
Radio-frequenci; circuits 187
the modulation index is very large compared with
unity, there are a large number of significant
sidebands and these occupy a bandwidth virtually
identical to i.e. the bandwidth over which the
signal sweeps. The usual approximation for the
bandwidth of an FM signal is BW =
2(/; ,
+
/,,) .
You can see in Figure 8.1b that the vectors
representing the two sidebands of an AM signal
are always symmetrically disposed about the
vector representing the carrier. As they rotate at
the same rate but in opposite directions, their
resultant is always directly adding to or reducing
the length of the carrier vector. The secon