ELEC 477Topics in Wireless System Design (Indep. Study) Spring 2008

Homework Assignment #7 – due in labTuesday, Apr. 8, 2008

(Prob. 4 removed 4/3/08)

  1. Two 20-dBm signals at 110 MHz and 120 MHz are applied to the input of a real amplifier that has the following specifications:

Input and output impedance = 50 

Input and output filter pass bands = 80-150 MHz

Gain = 20 dB (neglect roll-off within filter pass band)

Third-order intercept point = +25 dBm

1-dB compression point = +14 dBm

  1. What are the output levels (in dBm) of the first-order products? Would gain compression be an issue in this case?
  2. What are the output levels (in dBm) of the third-order (IM3) products?
  3. Repeat Part b for the case when both input signal levels rise to −5 dBm.
  4. Are significant second-order (IM2) products likely to appear at the output of the amplifier? Why or why not?
  1. The FM broadcast band spans the 88-108 MHzfrequency range. The most common FM receiver architecture uses a first IF of 10.7 MHz and an LO range of 98.7-118.7 MHz; however, this places the image band just above the FM band. If you were asked to recommend a new FM receiver architecture, what first IF frequency and LO range would you specify so that the low end of the new image band lies at twice the upper end of the FM band (i.e., at 219 MHz)? Choose an architecture that uses the difference product of the mixer.
  1. Up-conversion is a superheterodyne receiver architecture in which the IF is above the frequency range of interest. Suppose that you were to design an up-converting AM broadcast receiver with an IF of 2 MHz (center frequency of the IF filter). The AM broadcast band spans 550-1700 kHz (carrier frequencies). Specify the LO ranges you would need if you were planning to use (a) the mixer’s sum frequency output and (b) the difference frequency output. Which option is most likely to result in the best image rejection performance if the front-end filter is a low-pass type with a cutoff frequency of 1.9kHz?

Notes and Hints:

You may make reasonable assumptions and approximations in order to compensate for missing information, if any. Unless otherwise indicated, you may use Mathcad, Matlab, or Excel to make difficult or time-consuming calculations. If you do, remember to include a commentedprint-out of the file or screen display that shows your work. Use SI units, unless others are specified.