22 Chapter 4 Homework 1. For an AM DSBFC modulator with a carrier frequency f c = 200KHz and a maximum modulating signal frequency f m(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.
33 Homework Continued 2. For the AM wave form above determine:
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 V c = 20 V p, a load resistance R L = 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
99 Frequency Spectrum of An AM Double Sideband Full Carrier (DSBFC) Wave
10 Example 1 For an AM DSBFC modulator with a carrier frequency f c = 100KHz and a maximum modulating signal frequency f m(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.
12 Example 1 d. The Output Spectrum For An AM DSBFC Wave
13 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 )
56 Simplified Block Diagram of an AM Receiver 56
57 Simplified Block Diagram of an AM Receiver Receiver front end = RF section –Detecting the signal –Band-limiting the signal –Amplifying the Band-limited signa l 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
58 Noncoherent Tuned Radio Frequency Receiver Block Diagram 58
59 AM Superheterodyne Receiver Block Diagram 59
60 Bandwidth Improvement (BI) Noise reduction ratio BI = B RF / B IF Noise figure improvement NF IMP = 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 –NF Imp = 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 Aa 61
62 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