I'm reading an IEEE standard and it says that the first channel needs to be 50dB below the highest power in a 6 MHz operating channel in 100 KHz bandwidth. It was mentioned several times that the power spectrum measurement needs to be done over a 100 KHz bandwidth. Isn't it sufficient to just say that the power of the first channel should be 50dB below the highest power. I guess my question is what is the significance of the measurement bandwidth, such as the 100 KHz. Why do we need to specify a measurement bandwidth?.
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$\begingroup$ Bandwidth and SNR in one sentence? Might as well be an episode of the twilight zone. I'll be glad to see the answers to this one! $\endgroup$– SpaceyApr 9, 2013 at 17:54
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$\begingroup$ @Mohammad Where is the SNR here?. Also Bandwidth, SNR and Eb/No are all related to each other, so I don't get the "Bandwidth and SNR in one sentence?", Can you please elaborate. $\endgroup$– user4259Apr 9, 2013 at 18:19
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$\begingroup$ There are many ways of measuring SNR/BW, and more often than not, those metrics get called the same thing. Its a good question. I was just making a general comment. $\endgroup$– SpaceyApr 9, 2013 at 18:30
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1$\begingroup$ What are the characteristics of the signal for which the power will be measured? Most modulated signals don't confine themselves to a perfectly carved out region of frequency, so the signal power will increase with a measurement made with a wider bandwidth. You may get away with a lax statement that just gives relative power (this depends on the characteristics of the signal), but it is safer to specifically state how the power will be measured. IEEE is an engineering group. Engineering is all about repeatability. $\endgroup$– user2718Apr 9, 2013 at 21:03
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1 Answer
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The simple answer: when you take a power measurement using some piece of test equipment (such as a spectrum analyzer or an RF power meter), the reported value indicates the amount of power measured within some detection bandwidth of the instrument. For some devices, that might be a fixed value, determined by the filtering present in its RF front end. Other types of equipment, like a spectrum analyzer, will typically have adjustable bandwidth (which in this case would usually be called the instrument's resolution bandwidth).
To see how the bandwidth setting affects the measurement, try injecting a modulated signal into a spectrum analyzer and see how the power measurements vary as you change the resolution bandwidth: you can get very different answers. Take a common $\frac{\sin{x}}{x}$ spectral envelope as an example:
If you use a very small measurement bandwidth, then you're taking power measurements over very small swaths of frequency. This allows you to observe the detail in the spectrum's shape.
As you increase the resolution bandwidth, your instrument will take more and more of the signal's spectrum into account for the power measurement, and the reported values will become more of an overall measure of the total received power.
As was indicated in the comments already, the specification calls out a specific bandwith to ensure repeatability. Specifications are used to validate implementations as conforming to an existing standard. This particular scheme for measuring the emission mask of a transmitter implementation is likely in use because that was the one that was chosen when the standard was designed. Due to the imprecise definitions of concepts like SNR and bandwidth, there are many ways that you could define such a requirement, but this is the one that the authors chose. That doesn't make it right or wrong; it just is.
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$\begingroup$ I just saw your response, thanks for the answer. $\endgroup$– user4259Apr 14, 2013 at 20:47