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A coherent detector operates by mixing the
incoming data signal with a locally generated carrier reference and selecting the
difference component from the mixer output. Representing the modulated data signal as
a(t) cos wct
and the reference carrier as
cos(wct + q)
, the mixer output becomes: a(t) cos wct cos(wct + q) = 0.5 a(t)cos(q) + 0.5 a(t)cos(2wct + q) If the carrier is phase coherent with the incoming modulated carrier signal (that is, there is no frequency or phase difference between them, q = 0o), then the output is proportional to a(t) and perfect detection is achieved. |
If q = 90o, however, then cos(90o) = 0
and no output is obtained! It is thus essential to ensure that the carrier
oscillator in the receive modem unit is in some way phase-locked
to the carrier oscillator in the transmitter modem. Although coherent detection appears much more complicated than non-coherent detection, it is able to recover the data signal more accurately in the presence of noise. |