Experiments Using the HP8714 RF Network Analyzer

Transcription

1 Experiments Using the HP8714 RF Network Analyzer Purpose: The purpose of this set of experiments is two folded: to get familiar with the basic operation of a RF network analyzer, and to gain a physical understanding of the transmission line theory. Introduction: In experiment #1, the procedure for making a reflection measurement is listed, along with a brief description of the functions for each step. Following these steps, you should be able to complete the rest experiments. For a deeper understanding of the principle of the network analyzer, you are referred to the user s guide. This is not considered as an important part of this lab. Therefore as long as you can follow the steps and obtain the required measurement data, that will be enough. The rest experiments are concerned with several common transmission lines: the coaxial line, the microstrip line, and the two-wire line. It is intended for you to understand the following concepts: the characteristic impedance of a transmission line calculation of the characteristic impedance from geometric and material parameters the electric length and its relation to the physical length the reflection coefficient and its relation to the load impedance the use of smith chart to obtain the above parameters It is also desirable for you to observe the difference between theoretical prediction and the measurement result. For example, the textbook discusses only lossless transmission line, but in reality all transmission lines induce loss. Does the loss play an important role in your measurement result? It is expected that you will address this kind of question in your lab report. Pre-lab assignment: 1. Read this manual and chapter 2 and 3 of the network analyzer user s guide (the file network.pdf). 2. Read this manual and chapter 2 and 3 of the network analyzer user s guide (the file network.pdf)! 3. Make sure you know the definition of the terms mentioned in the introduction. Experiment #1: The procedure for making a reflection measurement Step 1: Turn on the power switch As the network analyzer boots up, various system info are shown on the screen. You can see that the analyzer has many micro-processors: MC683 processor, MC68882 floating point coprocessor, TMS341 graphic processor, and DSP32C32 digital signal processor. System RAMs are tested. Firmware version is shown What can we learn from these messages? Among other things, we know that the analyzer behaves somewhat like a PC. The difference is that the software is embedded inside the analyzer, and there is one significant thing about the analyzer it can generate and detect high frequency microwave signals. Nevertheless, one can say that operating the analyzer is no more difficult than using a word processing software. Instead of using keyboard and mouse, now we use the hard-keys (we call

2 them key later on) on the front panel and the softkeys displayed on the screen to control the behavior of the analyzer. Just like some infamous software, many important controlling features of the analyzer are hiding deeply in sub-submenus. These features are important because they affect the accuracy of your measurement results. So always remember this: it is easy to obtain a result from the analyzer, but the accuracy of the result will be highly dependent on your understanding of the physics of the device you are testing. You can say the same thing about using word processing software: you can easily type in sentences to form a paper, but your grade will be decided from the idea reflected in the paper. So, as you begin to work on a network analyzer considered by many to be a very complex instrument you want to keep a positive attitude: The analyzer can generate and detect high frequency RF signals. These signals are somehow digitalized processed inside the analyzer. Results are displayed on the screen in forms of figures or numbers. The behavior of the analyzer is controlled through the Keys of Softkeys. They are just like the menus and sub-menus in an usual software. To obtain an accurate or physical result, you need to understand the physics of the DUT(device under test). You will also need to watch out for some controlling parameters, which determine the way the analyzer samples and processes the various RF signals. Step 2: Press ENTER PRESET The ENTER key is used to clear the system info shown in the center of the screen. At this time, the display is formatted according to previous user s setting. And many of the controlling parameters might not be set at the values that you want. Pressing the PRESET key will set the analyzer to a default operating condition so that you can make change to only some of parameters, knowing that the rest are at the right value. The information shown on the screen at this time is explained in the following figure:

3 There are two measurement channels--think of them as two windows that can be set to display different parameters in different format. The active channel is indicated by a solid right arrow. Use key MEAS1 or MEAS2 to select the active channel. The type of measurement parameters for both channels are displayed. In this figure, channel 1 is showing the Transmission coefficient, and channel 2 is Off db This shows the display format which can be controled by the FORMAT key This shows the Scale/Div--the scale per division, which is controled by the SCALE key This is the reference level. It is the reference line in Cartesian format or the outer circle in polar format. Notice that the reference line has a solid arrow on its left 1:Transmission Log Mag 1. db/ Ref. db 2:Off Reference level 1: Start and Stop frequency Start.3 MHz Stop 3. MHz 1 Step 3: Press Freq softkey Start 3 softkey MHz Stop 3 GHz This is to set the desired start and stop frequency. db : -1 1:Transmission 2:Off Log Mag1. db/ Ref. db Active entry area: the number you enter for operating parameters will show up here FREQ Start Stop Center Span CW This denotes the frequency menu These are the softkeys under the frequency key(menu) Press the buttons on the right hand side to select Disp Freq Resolution -4 1 Start 3. MHz Stop 3. MHz

