environmental noise possible.
Here two unbalanced loop-back noise floor measurements, -20dbV in, on battery power (!), no line connected, in a noisier environment.
The setup is: out to 1.5m RG58 BNC cable to BNC-XLR adapter at input.

First the BNC cable is routed as a big loop
bat nf loop.jpeg
Second the BNC cabel is routed such that it encloses only minimal surface.
bat nf closed.jpeg
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