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