EE470 Electronic Communication Theory Exam II Open text, closed notes. For partial credit, you must show all formulas in symbolic form and you must work neatly!!! Date: November 6, 2013 Name: 1. [16%] The superheterodyne receiver shown in Figure PR-1 is to be used to receive frequency modulated signals with carrier frequencies between 96 MHz and 104 MHz. The carrier frequencies are spaced 200 khz apart. The IF frequency is 10 MHz. (a) [8%] Assuming high-side conversion is used, what is the range of local oscillator frequencies in order to receive signals between 96 MHz and 104 MHz? (min) (max) (b) [8%] When tuning in a station at 100 MHz what is the local oscillator frequency? Where is the image located? f IMAGE Antenna RF Mixer f IF IF FM Demod Baseband x(t) LO Figure PR-1: Superheterodyne FM Receiver EE470 - Richardson 1 of 5 Fall 2013
2. [16%] The 40 MHz lower sideband signal shown in Figure PR-2(a) is to be converted to a 60 MHz upper sideband signal shown in Figure PR-2(b) using the system shown in Figure PR-2(c). (a) [8%] Sketch the spectrum of w(t) (the output of the mixer). Indicate the value of all significant frequencies and amplitudes on the graph. (b) [8%] What is the minimum BW and the corresponding value of f o (the bandwidth and center frequency of the band pass filter) in order to generate the desired upper sideband signal? 10 X(f) 40 30 (a) 30 40 f (MHz) Y(f) 5 70 60 (b) 60 70 f (MHz) x(t) w(t) BPF y(t) f o, BW 100 MHz (c) Figure PR-2: Sideband Conversion EE470 - Richardson 2 of 5 Fall 2013
3. [24%] An FM transmission system transmits at a carrier frequency of 90 MHz. The modulating signal has a bandwidth of W 15 khz. The frequency deviation of the transmitted signal is f 60 khz. The indirect FM system shown in Figure PR-3 is used to generate the transmitted signal. The narrowband FM (NBFM) component uses a carrier frequency of 1 200 khz and has a maximum frequency deviation of 100 Hz. (The NBFM frequency deviation is adjustable.) (a) [4%] What is the deviation ratio, D, of the transmitted signal y(t)? (b) [5%] Use Carson's rule to estimate the bandwidth of the transmitted signal y(t). (c) [5%] How many frequency doublers are needed in the multiplier stage? (d) [4%] To what value should the frequency deviation of the NBFM component be adjusted? (e) [6%] What value of is required? Narrowband frequency modulator x(t) 1 Phase Frequency T w(t) v(t) y(t) Modulator Multipliers 1 Figure PR-3: Indirect FM Generation System EE470 - Richardson 3 of 5 Fall 2013
4. [24%] The frequency synthesizer shown in Figure PR-4 (below) uses phase-locked loops (PLL) and frequency dividers to generate all local oscillator frequencies ( ) that are needed for a superheterodyne receiver. (The local oscillator frequency changes between frequencies f 7 and f 8 as the receiver is tuned.) Find the unknown frequency divider values (n) and frequencies (f) by completing the table below. Note that the system uses only one variable frequency divider (in the lower PLL). The divider in this PLL varies between n 4 and n 5 as the receiver is tuned. The output frequency of this PLL varies between f 4 and f 5. The synthesizer uses a 120 MHz master oscillator. Notice that the synthesizer uses two filters a low pass filter and a high pass filter. You may assume that the LPF eliminates all frequencies above its value while the HPF eliminates all frequencies below its value. Frequency Frequency Value Divider Divider Value f M 120 MHz f 1 n 1 20 f 2 500 khz n 2 f 3 90 MHz n 3 f 4 n 4 2 f 5 10 MHz n 5 f 6 f 7 f 8 f 7 f 8 ~ f M LPF f 6 HPF n 1 f 1 PD VCO f 3 120 MHz 30 MHz n 3 n 2 f 2 PD VCO f x f 4 f x f 5 n x n 4 n x n 5 Figure PR-4: Local Oscillator for Superheterodyne Receiver EE470 - Richardson 4 of 5 Fall 2013
5. [20%] Answer the following questions by circling the correct answer. (a) [2%] v i (t) and v q (t) are known as the components of a bandpass signal. i. in-phase and quadrature ii. envelope and phase (b) [2%] Standard AM can be demodulated using a simple detector. i. phase ii. envelope (c) [2%] Frequency translation is accomplished by a signal with a sinusoid (in the time domain). i. convolving ii. multiplying (d) [2%] A frequency multiplier does not increase the frequency deviation of an FM signal. (e) [2%] Indirect FM generators use to increase the frequency deviation of an FM signal. i. multipliers ii. mixers iii. differentiators (f) [2%] Mixing (heterodyning) does not change the frequency deviation of an FM signal. (g) [2%] An interfering sinusoid with frequency near the carrier frequency will cause interference in AM but not FM (h) [2%] A direct conversion receiver demodulates a signal to. i. baseband ii. IF (i) [2%] A modulated signal with 1000 khz is to be beat down to an IF frequency of f IF 455 khz. What LO frequency could be used? i. 90 khz ii. 455 khz iii. 1455 khz (j) [2%] Stereo FM modulation was designed to be incompatible with older mono-fm receivers. EE470 - Richardson 5 of 5 Fall 2013