# RF mask (power spectrum measurement bandwidth)

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?.

• 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! – Spacey Apr 9 '13 at 17:54
• @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. – user4259 Apr 9 '13 at 18:19
• 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. – Spacey Apr 9 '13 at 18:30
• 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. – user2718 Apr 9 '13 at 21:03

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: