EXPERIMENT EM3 INTRODUCTION TO THE NETWORK ANALYZER

Transcription

1 ECE 351 ELECTROMAGNETICS EXPERIMENT EM3 INTRODUCTION TO THE NETWORK ANALYZER OBJECTIVE: The objective to this experiment is to introduce the student to some of the capabilities of a vector network analyzer. EQUIPMENT: QUANTITY DESCRIPTION 1 E5071C Network Analyzer 2 HP Coax Cables Terminations 1 DS 109 Double-stub Tuner 1 Microstrip Low Pass filter 3 BNC-to-N Adapters 2 SMA-to-N Adapters load with GR-to-N Adapter INTRODUCTION: The E5071C network analyzer is a versatile, two-port radio frequency (RF) measurement device which allows the user to measure the magnitude and phase of the signal at the input and/or the output port of any device under test. Based on these measurements a variety of results can be displayed, via the scattering parameters. For any two-port network, the scattering (or S ) parameters are defined as a ratio between various incident and reflected powers. In particular, S11 and S22 are the reflection coefficients at ports 1 and 2, respectively, when the opposite port is terminated in its characteristic impedance, Z0. Similarly, S21 is the gain from port 1 to port 2, when port 2 is terminated in Z0. Lastly, S12 is the gain from port 2 to port 1, when port 1 is terminated in Z0. Note that S21 and S12 basically provide transfer functions of output to input values. Based on these parameters, the instrument provides the user with a host of options to investigate, such as the input impedance and voltage standing wave ratio (VSWR) over the desired frequency range. EXPERIMENT: To protect the terminal ports of the network analyzer, all measurements should be done with the Agilent coax cables in place, and the device under test should be connected to these cables rather than directly to the input ports. PLEASE DO NOT REMOVE THESE CABLES. 1

2 PART A: 50- LOADS In this part of the experiment, the input impedance of two 50- terminations will be measured at several frequencies. 1. The Data table: The table below will help you to understand the requirements that follow. Frequency (MHz) Impedance, R+jX (Ω) Sample 1 (50 Ω Load) Sample 2 (50 Ω Load) 2. Before turning on the network analyzer, insert a portable memory drive into the USB cable connected to the network analyzer. This will be used to save some of the results from this lab. Next, turn the network analyzer on. Channel 1 should be active, as indicated by the light next to the Chan 1 key (number 1 in Figure 1). In order to be used properly, the analyzer must be calibrated for the intended application. This has been done ahead of time for you, and the calibration states have been saved. These states are accessed via the Save/Recall function. Please press the Save/Recall key (number 12 in Figure 1) then use the mouse to select Recall State from the Screen. Then use the mouse to select State 02 on the screen. This will load the stored calibration. 3. Using appropriate N-to-BNC adapters, connect one of the 50- terminations to port 1. We want to use the frequency range of MHz. This may already be selected through the calibration. However, just for practice, set this range by first pushing the Start key (number 9 in Figure 1), followed by entering 100 MHz using the keypad in the ENTRY section (see number 4 in Figure 1, and note that M stands for MHz). Repeat this sequence for the Stop frequency (number 10 in Figure 1) of 500 MHz. 4. Select the desired measurement by pressing the Meas key (number 5 in Figure 1) and selecting S Select the desired display format by pressing Format (number 6 in Figure 1) and selecting Smith from the screen using the mouse. Then select the R+jX option from the screen using the mouse. You should see a Smith chart plot of the input impedance (or reflection coefficient) for this frequency range. 2

