Lab 6-1: Microwave Multiport Circuits In this lab you will characterize several different multiport microstrip and coaxial components using a network analyzer. Some, but not all, of these components have a counterpart in the lowfrequency region. The purpose of this lab is to learn how to perform multiport network analysis, how to diagnose a circuit functionality and how to evaluate the quality of your measurement. Part I Calibrate the VNA Using a frequency range of 1 GHz to 6 GHz, and at least 401 frequency steps, perform a full twoport 3.5mm SOLT (short-open-load-thru) calibration of the Agilent 8753ES network analyzer. Remember, this calibration consists of a one-port (or reflection) calibration for both ports, as well as a transmission calibration in which a thru standard is connected between ports 1 and 2 of the network analyzer. In this case, a thru could be either a direct connection between a (male) connector at port 1 and a (female) connector at port 2, or using a thru adapter if your ports have the same connector type. There is a third calibration step, isolation, which can be omitted for this class. After you complete the calibration, check the calibration by observing all S-parameters when each of the calibration standards is connected. Save the calibration. Part II Two Port Networks In this part of the lab, you will measure the performance of various two-port microstrip circuit filters using the network analyzer. For all of the circuits, measure the following performance parameters: 1. How large is the insertion loss in the pass-band? (a) The insertion loss is the average value of the transmission coefficient, S12 (db), in the passband (Figure 1 Label A) 2. What is(are) the corner frequency(ies)? (a) The corner frequency is the frequency at which the transmission coefficient, S12, decreases by 3 db from its insertion loss in the passband. (Figure 1 Label B) 3. How large is the pass-band bandwidth? (a) The pass-band is measured from corner frequency to corner frequency (or, in the case of a low/high pass filter, from the maximum/minimum frequency measured by the VNA to the corner frequency). (Figure 1 Label C) 4. How large is the stop-band attenuation? (a) The stop band attenuation is the amount the transmission coefficient decreases from the insertion loss. (Figure 1 Label D) 5. At what frequency(ies) is this circuit designed for? 1
(a) The design frequency occurs at lowest value of S11 (in pass band) and highest value of S11 (in stop band). (Figure 2 Label F) **NOTE: include labeled plots with the parameters mentioned above in your report. The data table on the next page organizes these measurements. S12, db -3-5 -10 A B C 3 db B D -20 Figure 1: Label S12 Graphs for Lab Report -3 E -5 S11, db -10 Poorly Matched (>-10 db) -20 F Figure 2: Label S11 Graphs for Lab Report Q1: Measure (and save) the reflection and transmission S-parameter plots of Circuit #1. At what frequencies do you observe dips in S11 and S12 amplitude plots? Why do these dips occur? Estimate the length of the open stub if the relative permittivity of the substrate is 4.4. Q2: Circuit #2 is a low-pass filter. Explain how the circuit works based on physical principles. Check if the circuit is lossless by calculating S11 2 + S21 2 at a frequency point. Do you 2
Circuit Circuit 1 Function (Filter type) Insertion Loss (+db) (A) Corner (GHz) (B) Pass Band Range (GHz) (C) Stop Band Attenuation (db) (D) Design (E) Circuit 2 Low Pass Circuit 3 Circuit 4 Circuit 6 Table 1: 2 Port Networks get close to unity? Remember that the S-parameters are defined w.r.t. voltage, i.e. the value in db is 20 log S. Explain any errors in your measurement. Q3: Measure Circuit #3, explain its functionality and quantify all relevant parameters. How is it different from Circuit #2? Q4: Measure Circuit #4, explain its functionality and quantify all relevant parameters. Q5: Measure Circuit #6, explain its functionality and quantify all relevant parameters Q6: Do not measure the S-parameters for Circuit #7, instead, compare its topology to that of Circuit #4 and Circuit #6 and explain what is Circuit #7s functionality. Include a sketch (or picture) of Circuit #7 in your report to support your explanation. Part III Three-Port Networks In this part of the lab, you will compare the performance of 2, three-port microstrips. One microstrip includes a lossy element (a resistor in Circuit #10), whereas the other does not. Keep in mind, a three-port network cannot be lossless, matched AND reciprocal (it can be up to two of these!) Q7: What is the function of Circuit #9? How many measurements do you need to perform to fully characterize three-port networks? Justify your response. Q8: Attach the matched loads to ports 2 and 3, and make a plot of S11. What frequency is this circuit designed for? (Label A, Figure 2) How big is the 10-dB (about 2:1 VSWR) bandwidth of Circuit #9? (Label B, Figure 2) 3
Q9: Connect ports 1 and 2 of Circuit #9 to the VNA and terminate port 3 with a matched load. Measure the transmission coefficient S21 from port 1 to port 2. Does this circuit work well as a power splitter? Q10: Connect the network analyzer to port 2 and 3 of the circuit, and place a matched load at port 1. Instead of splitting the power of port 1 between ports 2 and 3, the circuit is now connected as a power combiner. Look at the magnitudes of S-parameters S23 (the isolation), S22 and S33 on the display (included these plots on your report). Does it function well as a power combiner? Is this a matched circuit? Is this circuit lossless? Is it reciprocal? Explain. Q11: What are the S-Parameters of an ideal power combiner? Is an ideal power combiner a matched network? Lossless? Reciprocal? Explain. Q12: Circuit #10 is a Wilkinson power divider. Measure this circuit as a three-port and quantify: 1. Power division (coupling coefficient, S31) 2. Return loss (how well the circuit is matched, S11, S22, S33) 3. Isolation between ports 2 and 3 (S23) 4. Insertion loss (S12, S13). 5. 10 db Bandwidth (Figure 2, Label B) 6. Relative phase between the two coupled ports Part IV Four-Port networks In this section of the lab, we will analyze the performance of Circuit #11 (a hybrid coupler) and use this information to analyze the topology of Circuit #12. Q13: How many measurements do you need to make to fully characterize Circuit #11 s performance? Explain why you chose this number and include a sketch or image of Circuit #11. Q14: There are only two ports on the network analyzer. What do you do with the rest of the ports in the circuit while you are doing the measurement? If you had no matched loads, could you still characterize the circuit, and if yes, what would you do? Measure the S-parameters. Make use of all the symmetries you can to reduce the number of plots. Include plots of relevant amplitudes. Include the phase plots of only S12, S13, and S14. Q15: What are the relative amplitudes and phases of S11, S21, S31 and S41 at the design frequency? What is the phase difference between S21 and S31 at that frequency? Q16: How large is the 10-dB bandwidth? (Figure 2, Label B) Q17: Make a sketch of circuit #12 and include it in your report. Compare its topology to that of circuit #11s and deduce circuit #12s performance. More specifically, what are the differences, if any, between the reflected, through, coupled, and isolated signals between circuit #12 and circuit #11. Make sure to talk about amplitude as well a phase of the relevant s-parameters. Do not include plots but use the network analyzer to verify your explanation. 4
Part V Coaxial Components In this part of the lab, you will characterize a few coaxial components. Recall the VNA 1-6 GHz calibration state. Circuit #14 is unspecified components whose functions you will determine using the network analyzer. Q18: What does circuit #14 do, and why do you think so? Provide plots with your explanation. Q19: If any other coaxial components are provided, characterize them and provide relevant plots, and explain what the function is. 5