Page 208 - Fundamentals of Communications Systems

6.30 Chapter Six

1.5

1

0.5

x c (t) 0

−0.5

−1

−1.5
0 0.5 1 1.5 2
Time ∗ f m
Figure 6.32 A LC-AM signal with P LC = 0.5. Sinusoidal signal.

Problem 6.7. The Federal Communications Commission (FCC) makes each
station strictly limit their output frequency content. Assume AM stations pro-
duce a usable audio bandwidth of 10 kHz. AM stations in the same geographical
area are normally spaced at least 30 kHz apart. As an example of why limits
on the frequency content are necessary the following problem is posed.
Consider two stations broadcasting in the same geographic area with a
receiver tuned to one of them as shown in Figure 6.33. Station A is broad-
casting a 1 V peak sinewave of 2 kHz, m A(t) (a test of the emergency broadcast

x (t)
A
Station A

Σ r (t) BPF I/Q Down- |Σ| m (t)
c
ˆ
A
converter
x (t) f
Station B B c

Figure 6.33 A model for two transmitting AM radio stations for Problem 6.7.