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The Philosophy and Consequence of Measurement – Power (part 3)


""‘When you cannot measure… your knowledge is of a meagre and unsatisfactory kind’. Lord Kelvin

We continue looking into the need and specifications for test equipment by looking at 6Ps (Precision, Power, Performance)- (Productivity, Portability, Package) and from Precision we continue our look at Power measurements which is basically how much oomph a system needs to carry the quantity as far and as for long as needed.

We split these in two (without a divider)

- Electrical Power Measurements which can be measured with some form of DMM or Oscilloscope; clamp; Wattmeter
- Spectral Power Measurements using a Power meter or Spectrum Analyser

So, in this article we’ll specifically look at Spectral Power or the Power Spectrum. How can our measurement device actually answer the question ‘How much of the signal is at a frequency of x?’ and therefore we are most likely to be using a Power Meter or Spectrum Analyser

We could also be looking at a variety of different signals

- Periodic signals which give peaks at a fundamental and its harmonics;
- Quasiperiodic signals which give peaks at linear combinations of two or more
- Irrationally related frequencies (often giving the appearance of a main sequence and sidebands)
- Chaotic dynamics which give broad band components to the spectrum.

These however are all statements about the ideal power spectrum, if infinitely long sequences of continuous data are available to process. In practice there are always limitations of restricted data length and sampling frequency, and it is important to investigate how these limitations affect the appearance of the power spectrum.

So, in essence we’d be looking to measure the averaged power spectrum of an input signal and in practise using various averaging modes such as RMS averaging, vector averaging, or peak hold, as well as the number of averages. You would then be able to observe the influence of these averaging parameters, typically on the noise floor, and notice that vector averaging requires the use of a trigger in order to lower the noise floor without lowering the fundamental along with it.

To assist us in that quest we’d be looking for an accurate measurement device with the correct range and the correct functionalities for us as engineers to apply our skills to hone in on the signal in question and best describe it to make informed decisions.

What the measurement of power in a spectrum tells us and what decisions it enables us as engineers to make we’ll discuss in our next article.

Please leave your comments and inputs. These articles are solely an opinion on the measurements we are interested in.

Rejoice in making your measurements. Demand more in measurement and enjoy this series.