Actions
Forum Thread Details
  • State Not Answered
  • Replies 21 replies
  • Subscribers 2561 subscribers
  • Views 2669 views
  • Users 0 members are here
Related

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.

Parents
  • 0

    Lots of good tips here. Thanks

    I just tried some measurements of "nothing" (i.e. a shorted input) in my garage and got a better reading.of 36mV peak-to-peak and 8mV RMS. I was about to try an extension cord down the end of the garden but it started raining heavily.

    It does seem that my house has become very electrically noisy. I suspect it's the outlets with the built-in USB power that I've fitted. They're convenient but the SMPS supply inside is unlikely  to be the best. I had one near my bed that would just be audible when my phone had finished charging and the current draw was very low. I thought I was imagining the noise that used to bother me at night for a while.

Reply
  • 0

    Lots of good tips here. Thanks

    I just tried some measurements of "nothing" (i.e. a shorted input) in my garage and got a better reading.of 36mV peak-to-peak and 8mV RMS. I was about to try an extension cord down the end of the garden but it started raining heavily.

    It does seem that my house has become very electrically noisy. I suspect it's the outlets with the built-in USB power that I've fitted. They're convenient but the SMPS supply inside is unlikely  to be the best. I had one near my bed that would just be audible when my phone had finished charging and the current draw was very low. I thought I was imagining the noise that used to bother me at night for a while.

Children
  • 0 in reply to Fred27

    USB outlets. I've always considered them very practical but I assumed they could introduce some noise into the network so I've not used one! Removing them would be a good test.

    A shorted input should measure a lot closer to zero. I don't know if your scope has a math function that can operate against a reference, but *maybe* you could try recording a reference waveform with the shorted input and then subtract it from the one at the power supply terminals. Won't be perfect but in "average" could give you a closer figure.