Page 204 - Fundamentals of Communications Systems

6.26 Chapter Six

−25 −20
−30
−35 −30
−40
−40
G x c (f), dB −50 G y H (f) −50
−45
−55
−60 −60
−65
−70
−70
−75 −80
−1−0.8−0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 −5000 −3000 −1000 0 1000 3000 5000
Frequency, f, Hz × 10 4 Frequency, f, Hz
(a) The transmitted spectrum (b) The spectrum of the highpass filter output
Figure 6.29 Signals in a SSB-AM transmitted reference system. f c = 7 kHz

is chosen to be about 200 Hz as this does not affect the audio quality of the signal.
The measured transmitted signal spectrum is shown in Figure 6.29(a). The transmit-
ted reference signal is the tone at the carrier frequency. The signal out of the notch
filter in the demodulator of Figure 6.28 is approximately a standard baseband SSB-AM
modulation. The measured power spectrum of y H (t) is shown in Figure 6.29(b) for a
notch bandwidth of 100 Hz. The transmitted reference tone has been greatly attenuated
but the modulated signal is roughly unaltered. The signal out of the low pass filter in
the demodulator of Figure 6.28 is approximately the phase shifted reference tone. The
measured power spectrum of y L (t) is shown in Figure 6.30(a) for a notch bandwidth of

−20 −90
−30 −100

−40 −110
G y L (f) −50 arg{Y L (t)}, degrees −120

−60 −130

−70 −140
−80 −150
−5000 −3000 −1000 0 1000 3000 5000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Frequency, f, Hz Time, t, sec
(a) The spectrum of the lowpass filter output (b) Measured phase of the demodulation
reference signal
Figure 6.30 Signals in a SSB-AM transmitted reference system.