[time-nuts] How to measure regulator noise?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jan 8 20:19:37 EST 2008

Don Collie wrote:
> Hi Bruce and Henk,
>     Your point about Bandwidth is a valid one, and as you say, it depends on 
> the application.
> The 400H has corner frequencies of about 10Hz, and 4Mhz, so the noise BW is 
> a bit wider than this [Y/N?]. This setup can`t measure noise at very low 
> Frequencies, or Frequencies much greater than the [nominal]
> 4Mhz, because the noise frequency components simply can`t get through.
> For my purposes the setup I described is adequate - I mainly want to compare 
> one regulator with another [although it would be nice to be able to do so 
> over the frequencies
> 0Hz to infinity]. I suggest that if a regulator proves itself quieter over a 
> restricted BW such as I described, it is *probably* quieter at other 
> frequencies too.
> Take that Bruce!  ;-)..............................Don.

If you have a PC sound card then it can be used with a suitable preamp
to measure regulator noise spectral density over the [20Hz, 20kHz ] range.
Except for switching regulators there's usually not much noise beyond
Often linear regulator output noise may peak somewhere within this range.
The location and the amplitude of the noise peak depends on the
regulator output current and the characteristics of the regulator output
The originator of this thread needs to know the regulator noise spectrum
detail in order to properly evaluate various regulators for use in low
phase noise equipment.
The high frequency noise of a regulator is not a reliable guide to its
flicker noise characteristics.
In low phase noise design guessing isnt good enough, actual measurements
are required for verification.
Whilst it is relatively easy to filter out high frequency power supply
noise, filtering out low frequency noise is much more difficult.

Single ended circuits (most RF circuits) have little or no inherent
power supply rejection and they are particularly susceptible to phase
noise generated by power supply noise modulating the RF circuit's phase


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