On 1 May 2006 06:47:45 -0700, "electro"
wrote:

under unlicensed 2.4GHz FCC part 15 for digital modulation
system(non-spread spectrum), it says that the
1. maximum conducted power is 1W
2. the spectral density be not greater than 8dB for every 3khz band
during continous transmission
3. the 6-dB bandwith is at least 500khz

question:
1. what is spectral density in this sense and its significance?
2. how can i measure my spectral density?
3. how does 1, 2 and 3 relate to each other?

This is just a guess, but let's see what we can do to make some sense
out of this.

If you were hoping that the 1 watt of power would be evenly
distributed across the 3 kHz bandwidth, that would be for the purpose
of making sure that no one would be putting all of his 1 watt into a
single frequency, which would be quite a strong concentration of power
in comparison to having it spread out over the 3 kHz. The idea being
that the power when concentrated is very noticeable and might cause
QRM, whereas the same wattage spread out evenly over 3 kHz would
barely exceed the background noise and would not be noticeable.

So, you might want to require that signals not be very "peaky", or
concentrated. In order to put limitations on "peakiness", you might
require it to be "at least so wide" and "no higher than x" in units of
spectral density.

Assume that in the above requirement that the 500 Hz bandwidth
requirement is to be interpreted as their requirement that avoids the
signal being concentrated in a narrow band of frequencies. Also assume
that the maximum power density in that 500 Hz bandwidth should be
commensurate with the entire 1 watt being distributed over only 500 Hz
instead of 3 kHz. Notice how we have traded off the requirements so
that they are being applied to 1/6th of the 3 kHz. If we are allowed
to concentrate all of our power in 1/6th of the bandwidth, then it
follows that we would be required to keep the power density in that
bandwidth at no higher than 6X the level it would have when spread out
over the entire 3 kHz.

I find that it is probably not a coincidence that the 6:1 ratio is
almost exactly 8 dB in power ratio.