ECE 451 Automated Microwave Measurements Laboratory

Transcription

1 ECE 451 Automated Microwave Measurements Laboratory Experiment No. 5 Automated Scalar Reflectometer Measurements Using a Directional Coupler And Two Detectors to Obtain Both Incident and Reflected Information for Simple Scalar Error Corrections Introduction In this experiment, each group will write two simple programs. The first program will be written to measure the S11 in db as a function of frequency of a short, an open, a student unknown, and a shorted 15 foot length of RG/8U coaxial cable. In each case, both the incident and reflected power will be measured. Next, the ratio of reflected to incident power will be taken. The incident power, reflected power and their ratio for the device being measured are stored in a LVM file. The second program will be written to read the stored measured data and to make simple scalar error corrections on the student unknown and the shorted cable using the short and open data. The data from the simple scalar error corrections made on the student unknown and shorted cable will also be displayed. SOURCE HP8350B 719 HPIB DVM 1 HP3457A (INCIDENT) 722 DVM 2 HP3457A (REFLECTED) 715 DET DUAL DIRECTIONAL COUPLER HP778D SHORT OPEN DUT DET TEST PORT Figure 1. Scalar Reflectometer without Log Levelers 1

2 Procedure 1) Connect the instruments as shown in Figure 1. The measurements should be made over the 300 to 1300 MHz frequency range every 5 MHz (201 points). 2) Write a program to collect incident and reflected detector voltage data over the 300 to 1300 MHz frequency range, changing the measured voltage values to power in dbm at each frequency, In the frequency loop, fill three arrays: (a) incident power in dbm, (b) reflected power in dbm, and (c) the ratio of reflected to incident power in dbm (to take the ratio, subtract the incident from the reflected power values in dbm). When finished with the measurement loop, the program should store all three arrays in a LVM file for each device. You are not required to write the program from the start. You can copy and rename the first program written in the lab 4 in order to modify it. Compare the flow chart in Figure 2 from this lab with Figure 2 from the lab 4. You will notice that it requires sweeping the frequency at constant source output power where as power was swept at constant voltage in lab 4. Additionally, 3 sets of arrays are displayed and stored in the LVM file for each run. 3) Run the program four times, measuring a short, an open, a student unknown, and a shorted 15-foot coaxial cable. The names of the arrays storing power, frequency and file for storing devices should be indicative of its contents, for example: Arrays to store: Sweeping frequency freqsweep Incident power incpwr Reflected power..refpwr Ratio ratio Files to store: Short..short.lvm Open..open.lvm Student s device under test sdut.lvm Shorted 15-foot cable.scx.lvm Obtain the uncorrected plots of the reflected power, and incident/reflected ratio of your student unknown. 4) Modify the measuring set-up by placing a 6 db pad between the source and the directional coupler. Use the same program, measure the student unknown and store the data (incident power, reflected power, and ratio) in an appropriate file (sdut6.lvm) 2

3 Obtain the uncorrected plot of reflected power using 6dB pad of your student unknown. 5) Write a second program, which reads the LVM files created in step (2) above. In order to perform corrections, the program must read three sets of files for short, open and device for which measurement corrections are necessary. Segment of this program is very similar to the tutorial 2 introduced in lab 4. The program should display the corrected data as a function of frequency. Six sets of corrected data S11 that are listed below are required for each device under test. Remember that there are two devices under test (DUT) your student unknown, and shorted 15-foot coaxial cable. Therefore, the second program must run two times. 1. Short corrected: Reflected power of DUT corrected with reflected power of short (refpwr_dut refpwr_short) 2. Open corrected: Reflected power of DUT corrected with reflected power of open (refpwr_dut refpwr_open) 3. Average corrected: Reflected power of DUT corrected with an average of reflected powers of short and open (refpwr_dut (refpwr_short + refpwr_open)/2) 4. Ratio short corrected: Ratio of DUT corrected with ratio of short (ratio_dut ratio_short) 5. Ratio open corrected: Ratio of DUT corrected with ratio of open (ratio_dut ratio_open) 6. Ratio average corrected: Ratio of DUT corrected with an average of ratios of short and open (ratio_dut (ratio_short + ratio_open)/2) 6) Even though there will be a total of 12 sets of corrected S11 in db for the two unknowns as a function of frequency, only obtain 5 sets of S11 corrected data for the followings. 1. Student unknown with correction (1) from above 2. Student unknown with correction (2) 3. Student unknown with correction (3) 4. Student unknown with correction (6) 5. Shorted 15 foot coaxial cable with correction (6) 3

4 You should now have a total of 8 plots. Compare the uncorrected student unknown data (reflected data only vs. power ratio) in plots obtained from procedure (3). Why is one of them smoother? Also, compare the uncorrected, reflected data only from procedure (3) vs. reflected data only taken with 6dB pad from procedure (4). Why is the plot with the 6dB pad smoother? What is the disadvantage of using the 6dB pad? 7) From corrected S11 in db plot of the shorted cable, obtain one-way attenuation of the cable at your 7 assigned frequencies, and compare it in a table with the results obtained from the calculation #4 of Experiment 2. 8) Modify the first program by deleting unnecessary segments with a new measurement set-up shown in figure 2 to measure incident voltage of the shorted, 15-foot cable as a function of frequency. Find the frequency where the greatest voltage variation occurs from the incident voltage vs. frequency plot. At that point, find the Vmax/Vmin ratio. From procedure (6), obtain the cable attenuation at that frequency. Calculate the VSWR of your source (HP8350B). HPIB BUS SOURCE SWEEPER HP 8350B (S/S MODE) DVM HP 3457A SHORT GRAY CABLE BNC WILTRON DET. FEMALE CONNECTOR 20 db COUPLER HP 778D MALE CONNECTOR SHORTED, 15 FOOT, RG8/U CABLE 50 OHM TERM. Figure 2. 4

5 Level Sweep Oscillator 8350B Number of frequency points Store frequency point Calculate frequency step DVM 1 DVM 2 For-loop (number of frequency points) Current loop value Start frequency Incident Stop frequency Calculate current frequency Set the current frequency of 8350B Read DVM 1 Convert input power to dbm Incident Store frequencies Read DVM 2 Calculate ratio Ratio End of for-loop Convert reflected power to dbm Ratio Set Source to safe state Reflected Set DVM1to local Set DVM2 to local Reflected Saving Data 5