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Understanding External Power Supply Requirements

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EE_Rob

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Hello, I have never really seen power supply requirements list like this before. Could someone please try to explain what the at 1MHz in 50kHz bandwidth means with respect to the design of a power supply? Are these requirements for noise at these specific frequencies, and therefore alluding to the fact that I need some filtering?

Thank you.

The external power supply needs to guarantee the following:

The supply noise should be less than the limits in the following profile:
  • 100 mVpp from 0 to 50 kHz
  • 5 mVpp at 1 MHz measured in 50 kHz bandwidth
  • 10 mVpp at 1 MHz measured in 1MHz bandwidth
  • 5 mVpp above 5 MHz measured in 1 MHz bandwidth
 
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These are SPECTRUM ANALYZER setups.

Are you familiar with an spectrum analyzer?
Such that I can explain. Otherwise it would not make sense to you.
 

Yes I am familiar with a spectrum analyzer, please continue.
 

Its interesting that that the noise requirements are peak to peak values, not rms.

So they are very concerned about switching spikes and high frequencies, not just low frequency ac ripple.

Many switching supplies quote rms noise which can completely disguise the presence of very narrow high amplitude switching spikes, so there is a reason....
 

5 mVpp at 1 MHz measured in 50 kHz bandwidth
10 mVpp at 1 MHz measured in 1MHz bandwidth
5 mVpp above 5 MHz measured in 1 MHz bandwidth

What I can understand here is that (for the second requirement) you set a frequency span of 1 Mhz with a resolution bandwidth of 50 Khz, set the vertical display in linear mode and measure the maximum peak to be 5 millivolt. That is my understanding, I would ask for a clarification.

Similarly for the third and fourth requirements. Just change the span and res BW.

For the first requirement, I am really not sure. Although I suspect that it could be perhaps be measured with an oscilloscope preceded by a low pass filter.
 

I'm not sure whether those "bandwidths" are referring to the RBW or the total frequency span for the measurement. For the first two, the frequency span makes sense, but not for the third one.

And a SA can't tell you the peak to peak voltage (unless the signal is just a pure sinewave, not the case here). In practice, applying a filter to a signal can change its peak to peak voltage through the phase response, even if the magnitude response is very flat. So you could get differing results depending on how you limit the measurement bandwidth.
 

Ok thank you all for the help. I will speak with the vendor and try to get some clarification.
 

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