ECEN 4634/5634, MICROWAVE AND RF LABORATORY

Transcription

1 ECEN 4634/5634, MICROWAVE AND RF LABORATORY Final Exam December 18, :30-10:00pm 150 minutes, closed book, 1 sheet allowed, no calculators (estimates need to be within 3dB) Part 1 (60%). Briefly answer the following questions: Q1. (10/8 points) Several points are labeled on the Smith Chart. For each description below, write the label of a corresponding point on the chart in the box. Note: there may be more than one correct answer, give only one answer for each. Capacitor Contour of constant susceptance Real part of impedance is 0.2Z0 Г = 0.3 Open circuit Page 1 of 8

2 Q2.. (10/8 points) Sketch the resistance vs. power curve of a thermistor (qualitatively only, but label your axes). How does the curve change when the ambient temperature increases? Q3. (10/7 points) A sinusoidal signal with power of 36 dbm is applied to a 50 ohm load. What is the peak voltage across the load (in V)? Q4. (10/8 points) Where is the far field of a satellite TV dish antenna 1m in radius at 3 GHz? What is the approximate gain of this antenna? You can leave your answer in terms of π. Page 2 of 8

3 Q5. (10/7 points) A microstrip transmission line is designed to have an electrical length of 300 degrees at 10 GHz, on a substrate with effective dielectric constant εr=4. What is the physical length of this microstrip? Q6. (10/8 points) An amplifier has a power-added efficiency PAE1 = 30%. If a second amplifier with the same bias point, same output power, same gain, and same input match has PAE2 = 70%, how much less dc power is consumed in the second amplifier (expressed as a ratio)? Page 3 of 8

4 The following questions are for ECEN 5634 students only: Q7. (6 points) Why is a circuit unstable if Sii at any port i is larger than unity? Q8. (8 points) Write the scattering matrix of: (a) an ideal lumped element low-pass filter in the pass band (b) an ideal lumped element low-pass filter in the stop band. Page 4 of 8

5 Part 2: Problems (20% each for 4634, 15% each for 5634) P1. (20/14 points) A single-slope FM-CW radar with frequency sweep from 9.9 GHz to 10.1 GHz over 2 ms is used to detect an object 15 m away and moving 75 m/s away from the Radar. (a) What is the Doppler shift seen by the radar, due to velocity only, when the radar transmits at its center frequency of 10 GHz? (b) What is the total Δf measured by the radar, including shifts due to both range and velocity, at the radar center frequency of 10 GHz? Page 5 of 8

6 P2. (20/12 points) Match the TDR step responses, measured as reflected voltage, to the networks shown in the tables below. Note that some responses may be used multiple times, or not at all, and there may be more than one correct answer. Give only one answer for each network. Network Response Network Response Network Response (1) (2) (3) (4) (5) (6) Networks: TDR reflections due to step input: Page 6 of 8

7 The following additional problem is for 5634 students only: (10 points) P3. (14 points) A resonator is made from a series LC tank with values L= 10/π nh and C =16/π pf. The total tank loss can be modeled as a resistor of value 1 ohm. You want to use a Gunn diode with this tank to make a resonator. The diode is modeled as a piecewise linear IV curve as shown below: (a) From the IV curve, draw the Gunn diode resistance as a function of bias voltage. Label the plot axes. (continued next page) Page 7 of 8

8 (b) At what bias voltage range(s) does the diode cancel the loss in the tank? Are all possible bias points equally good for oscillation? Why or why not? (c) What is the frequency of oscillation (in GHz) when the tank and Gunn diode are combined? Page 8 of 8