For the purposes of noise calculations, both the VTF and the
Heterodyne receiver systems are equivalent to the common
feature which they share: A bandpass filter followed by a
demodulator. This is represented by the ideal system shown in
Figure 
. The band-pass filter is used to isolate
the desired signal and is typically called the ``predetection filter.''
The predetection filter precedes the demodulator and needs to have a
bandwidth wide enough to pass the desired signal without causing
distortion but as narrow as possible to reduce the noise passed to
the demodulator.
The output of the predetection filter, 
, is a combination of the
modulated carrier and narrow band noise:
where 
is the channel attenuation and 
is narrow band
white Gaussian random process with power spectral density
where B is the bandwidth of the predetection filter.
Figure: Receiver System Model for Noise Calculations