The Philosophy and Consequence of Measurement – Power

Fundamental Measurement Techniques: Power

Published: 2nd October 2014

Welcome to our new occasional series – ‘Fundamental measurement techniques’ beginning with Power.

We will look at the importance of power measurements and fundamental, basic types of power measurement as related to spectrum analysis i.e. Absolute, Average, Peak and Pulse power.

As with many fundamental measurements we recommend test equipment that provides accurate, repeatable, traceable and convenient measurement capability. A complete range of which are available from MCS Test Equipment.

Signal power is critical to performance at every level. In many case we regularly hear ourselves commenting to our mobile phones in our daily lives that there’s no signal, we just dropped out, can’t connect. We demand high quality and availability, all of which is dependent on power.

The measurement of power is made across design and manufacturing and the need to replicate those measurements at different times and places (from factory to field, twenty four hours a day; three hundred and sixty five days a year) is constant.

At DC and low frequencies voltage and current measurements are simple and power is easily calculated as:

Power = Current x Voltage or re-described using Ohm’s Law

Power = (Current squared) x Resistance

Power is defined as work done or energy transferred over time and the fundamental unit if power is the watt.

In electrical terms one watt is dissipated when a current of one ampere flows across a potential difference of one volt.

However at higher frequencies (approaching 1GHz) voltage and current measurements become more complex due to wave interaction (even in a lossless transmission line). Now, direct power is more easily measured and as a fundamental quantity. Absolute Power:- The comparison of transmitter and receiver power is a valuable measurement and to make them realistic and visible power it is displayed on a logarithmic or decibel scale. The decibel scale is dimensionless unless compared to a reference point so Absolute Power is ascribed as dBm or dB relative to 1 milliwatt

Average Power:- The instantaneous power of a modulated signal varies continually so, here a signal’s average power is most commonly measured. Test equipment for the measurement of average power is economical, intuitive and very importantly accurate and traceable to national and international standards.

Peak Power:- Peak power is a measure of the maximum signal power. This is measured by a peak power meter or similar functionality within a more complex measuring instrument.

Pulse Power:- For a pulsed RF signal such as with a radar system the fundamental measurement is that of the average power of the pulse divided by the pulse duty cycle- the pulse power. Various high quality instruments and sensors are available for this measurement. NB This technique is not applicable for digital modulation systems where the duty cycle is not constant and the pulse amplitude and shape is variable.

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