eeng 3810 chapter 4
DESCRIPTION
Basic Communication theoryTRANSCRIPT
22
Chapter 4 Homework
1. For an AM DSBFC modulator with a carrier frequency
fc = 200KHz and a maximum modulating signal frequency fm(max) = 10 KHz, determine :
a. Frequency limits for the upper and lower sidebands.
b. Bandwidth.
b. Upper and lower side frequencies produced when the modulating signal is a single-frequency 6 KHz tone.
55
Homework Continued
4. Repeat steps (a) through (d) in Example 4 in these lecture slides for a modulation coefficient of 0.5.
5. For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 20 Vp, a load resistance RL = 20 , and a modulation coefficient m = 0.8, determine the power of the modulated wave
Homework Continued
6.Determine the noise improvement for a receiver with an RF bandwidth equal to 100 KHz and an IF bandwidth equal to 20 KHz.
6
1010
Example 1
For an AM DSBFC modulator with a carrier frequency
fc = 100KHz and a maximum modulating signal frequency fm(max) = 5 KHz, determine :
a. Frequency limits for the upper and lower sidebands.
b. Bandwidth.
c. Upper and lower side frequencies produced when the
modulating signal is a single-frequency 3 KHz tone.
1313
Phasor addition in an AM DSBFC envelope
• For a single-frequency modulating signal, am AM envelop is produced from the vector addition of the carrier and upper and lower side frequencies. Phasors of the carrier,
• The upper and lower frequencies combine and produce a resultant component that combines with the carrier component.
• Phasors for the carrier, upper and lower frequencies all rotate in the counterclockwise direction.
• The upper sideband frequency rotates faster than the carrier. (usf > c)
• The lower sideband frequency rotes slower than the carrier. (usf < c)
1616
If the modulating signal is pure, single frequency sine wave and the modulation process is symmetrical, the % modulation can be derived as follows:
1717
Peak Amplitudes of Upper and Lower Sidebands
The peak change in amplitude of the output wave (Em) is equal to the sum of the voltages from the
upper and lower sideband frequencies. Therefore,
1818
Percent Modulation of An AM DSBFC Envelope (a) modulating signal; (b) unmodulated carrier; (c) 50% modulated wave;
(d) 100% modulated wave
2222
Generation of an AM DSBFC Envelope Shown in The Time Domain
sin(225t)
–½ cos(230t)
+ ½ cos(220t)
summation of (a), (b), and (c)
42
Single-side Band Full Carrier (SSBFC)
The carrier is transmitted at full power and only one sideband is
transmitted.
44
Single-Sideband Suppressed Carrier (SSBSC)
The carrier is suppressed 100% and one sideband is removed. Only one
sideband is transmitted.
46
Single-Sideband Reduced Carrier(SSBRC)
One sideband is removed and the carrier voltage is reduced to 10%
of its un-modulated amplitude.
47
Independent Sideband(ISB)
A single carrier is independently modulated by two different modulating signals.
49
Vestigial Sideband(VSB)
The carrier and one complete sideband are transmitted, but only part of the
other sideband is transmitted.
57
Simplified Block Diagram of an AM Receiver
• Receiver front end = RF section– Detecting the signal– Band-limiting the signal
– Amplifying the Band-limited signal• Mixer/converter
– Down converts the RF signal to an IF signal
• Intermediate frequency (IF) signal– Amplification– Selectivity
• Ability of a receiver to accept assigned frequency
• Ability of a receiver to reject other frequencies
• AM detector demodulates the IF signal to the original signal
• Audio section amplifies the recovered signal.
57
60
Bandwidth Improvement (BI)
• Noise reduction ratio
• BI = BRF / BIF
• Noise figure improvement
• NFIMP = 10 log BI
• Determine the noise improvement for a receiver with an RF bandwidth equal to 200 KHz and an IF bandwidth equal to 10 KHz.– BI = 200 KHz / 10 KHZ = 20
– NFImp = 10 log 20 = 13 dB
60
61
Sensitivity
• Sensitivity: minimum RF signal level that the receiver can detect at the RF input.
• AM broadcast receivers– 10 dB signal to noise ratio– ½ watt (27 dBm) of power at the audio output– 50 uV Sensitivity
• Microwave receivers– 40 dB signal to noise ratio– 5 mw (7 dBm) of power at the output
• Aa61
62
Dynamic Range
• Dynamic Range– Difference in dB between the minimum input level and
the level that will over drive the receiver (produce distortion).
– Input power range that the receiver is useful.– 100 dB is about the highest posible.
• Low Dynamic Range– Causes desensitizing of the RF amplifiers– Results in sever inter-modulation distortion of weaker
signals
62
63
Fidelity
• Ability to produce an exact replica of the original signal.• Forms of distortion
– Amplitude• Results from non-uniform gain in amplifiers and filters.• Output signal differs from the original signal
– Frequency: frequencies are in the output that were not in the orginal signal
– Phase• Not important for voice transmission
• Devastating for digital transmission
63