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Difficulty getting good ripple / noise measurements

Fred27

Im currently roadtesting the Rohde & Schwarz NGC103 and as part of my investigation I'm trying to measure the ripple / noise and compare it to other supplies. The problem I'm having is that I don't seem to be able to get a reasonable measurement. I get the feeling that isn't swamped by external interference.

I watched a helpful EEVBlog video and tried to optimise my setup but I don't seem to be able to get close to any meaningful measurement. In fact, there's not much different if I just connect the two banana plugs together and measure the noise on my 0V there!

Here's a photo of the best setup I've managed so far. I tried differential measurement with one probe on + and one on - and measuring the difference. Whilst I could see the common mode noise on both channes that were being cancelled out, the result was still noisier than this. I have AC coupling, a 20Mhz bandwidth limit and have tried via a 10:1 probe, a 1:1 probe, and probeless direct via banana to BNC as pictured.

Noise measurement setup

Noise measurement screenshot

Any suggestions? In fact, is measuring noise in a setup like this even useful? Would you just trust the datasheet on this? For reference, the datasheet says < 6mV peak-to-peak and < 1mV RMS. I'm getting over an order of magniture more.

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  • 0 in reply to anniel747

    That certainly helped get a better picture of what was going on. A spectragram revealed some intermittent noise around the ISM band here in the UK - 433MHz and just under 700MHz. I think that's to be expected. However, the big reveal was a lot of noise around 20MHz that disappeared when I went out into the garden. I suspect this is the USB power built in to my wall outlets. I also remembered that I was using an old-ish Ethernet over poweline adapter but that didn't seem to be causing any problems.

    However I still have a fair amount of noise at lower frequencies. The FFT says arounf 76kHz but I think that'll be a rough figure. I'm still not sure what this is, but it's still swamping any readings I'm getting from the PSU.

    image

  • 0 in reply to Fred27

    A 20 MHz peak seems to be high for switch mode supply harmonics. Maybe an artifact generated by a PC?

  • 0 in reply to Jan Cumps

    When I was testing this:  Building a Low-Cost Low-Noise Negative- or Positive-Rail Power Supply 

    I was getting ballpark 40 MHz noise coming from somewhere. I never managed to get to the bottom of it : (

    In my case I could power from a battery and put it all in a metal box, but I don't have any ideas for when the DUT is the power supply itself : ( maybe an isolation transformer powering the supply could partially help if that noise is coming from the mains, but maybe it wont have much improvement : ( Or, a mains filter (example one is mentioned here:  Building a Mains Breakout Box ) . Or, add a load of ferrite beads to the output wires of the power supply (using coax). I don't think it's unreasonable to use the ferrite on the supply output cables for the roadtest, I think that's a very valid way to use the power supply. And any customer buying the R&S supply for the low noise, would love to know which ferrites/how many of them, and how to wire up to the circuit under test, using whatever cable  decides on and measures with. Would be super-useful to all I reckon, even those with other power supplies.

    Regarding ferrites any of these might be good: farnell order codes 1838839 or 1651719  or  1703107, or even fatter ones (and pass the wire through a couple of times at least, and maybe use many of them, e.g. three or four at least.

  • 0 in reply to Jan Cumps

    n.b.: industry convention for noise measurements is 20 MHz. Many oscilloscopes have a "high-bandwidth/20 MHz" switch because of that. 

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