What is the unit of power spectrum for a voltage signal?

onemilimeter · Aug 26, 2009

I've an Agilent 35670A Dynamic Signal Analyzer. A signal is attached to Channel-1. The unit of the signal measured by the analyzer is as follows:

Time Capture (time domain): Volt (V)
Linear Spectrum (frequency domain): Volt-RMS (Vrms)
Power Spectrum (frequency domain): Volt-RMS (Vrms)

However, some online articles, for example the following:

http://zone.ni.com/reference/en-XX/help/371361D-01/lvanls/power_spectrum/

... show that for a voltage signal, the unit of the power spectrum of the signal is Volt-RMS squared (Vrms^2).

Which one is correct?

Thanks.

zappedagain · Aug 26, 2009

Power (watts) is proportional to voltage squared. Your meter is doing the conversion for you. While the proper unit is watts/Hz, or volt^2/Hz, your meter is translating it for you (to make it easier). There may be an option somewhere to change the units displayed.

This confusion came up long ago with photodetectors when looking at noise performance. Photodetectors convert convert optical power to current directly, so this is a square law conversion. To keep the relationship linear the wrong units are typically displayed.

John D

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

What is the unit of power spectrum for a voltage signal?

onemilimeter

Electrical

Recommended for you

zappedagain

Electrical

Similar threads

Part and Inventory Search

Sponsor