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The Philosophy and Consequence of Measurement – Power (part 4)


""‘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

And now we look into what the measurement of power in a spectrum tells us and what decisions it enables us as engineers.

The basics:- When a signal contains only one frequency (i.e. it is sinusoidal) determining that frequency, its amplitude and phase can be a straightforward process. However, real signals are usually not sinusoidal, contain many frequencies and may contain random elements. Determining the frequency content of such a signal requires more sophisticated methods.

Communication systems and data processing systems are required to handle an ever increasing number and variance of signals in the presence of and near to noise. The capacity and demand for more and more information as data, words and video is increasing daily.

To satiate this demand we must work nearer and nearer to limits once only dreamt of. Nearer to the noise floor and more within the presence of noise.

With that comes the demand and challenge of even more precise measurement and the need for sophisticated measurement tools.

Today’s basic spectrum analyser is awash with added functionality and the specification of high performance instrumentation is breathtaking. Just take a look at the range available here from MCS Test Equipment.

From cellular to cordless to wireless LAN (WLAN) systems, spectrum is a vital component in the system design process. Measuring power is important for circuit designers as well. Most communications systems fall into one of two technology categories:

1. Bandwidth Efficient, the ability of a modulation scheme to accommodate data within a limited bandwidth, or
2. Power Efficient, the ability of the system to reliably send information at the lowest practical power level.

For designers of some digital terrestrial microwave radios a high priority is good bandwidth efficiency with low bit-error-rate. They have plenty of power available and are not too concerned with power efficiency .

On the other hand, designers of mobile phones put a high priority on power efficiency because, quite simply, the phones run on a battery.

Every time one of these efficiency parameters (bandwidth or power) is increased, the other can decrease, or become more complex or less efficient.

The radio spectrum is very valuable and operators who do not use the spectrum efficiently lose out in a highly competitive market.

Through the demand for higher data security, better quality communications, and quicker system availability developers today face the constraints of available bandwidth, permissible power and inherent noise level of the system. The RF spectrum must be shared, yet every day there are more users for that spectrum as demand for communications services increases. Therefore the ‘simple’ measurement of power spectra is a problem of steadily increasing importance.

The idea of obtaining a spectrum from a measurement may therefore at times seem overwhelming, not least because signals in the natural world can contain infinitely many frequencies. However, such continuous signals can also be broken into infinitely many time steps and we can measure their behaviour in time by sampling them at regular intervals over some limited time.

In an exactly analogous way, measuring a spectrum is an exercise in sampling it at regular intervals in frequency over a limited frequency range. To understand how this comes about we need to consider the whole measurement process and have confidence in the instrumentation we are using and who we have bought it from.

Next we look at Performance as it relates to the instrumentation we are using and looking for.

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.