2. The Vector Network Analyzer

Transcription

1 ECE 584 Laboratory Experiments 2. The Vector Network Analyzer Introduction: In this experiment we will learn to use a Vector Network Analyzer to measure the magnitude and phase of reflection and transmission coefficients (S parameters) of several networks. Such measurements are of critical importance in the design and testing of microwave circuits. A discussion of the scattering matrix, or S-parameters, is presented in Section 4.3 of the text. The S- parameters of a network are the complex response of the network to a stimulus. S ij is the response at port i to a stimulus at port j, with any other ports terminated with matched loads. The network analyzer provides a stimulus and measures response, using two bi-directional test ports, reporting each S-parameter as a complex function across a user-specified frequency span. Reflection and transmission coefficients are discussed in Section 2.3. For Part 5 of this experiment you need to review Smith charts and understand their use. See Section 2.4 and the supplemental notes on the Smith chart handed out in class. You will measure the reflection and transmission coefficients of a circulator; for background, see Figure 7.2 and read Section 9.6. Equipment Needed: E5062A ENA Series Network Analyzer Calibration Kit USB flash memory coaxial low-pass filter coaxial circulator coaxial matched loads and shorts coaxial attenuator coaxial connector with "50 ΩΩ" resistor "black boxes" with unknown networks Note Be sure to save data for all results obtained in the lab. The network analyzer saves the data in a.csv file. This file can be imported into a data analysis and visualization program of your choice. These plots must be included in your lab report with clear axis labels. All axes in your submitted reports should contain labels and units. 1

2 1. Calibration: Set the frequency sweep to cover 1 MHz to 3 GHz. Perform a full two-port calibration as described below and in the instrument manual. You may omit the isolation test. Network analyzer calibration is different from ordinary instrument calibration as performed every year or two. With a network analyzer, every measurement setup must be calibrated. Calibration creates a reference plane for each of the two ports at the mating interface of the user-end connector of the test cable, and S-parameter magnitudes and phases are reported with respect to these reference planes. You will use the SMA-Female calibration kit provided in the lab for this exercise. This kit contains inexpensive coaxial short, open, load, and thru adapter chosen to be compatible with the SMA male connectors of the network analyzer test cables. Network analyzer measurement accuracy is very sensitive to the quality of the calibration standards and calibration methodology. Although proper Agilent or HP calibration standards are widely used, they are several orders of magnitude more expensive and very easy to break. The SMA kit characteristics (parasitic inductance, etc.) are good enough for introductory work up to 3 GHz. A calibration kit definition has been provided in the network analyzer to account for the electrical lengths in the short, open, and thru adapter. Always remember this simple rule for microwave connectors: Twist only the nut. Never rotate the bodies of two connectors against each other; this would cause grinding at the connector interface itself. Only the external threaded nut should be turned. In general, the nut should be tightened with a properly-selected torque wrench. However, for this lab, it is sufficient to make connections fingertight in order to reduce wear. Accordingly, after the connection is made, avoid putting any additional twisting force on the connectors or cables. 2-Port Calibration Procedure for ENA Series Network Analyzer 1. Set the start frequency and stop frequency (in this case, GHz to 3 GHz). 2. Press Cal 3. Check that Cal Kit is set to SMA Kit 4. Check that Port Extensions are turned off for both ports. 5. Press Cal Calibrate 2-Port Cal Reflection 6. Attach the Open to the end of the Port 1 cable, finger-tight. Press Port 1 Open. 7. Attach the Short to the end of the Port 1 cable. Press Port 1 Short. 8. Attach the Broadband Matched Load to the end of the Port 1 cable. Press Press Port 1 Load. 9. Repeat for Port Press Return and then Transmission. 11. Connect the SMA Female-Female Adapter between the two cables. Press Port 1-2 Thru. 12. Click Return and then Done. 2