3 6. Record the impedance of the 50- termination at 100 MHz by pressing Marker (number 8 in Figure 1) followed by 100 MHz in the same fashion used above. The marker arrow should appear on the screen. In addition, several values will appear on the top of the screen, one of which is the impedance, given in rectangular form. Record the frequency and impedance. 7. Repeat this process for frequencies of 200, 300, 400, and 500 MHz. 8. Repeat this process for the second 50- termination. PART B: 100- LOAD AND DOUBLE-STUB TUNER In this part of the experiment, the input impedance of a 100- load in series with an N-to- GR adapter (i.e., a short section of transmission line) will be measured, and a double-stub tuner will be used to match this load to the 50- characteristic impedance. 1. Data table: The data table below will help you to understand the requirements that follow. Requirement Impedance, R+jX (Ω) Length of Pole 1 Length of Pole 2 VSWR BW (VSWR 1.5) Center Frequency = 1.1 GHz Before Matching After Matching 2. Remove the 50- load and the N-to-BNC adapter, and connect the 100- load to port 1 via the N-to-GR adapter. If the Start and Stop frequencies are not 50 MHz and 1.5 GHz, change them to those values using the procedure mentioned above. You should still be measuring S11 with a Smith Chart display. If not, please use the procedure outlined above to select those options. Observe the display and record your suggested explanations of what you see. 3. Record the impedance at 1.1 GHz using the process outlined above (i.e., use the Marker key, etc.). 4. Disconnect the 100- load (and adapter) from the analyzer and connect it to port 4 of the double-stub tuner. Connect port 1 of the double-stub tuner to the 3

4 coax cable coming from port 1 of the network analyzer. Record the impedance at 1.1 GHz. 5. We are going to use the double-stub tuner to match the 100- load to the line. It will be helpful to observe the VSWR (i.e., SWR) while doing this. To plot the initial VSWR, press Format (number 6 in Figure 1) and choose the SWR option from the screen. Then use the print screen option using the keyboard (Ctrl+PSc/LRq or Ctrl+I button). This will make a copy of the plot that can be inserted into the software paint (start > paint). After inserting the image into paint, save the image as a jpg on the provided thumb drive. This will be used later for your report. Now match the 100- load to the 50- coax cable by adjusting the lengths starting with the stub closest to the load. Measure the lengths of the two stubs and record these lengths. 6. Print the final VSWR by repeating the appropriate steps from (4). Also copy an image of the final Smith chart (you can select the Smith chart by pressing Format and then Smith Chart ). 7. Determine the bandwidth for which the VSWR 1.5. To do this, again press Format (number 6 in Figure 1) and choose SWR. The marker at 1.1 GHz should have an SWR very close to 1.0, which is recorded in the upper right of the display. Using the Marker key and the tuning dial, determine the lower and upper frequencies for which the SWR 1.5. Record these frequencies, and their difference (which is the bandwidth). PART C: Microstrip Low-Pass Filter In this part of the experiment, the characteristics of a low-pass filter implemented on microstrip will be measured. 1. Disconnect the double-stub tuner from the analyzer. You will now test the microstrip low-pass filter. Set the Start and Stop frequencies to 500 MHz and 4.5 GHz, respectively. Choose 1601 data points by pressing the SWEEP SETUP key, selecting POINTS from the screen using the mouse, and pressing 1601 followed by x1. Press the MEASURE key, and select S21 from the screen using the mouse. This will allow us to observe the forward gain (or loss) of the filter. You are now all set to do your measurements. 2. Connect each port of the analyzer to one side of the microstrip low pass filter. Verify that you are using the log magnitude format of S21 (the format is specified in the upper left hand portion of the screen). If not, press the FORMAT button and select LOG MAG option from the screen using the mouse. You should now be observing the magnitude of the forward gain of the low pass filter. Since it is a filter, the gain is negative. 4

5 3. Low pass filters are typically characterized by the 3-dB frequency. Use the MARKER key and then select Marker 1 from the screen using the mouse to determine (and record) this frequency. Copy an image of this screen showing the frequency response of S21 and the 3-dB frequency. 4. Look at the phase of S21 by pushing FORMAT and selecting PHASE on the screen with the mouse. Make an image of this screen. In your report describe what you can infer about the filter from the phase plot. 5. Disconnect the low pass filter and the N to SMA adapters. Turn off the network analyzer. Figure 1: Network Analyzer 5