Re: [SI-LIST] : EMI

Neven Orhanovic (neven@icx.com)
Fri, 31 Oct 1997 13:52:48 -0800 (PST)

Derek Walton makes a good point. By modulating the original signal
we get a phase modulated (or frequency modulated) signal with
the same power replacing the original CW signal. This is like
comparing the output of two transmitters of the same power that
use different modulation techniques (e.g., one produces wide
band FM and the other unmodulated or "slightly" modulated CW).
The WFM signal will have a smaller signal amplitude in the frequency
domain but weather it will generate less interference depends on
the demodulating circuit (victim). In the amateur radio world,
an FM signal transmitted in the HF band (3-30 Mhz) will usually
generate more interference on nearby receivers than its equal power
CW counterpart (good old telegraphy signal). This is one of
the reasons why FM is only allowed above 28 MHz in the amateur
radio HF bands. Narrow band signals are also much easier to get
rid of (notch filter) than their wide band cousins.

I would expect the same to be true for the typical 800 MHz band
FM cellular phone. It should be less affected by a 800-and-something
MHz CW signal that will not make it through its phase discriminator
than by the same power FM signal that it can demodulate.
(We will know for sure once the clock frequencies of home PCs get
into the 800 MHz cellular phone range. :-) )

In my opinion, both radiated power density and total radiated power
within given frequency bands should be considered when evaluating
how much a device pollutes the RF environment.

Neven Orhanovic, AB7CU
Interconnectix