3 2. Verification: Measure the return loss of a matched load, and verify that the return loss is at least 20 db over the frequency band, using Log Magnitude format. Connect a coaxial attenuator between the two ports and verify that S 21 drops by the proper amount. On each port, measure the Short. View S 11 (or S 22 ) in Log Magnitude, Phase, and Smith Chart formats. The return loss of the short should be close to 0 db (total reflection). If the results are incorrect, go back and do the calibration again. Next, determine whether the phase response of the short is what you would expect. What is the phase of the reflection coefficient of an ideal short? Why would your measurement of S 11 (a function of frequency) differ? To understand the difference, keep in mind that the reference plane of the test port is the mating interface of the test cable's connector, but the SMA Short device has a few millimeters of coaxial transmission line between the mating interface and the reflective plane. Optional: Using Port Extensions (in the Cal menu), you can artificially move the reference plane of your measurement port to obtain constant phase across the band. Consider the propagation velocity through coaxial transmission line, approximately 75% x 3 x 10 8 m/s. Note the amount of extension (seconds) that was required to simulate an ideal short at the port reference plane. This is simply for illustration, and you should turn port extensions OFF again before continuing. Once you are confident of the calibration results, save your measured S-parameters for the matched load and short on USB flash memory using csv format. Press Save/Recall and choose Save trace data, browse to the USB flash drive, and save in a subdirectory for your name. 3. Measure the coaxial low-pass filter: Measure S 11 and S 21 of the low-pass filter from GHz to 3 GHz, and plot your results. Also take a look at S 12 and S 22. Recall that the scattering matrix is defined such that unused ports must be terminated with matched loads. For a two-port device, when you measure S 11, the network analyzer will cause its Port 2 to be a matched load automatically. Now, redo the S 11 measurement without the required matched load connected to the filter output, and compare with the first result; i.e., simply remove the Port 2 cable from the filter in order to remove the automatic matched load. Be sure to use scales ranges (db/division) to display meaningful results. Note that the network analyzer has the capability of displaying Γ, SWR, Z, and Y in various formats. Try some of these options. 4. Determination of "Black Box" networks: Several "black box" microwave networks having two or three ports are available in our lab. Measure the S parameters of each of these networks, and try to determine the type of circuit or component that is inside the box. Is the network reciprocal? Lossless? Matched? Are any of the ports isolated? Any unused port must be terminated with a 50 ΩΩ matched load according to the definition of the scattering matrix. 3

4 5. Measure the coaxial circulator: One of the black boxes contains a circulator. Using the same procedure as above, measure S 21, S 32, and S 13, and then the reverse paths S 12, S 23, and S 31, for the circulator. Since this is a 3-port device, you must separately terminate the unused port with a matched load. Check S 11, S 22, and S 33. Use a scale of 1 db per division to get accurate results for the insertion loss measurements. What happens to the measurements when the unused port is not terminated? 6. Additional Measurements Perform additional measurements from components found in the lab: Measure the input impedance of the filter using the Smith chart display Measure the group delay of the filter Measure the attenuation vs. frequency of a piece of coaxial cable Measure the impedance of an ordinary 50-ohm resistor 4

5 Write-up: Calibration and verification: Explain what calibration achieves and how. Provide plots of measured S-parameters of the matched load, attenuator, and short to verify your calibration and explain the results. Return loss of matched load: What were the best and worst return losses measured over the sweep range? List the frequencies where these occurred, and the corresponding SWRs. To help understand these results, complete the following table to convert between return loss, reflection coefficient magnitude, and SWR: Return Loss Γ SWR 0 db 1 db 2 db 3 db 5 db 10 db 20 db Low-pass filter: What is the measured 3 db cutoff frequency for the filter? What is the roll-off of the attenuation of this filter in the stop-band (db/octave)? What is the frequency range for which S 12 < 20 db? What is the frequency range for which the input SWR is less than 2.0? Is there any difference in S 11 when the output is terminated with a matched load versus having port 2 connected to the network analyzer? What causes this difference? Black boxes: Discuss your measurements, and how you arrived at your idea of what is inside the box. Draw a circuit diagram of the network. Circulator: Over what frequency range is the insertion loss less than 0.5 db? Over what frequency range is the return loss greater than 20 db? What is the minimum isolation over this latter range? What is the effect on the above quantities when the matched load is removed? Why does this happen? Present and discuss your results for the additional measurements you have made. Compare the coaxial cable s measured attenuation to a calculated value or data from the manufacturer. Compare the resistor's measured impedance to its nominal DC value of 50 ohms. 5

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