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Raspberry Pi
Forum Does the Pi 4 power adapter choice influence analog HAT noise?
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  • Replies 7 replies
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  • RoadTest
  • raspberry
  • pi4
  • psu
  • mcc172
  • fft
  • noise
Related

Does the Pi 4 power adapter choice influence analog HAT noise?

Jan Cumps
Jan Cumps 1 month ago

shabaz noticed that he got different noise figures when using an analog HAT on a Pi4 - in a road test (look for section Measurement HAT Noise Performance) and here. I originally used a power brick for a Lenovo laptop. But I now have an official Pi 4 power brick. I'm going to use FFT software I made to see if the noise floor differs, or if I see spikes.
later addition: a third test with a linear lab supply.

I'm using a MCC172 DAQ HAT, and have terminated the 50 Ohm input with a matching resistor. Let's see if I can spot differences.

The dynamic range of the DAQ channel is 5 Vpp AC

All FFTs below are based on 20480 samples, 1024 samples per second. The FFT shows the results of (20480 Sa / 1024 Sa/s) 20 seconds worth of data.
The waveform display just shows the 1024 samples of the last second (because I took the screen shots at the end of the collection cycle, the waveform display updates real time and it fits 1024 data points), 

Waveform amplitude in V, X axis is sample count.
FFT amplitude in dB, X axis in Hz.

Test 1: with the official Pi 4 PSU

Test 2: with a Lenovo laptop brick

Test 3: with a Rigol linear lab power supply

Raw data analysis

Here is the same test again, now logging the raw samples to a spreadsheet (and attach them to this post. You can then do your own analysis).
The X axis, just like on the LabVIEW captures above, are the sample counts. Delta between each sample is 1/1024 s.

Pi4 PSU:

Lenovo brick:

Linear PSU:

The 3 methods next to each other, on the same scale:

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Top Replies

  • Jan Cumps
    Jan Cumps 1 month ago in reply to shabaz +1

    scan speed is 1024 points per second. The FFT uses 20480 -> 20 seconds of samples.

  • Jan Cumps
    Jan Cumps 1 month ago in reply to Jan Cumps +1

    (same number of samples as what's collected in the spreadsheets and shown in the graph at the end)

  • Jan Cumps
    Jan Cumps 1 month ago +1

    Comparison of the 3 tests on the same scale:

    These are the same samples captured before, now put in a single series. The vertical green bars below the X axis show where data from the next test begins.

  • Jan Cumps
    Jan Cumps 1 month ago

    How can I upload a csv or a zip with csv in it?

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  • shabaz
    shabaz 1 month ago in reply to Jan Cumps

    Hi Jan,

    I struggle to find where to upload stuff too, maybe it's a button on the page but often I've not found it, and resort to sticking the CSV etc on GitHub, not always ideal.

    Regarding the measurements, is the FFT using 1024 points? or 20480? I think it's just 1024, right? If so, if that can be increased, then the noise floor will be lowered, in case there is anything buried deep.

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  • Jan Cumps
    Jan Cumps 1 month ago in reply to shabaz

    scan speed is 1024 points per second. The FFT uses 20480 -> 20 seconds of samples.

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  • Jan Cumps
    Jan Cumps 1 month ago in reply to Jan Cumps

    (same number of samples as what's collected in the spreadsheets and shown in the graph at the end)

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  • Jan Cumps
    Jan Cumps 1 month ago

    I'm going to do another test, with a linear power supply.

    I created this small gizmo, connecting Pin 2 and 4 together for 5 V and 6 and 9 for ground. With a decent gauge cable.
    If needed, I'll put some decoupling over them, but I'll try without first.

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  • Jan Cumps
    Jan Cumps 1 month ago in reply to Jan Cumps

    Results with linear PSU, same setup, sample settings and quantities:

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  • Jan Cumps
    Jan Cumps 1 month ago

    Comparison of the 3 tests on the same scale:

    These are the same samples captured before, now put in a single series. The vertical green bars below the X axis show where data from the next test begins.

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