Name: _____________________________
Final Exam
Autumn 2005
Expect to receive full credit only for answers that are thoroughly substantiated.
1. (40) Assume an FM modulator with f d  8 Hz/V, a carrier 10 cos(1000t ) and the
message given by 10 cos( 20t ) . The modulated FM signal is passed through an
ideal band pass filter with center frequency 500 Hz, bandwidth 70 Hz.
a.) What is the peak frequency deviation?
b.) What is the phase deviation?
c.) What is the modulation index?
d.) What are the powers at the input and at the filter output?
e.) Draw to scale the single-sided spectrum of the signal at the filter output. Label
the amplitude and frequency axes.
2. (40) The signal 3 sin( 8000t ) is quantized and transmitted using a PCM system. If
each data sample at the receiving site must be know within 0.10 of the peak value,
a.) How many binary symbols must each coded word contain? Make sure to
choose a practical value.
b.) What is the bandwidth of the resulting PCM signal?
c.) Discuss the types of noise present in the receiving system
Noise from Quantization: One of the types of error introduced into an
analog audio signal by encoding it in digital form. The digital equivalent
of tape hiss, quantization noise is caused by the small differences between
the actual amplitudes of the points being sampled and the bit resolution of
the analog-to-digital converter.
Noise from the Channel (word error): This error is caused by the additive
probability of having an error in each bit.(p. 318-319)
d.) Can the S/N ratio always be improved by simply increasing the signal
(message) level? Why?
If “signal (message) level” is related to the value of PT (ask the professor),
then the following applies: Ver parrafo que dice “If the additive noise in
the channel……”.
3. (20) Answer the following questions.
a.) How many ways are there to improve SNR in an FM system? Discuss.
(Hint: Table 6.1).
Sec. 6.3.4 Improve output SNR Example 6.3
Preemphasis and deemphasis can be used to partially combat the effects of interference.
These techniques can also be used to great advantage when noise is present in angle
modulation systems
Summary Chap 6 #10
b.) Describe briefly the threshold effect in the case of interference in linear
modulation. Discuss separately the cases of coherent and envelope
Summary Chap 6 #4 -> coherent and envelope demodulation.