4 Step 4: Press Menu softkey Number of Points 2 1 Enter This tells the analyzer to make measurement at 21 frequency points within the chosen range Step 5: Press SWEEP softkey Sweep Time 2 softkey Sec This tells the analyzer it should sweep the chosen number of frequencies in 2 seconds, no more, no less. Step 6: Press Power Check if the power is.dbm This sets the output RF signal power Step 7: Press MENU softkey Trigger Softkey Continuous Notice that a small arrow on the bottom of the screen is moving from left to right, and this process is repeated continuously. Step 8: Press AVG softkey Average OFF This tells the analyzer not to take average from several measurements for any one particular frequency. In other words, the analyzer will make one measurement at each frequency point and output the result. Step 9: Press MEAS1 softkey Reflection Step 1: Press CAL softkey User 1-Port This starts a one-port reflection calibration. The analyzer will prompt you to connect three standards (open, short and load) to port 1. You will need to press softkey [Measure Standard] after connecting each standard. The analyzer will measure the response of each standard and then calculate some error correction coefficients. These coefficients will be used in some algorithm to error-correct the response of your device(dut). Why doing the calibration? In simple words, the analyzer is not smart enough to make an absolute measure the response of the DUT. This is because the RF signals in the internal circuit are affected by many other factors, such as the leakage in the internal RF circuit. To isolate the response of the DUT, the analyzer uses a comparison measurement, which means it compares the internally detected signals when the DUT is being test, to those when the Standards are connected. The so-called Standard means that it has a well-known response as is indicated by the name Short, Open or Load (matched load). These Standards are difficult to make and hence they are expensive. Unfortunately, since we don t have our own standards for the ESC4 lab, we cannot provide you the opportunity to actually make the calibration measurement. Instead, you will use the calibration data previously saved in the analyzer. To do this, follow step 1. Step 1 : Press SAVE/RECALL, select file STATE.STA softkey Recall State

5 DOS volume Page 1/1 INT: Bytes Free: FILE NAME TYPE SIZE LAST CHANGE..<PARENT> <DIR> STATE.STA DOS Feb- 18:48:44 Use the arrow key want to select. to high-light the file you SAVE RECALL Save State Re-Save State Define Save Recall State Programs Select Disk File Utilities FastRecall on OFF This step recalls the calibration data (or error correction coefficients) saved in the file STAT.STA. Step 11: Connect the DUT(device under test) to the left test-port. Now we get ready to measure the reflection response of our device. Step 12: Press MEAS1 softkey Reflection Format Smith Chart This tells the analyzer to display the reflection coefficient in a smith chart format. To display the transmission coefficient, you will need to substitute softkey [Reflection] with softkey [Transmission]. Refer to page 3-59 of the network analyzer user s guide for a discussion of how to interpret the result. It is in chapter 3, under View and Interpret the Impedance Measurement Results. Record the results. 1:Reflection Smith 1 U FS 2:Off j5 j25 j1 FORMAT Log Mag Lin Mag j1 j25 SWR Delay Phase -j1 -j25 Smith Chart Polar -j25 -j1 -j5 Start 3. MHz Stop 3. MHz More Format

6 Experiment #2 Two Coaxial lines of different length 1. Obtain the radius of the inner conductor and the inner radius of the outer conductor of the coaxial lines. 2. Calculate the characteristic impedance of the coaxial line using the following formula: η b µ Z = ln where η = ε r =2.1 2π a εrε 3. Following the steps in experiment #1(you need to perform step 11 and 12 only), measure the transmission coefficients for the two coaxial lines. 4. Calculate the difference in electric length of these two coaxial lines. 5. Determine if there is any loss induced by these two coaxial lines. Experiment #3 Microstrip line 1. Obtain the width w of the metal strip, the distance d of the strip from the ground plane. 2. Use the following procedure to calculate the characteristic impedance of the microstrip line: εr + 1 εr 1 1 ε e = + where ε r = (1 + 12d / w) 6 8d w if w/d 1, then Z = ln( + ) ε w 4d e 12π if w/d 1, then Z = εe [ w/ d ln( w/ d )] 3. Connect the microstrip line to the analyzer and measure the transmission coefficients. Observe if there is any loss. 4. Connect one end of the microstrip line to test port 1(RF out), leave the other end open, and measure the reflection coefficient and the impedance Z open of this load. 5. Connect one end of the microstrip line to test port 1(RF out), use a wire to short-circuit the other end, and measure the reflection coefficient and the impedance Z short of this load. 6. Use the following formula to calculate the normalized characteristic impedance of the microstrip line. Z = ( Z short Zopen ) 7. Compare the results in step 2 and step 6. Experiment #4 Two-wire line 1. Obtain the radius d of the wire and the distance D between the two wires. 2. Use the following procedure to calculate the characteristic impedance of the microstrip line: η 2D Z = ln where ε r =2.8 and D>>d π d 3. Connect one end of the two-wire line to test port 1(RF out), leave the other end open, and measure the reflection coefficient and the impedance Z open of this load. 4. Connect one end of the two-wire line to test port 1(RF out), use a wire to short-circuit the other end, and measure the reflection coefficient and the impedance Z short of this load. 5. Use the following formula to calculate the normalized characteristic impedance of the microstrip line. Z = ( Z short Zopen ) 6. Compare the results in step 2 and step 5.