EE 3324 Electromagnetics Laboratory

Transcription

1 EE 3324 Electromagnetics Laboratory Experiment #10 Microstrip Circuits and Measurements 1. Objective The objective of Experiment #8 is to investigate the application of microstrip technology. A precision bench-top microstrip training apparatus is used to investigate the measurement of return loss, reflection coefficient and VSWR. The procedures for impedance matching microstrip circuits are also investigated. 2. Introduction Microstrip transmission lines, as shown in Figure 1, are simple low cost transmission lines that are easily fabricated using printed circuit board techniques. Microstrip transmission lines are commonly used in microwave applications over a frequency range of approximately 1 GHz to 40 GHz. In addition to basic transmission lines, microstrip technology can be used to fabricate components such as antennas, attenuators, filters, amplifiers, and oscillators. The Feedback MST532 Microwave Trainer contains the fundamental passive and active microwave integrated circuit (MIC) components used in modern microwave radio, radar and satellite communications. The individual components of the MST532 Microwave Traniner are fabricated using microstrip technology and encased in nickel plated aluminum packages with coaxial Figure 1. Microstrip geometry. connections (miniature SMA coaxial connectors). The system components used in this experiment are shown in Figure 2. The characteristic impedance of the trainer system is 50. The MIC components of the MST532 are designed to operate in the S-band (2-4 GHz). The MST532 microwave source is a voltage controlled oscillator which is tunable over the range of GHz The MST532 trainer also contains coaxial components as shown in Figure 3. These coaxial components include a 50 matched load, a short circuit termination and an open circuit termination. As with any transmission line, microstrip transmission line terminations must be matched to the transmission line characteristic impedance in order to prevent reflections, and thus standing waves, on the microstrip. The standard transmission line formulas for parameters such as reflection coefficient and standing wave ratio are used to define the corresponding microstrip parameters. The return loss of the microstrip is defined as the ratio of the power reflected from the load (P r ) to the power incident on the load (P i ). The incident and reflected powers are proportional to the square of

2 the voltages associated with the incident and reflected waves (V i 2 and V r2 ). Return loss is normally expressed in units of db such that (1) where is the microstrip reflection coefficient. Figure 2. MST532 Microwave Trainer system components.

3 Figure 3. MST532 Microwave Trainer coaxial components. 3. Equipment List Feedback MWT532 Microstrip Trainer DC Power Supply Multimeter 4. Procedure 4.1 Measurement of Return Loss 1. Set up the system shown in Figure 4 with the short circuit coaxial termination, SC, denoted by a white spot on its casing, connected to point X, port 2 of the directional coupler. Connect a coaxial matched termination (MT) to port 3 of the directional coupler and connect the crystal detector (D) to port 4 of the directional coupler. Connect the output of the detector to the multimeter using a coaxial cable (to the to measure the detector DC output voltage). Port 2 of the circulator is connected to port 1 of the directional coupler using the SMA plug to plug connector (PPG). Connect a coaxial matched termination (MT) to port 3 of the

4 circulator. The operation of the directional coupler is a follows. A fraction of the microwave energy entering port 1 exits port 3. (power out of port 4 is negligible) A fraction of the microwave energy entering port 2 exits port 4. (power out of port 3 is negligible) The circulator can be used as an isolation device. The basic operation of the circulator, as the names implies, is to direct microwave energy in different directions as follows. Microwave energy entering port 1 exits port 2. Microwave energy entering port 2 exits port 3. Microwave energy entering port 3 exits port 1. By connecting the circulator as shown in Figure 4, with a matched termination on port 3, the circulator is used as an isolation device. All of the microwave energy entering port 1 passes through the circulator out port 2. Any reflected energy entering port 2 is directed to port 3, where it is absorbed by the matched termination. Thus, the circulator is providing isolation for the microwave source (VCO - voltage controlled oscillator) connected to port 1 of the circulator. Figure 4. Experimental setup for return loss measurements. The VCO requires 3 input connections from two DC power supplies. The negative terminals of the DC power supplies should be connected together and then connected to the ground input to the VCO (black). One DC power supply should be set to +15V and its positive terminal connected to the power input terminal (red) of the VCO. The positive terminal of the other DC power supply serves as the tuning voltage and should be connected to the tuning voltage input to the VCO (white). Set the modulator switch on the VCO to OFF. 2. Measure the reflected power coupled out of the directional coupler at port 4 at frequencies of 2.4, 2.5, 2.75, 3.0 and 3.25 GHz. The value of tuning voltage required for each frequency is obtained from the supplied VCO calibration curve. The measured crystal detector output voltage can be translated to microwave output power using the calibration curve for the detector. 3. Disconnect the short circuit termination and replace it with the open circuit termination, OC,

5 denoted by a blue spot on its casing. Repeat the reflected power measurements of part Determine the average value of the measured reflected powers for the short circuit and open circuit loads at each frequency. This average value represents the incident power at each frequency. 5. Disconnect the open circuit and connect the microstrip matched load (ML). You will need a plug to plug connector (PPG) to connect the matched load to the directional coupler. Measure the reflected power at port 4 of the directional coupler for the five frequencies used in part Disconnect the microstrip matched load and connect a 50 coaxial termination at point X. Measure the reflected power at each of the five frequencies. 7. Disconnect the 50 coaxial termination and connect the low pass filter at point X using a plug to plug connector (PPG). Terminate the filter output with a 50 coaxial termination. Measure the reflected power at each frequency from 2.45 to 3.65 GHz in 50 MHz steps. From your measured results, calculate the reflection coefficient, the return loss in db, and the standing wave ratio for the three components investigated (microstrip matched load, coaxial matched load, low pass filter). Compare the characteristics of the microstrip and 50 coaxial matched terminations. Estimate the cutoff frequency of the low-pass filter based on your measured results. 4.2 Matching Measurements 1. The same experimental setup used for the return loss measurements will be used for the matching measurements (Figure 4) except the unit containing an unknown load, a /4 transformer and a shunt stub tuner (ZT) will be used. Set up the system shown in Figure 4 with the short circuit coaxial termination, SC, denoted by a white spot on its casing, connected to point X, port 2 of the directional coupler. Measure the reflected power coupled out of the directional coupler at port 4 at frequencies of 2.7 to 3.3 GHz in 100 MHz steps. 2. Disconnect the short circuit termination and replace it with the open circuit termination, OC, denoted by a blue spot on its casing. Repeat the reflected power measurements of part Determine the average value of the measured reflected powers for the short circuit and open circuit loads at each frequency. This average value represents the incident power at each frequency. 4. Disconnect the open circuit and connect the line A (the unknown resistive load R) of the matching unit (ZT). Measure the reflected power at port 4 of the directional coupler for the frequencies used in part Disconnect line A and connect line B (the /4 wave transformer) at point X. Measure the reflected power at each of the frequencies used in part Disconnect line B and connect line C (the shunt stub tuner) at point X. Measure the reflected power at each of the frequencies used in part 1. From your measured results, calculate the reflection coefficient and standing wave ratio for the unknown load resistance termination. From the reflection coefficient, determine the value of the unknown load R. Perform these calculations at each of the measured frequencies. Also, calculate

6 the reflection coefficient and standing wave ratio for the /4 wave transformer and the shunt stub tuner at each frequency. Plot graphs of the measured standing wave ratio vs. frequency for the three terminations and discuss your results.