Page 35 - Analog and Digital Filter Design
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32 Analog and Digital Filter Design
MODULATING SIGNAL
4! 21 I
-2
TI ME
CARRI ER SI GNAL
w 21
AMPLITUDE MODULATED
CARRl ER
W 21 1
n A I
TI ME
Figure 1.12 (a-c)
Multiplying Signals in the Time Domain
In the frequency domain, multiplying one signal by another (known as mixing in
radio frequency design terms) causes frequency shifting. Suppose the two signals
cos A and cos B, described above, are cos (colt) and cos (ozt). Each of these signals
has energy and produce lines on the spectrum analyzer display at a single fre-
quency, o, and w2. When mixed together there are two new signals produced with
energy at new frequencies, which are the sum and difference frequencies given by
+
ol w2 and o, - wz. An example of this is shown in Figure 1.13.
