Jeff Liebermann wrote:
> Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> hath wroth:
>
> >Nope. Quiz time:
> >
> >You have a spectrum analyzer that shows an 802.11g spread spectrum
> >signal that is approximately 25Mhz wide. Peak power on the scale is
> >0dBm to make the calculations easy. The spectrum analyzer IF
> >bandwidth is 500KHz. What is the "real" power (ignoring duty cycle)?
> >
> >I'll supply the answer tomorrow. Note that this is not a trick
> >question and the numbers are quite real.
>
> Not even a guess? Oh well. This is a good example of why you can't
> directly use a spectrum analyzer in place of a power meter or
> bolometer with wideband (SS, video, UWB, etc) signals.
>
> The indicated power level on the display is 0dBm. However, the power
> is splattered over a bandwidth of 25Mhz. We want the equivalent
> carrier power, or the power of the de-spread signal. So we add:
> 10 * log(25MHz/0.5MHz) = 10 * log(50) = 17dB
> So, the real power is:
> 0dBm + 17dB = +17dBm
>
> Even this is not exact because the power spectra is not equal power
> across the 25Mhz bandwidth and the IF filter is not a brick wall at
> 500KHz. There's also quite a bit of power outside of the 25MHz
> window. The rms (heating) power is also different because of the
> transmit duty cycle.
>
> --
> Jeff Liebermann jeffl@comix.santa-cruz.ca.us
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

This depends on the "intelligence" of your spectrum analyzer. For the
audio spectrum analyzer I have, you just push a button and it computes
the energy in the bandwidth since it has all the information available
to do such a computation. I don't know if there are RF spectrum
analyzers with such intelligence, but I don't see why not.

In datacom (again, back to voice band communications), some modems used
guard tones to indicate the line contained data, not voice. The CCITT
specification is based on energy, so making such measurements (energy
in a bandwidth) is a desirable feature. That is, the tone had to be a
certain value relative to the data energy, which was spread out.