Amplitude Modulation
Coherent DemodulationCoherent Demodulation is accomplished by remodulating Ideal Coherent DemodulationLet
where
and
Let the LO output be
For AM-TC, the DC component of
Effect of LO OffsetNow let the LO output be
If Pilot Tone is the most common method to generate coherent carrier phase reference at the receiver. Non-coherent DemodulationThe simplest form of non-coherent demodulation is envelope detection. Envelope DetectionFor AM-TC, a technique that does not require a coherent carrier reference can be used if sufficient carrier power is transmitted. This method is called envelope detection. The envelope
which is unambiguous as long as An envelope detector produces an output Refer to Figure 3.3 page 148. InterferenceThe interference modeled as a sinusoid at a frequency slightly different than that of the carrier. The received signal is
Coherent Demodulation
DSB (AM-SC)
The signal and interference powers are
so that the signal-to-interference ratio is
where DSB-TC
The signal and interference powers are
so that the signal-to-interference ratio is
where Envelope DetectionThe received signal is expressed as
where
Clearly, the envelope is
This is exactly the result for coherent demodulation. Note that the approximation used are valid only when
which is the case when the signal power is much stronger than the interference power.
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using a local
oscillator (LO) which is at the same frequency and in phase with the original
carrier.
. Then
is removed to
yield
. Then
which
is a phase offset, then the effect is simple attenuation for DSB modulation and
phase distortion for SSB. If 
where 
is a slight frequency
offset, then distortion is introduced which is called ``warble'' for DSB. For
SSB, the spectrum of the information signal is translated in frequency from
baseband by 
Hz which sounds like the ``Donald Duck'' effect.
of the general signal 
is defined
mathematically as the magnitude of 
. In this case
.
when the input is 
and can be
realized using a simple diode with an RC network.
is the carrier-to-interference ratio.
is the carrier-to-interference ratio.
which may be
expressed as
