ECE 451 Automated Microwave Measurements Laboratory

Transcription

1 ECE 451 Automated Microwave Measurements Laboratory Experiment No. 9 Vector Network Analyzer Measurements of Nonlinear Devices This experiment contains two portions: measurement and simulation of nonlinear characteristics of an amplifier. You will learn how to read component data sheets and verify functionality by measuring key figures-of-merit using the Vector Network Analyzer. We will be using an evaluation board for a monolithic power amplifier for nonlinear characterization in this experiment. In the first part, you will measure the S-parameters of the amplifier module to verify the small-signal parameters match with the data-sheet. Additionally, you will perform a powersweep on the VNA to characterize the 1-dB compression point for the amplifier. In the second part, you will take the measured large-signal S-parameters of the amplifier and simulate the nonlinear behavior using the Harmonic Balance simulator in the Agilent ADS software. Lastly, you will compare the simulated 1-dB compression point with the measured data and perform a circuit envelope simulation to predict the TOI points of the PA. I. Motivation Modern communications systems require high-performance active RF components to meet the increasing demands for fast data rates and reliable communication. We often strive to design circuits that are highly linear as linear systems provide the most flexibility in design and can be modeled easily. However, most systems in nature are inherently non-linear when realized physically, thus a key challenge for engineers is to effectively characterize the non-linear behavior in the circuits/systems designed so that they can accurately model the physical behavior of these circuits in simulation. Non-linear device behavior causes interference, reduces effective bandwidth and overall leads to wasted frequency spectrum, a scarce resource in the RF/Microwave world. Power-Amplifiers are the fundamental most vital and power hungry components in the transmit chain and thus need to be carefully designed such that the system overall is highly linear. Typically, we design PAs to operate only within their linear region of operation but that results in an inefficient use of the available power. PA design is different than traditional microwave amplifier design as the goal is not to simply use simultaneous conjugate for max power, instead the designer has to satisfy figures-of-merit like Power-Added-Efficiency (PAE) and TOI points that dictate the level of non-linearity in the amplifier. Since, PAs operate in the large signal regime they are often driven into the non-linear regime and thus characterization of the non-linear behavior is of utmost importance to the RF engineer. 9-1

2 II. Small-Signal S-Parameter Measurement Procedure 1) Log into the network analyzer using your NetId and Active Directory Password. The network analyzer software will open as soon as the login is complete. 2) Create the setup as shown in Figure 1 below. Make sure to turn press Output On before measuring the amplifier s S-parameters in the VNA. Figure 1: Measurement Setup for the PA 3) Click on the yellow Start Button and set it to 300 MHz. Click on green Stop Button and set it to 6 GHz. Now select Channel < Power and enter -15 dbm. Next select Sweep < Number of Points and select 1601 points. 4) Use the Calibration Wizard to do a full 2-port SOLT calibration with the 85052D 3.5mm Calibration Kit. 5) With the power supply output still off, set the current limit to 50 ma and the voltage to 12V (use the 25V controls and output). 6) With the power supply and PNA connected and calibrated turn the output of the power supply on. 7) On the left side of the screen, right click the box that says S11, go into Format and select Log Mag. This displays Log-Magnitude for each measured frequency point. Each point is connected by a straight line. 8) To save data select File < Save As. Find a directory on your personal drive, give the file the appropriate name and select Trace (*.s2p) as your Save as Type. Sample name can be PA_SmallSig.s2p 9-2

3 9) Open ADS, create a new workspace, and import all of these datasets in, giving them the same names as you saved them in above (use the same procedure from Experiment 4). See the Notes on Using ADS at the end of the procedure for some advice on dealing with multiple datasets. 10) In the Data Display plot S-parameters of the amplifier and put markers on S21 as well as S11 for each of the points listed in the data-sheet to compare measured data with manufacturer s data. III. 1-dB Compression Point Measurement Procedure a) Prepare the PNA for a Power Sweep: 1. Log into the Agilent E8357A PNA. Start up the Network Analyzer software if it doesn t start automatically 2. Add 10dB of attenuation to port 1 by selecting Channel-Power and Attenuators and change the attenuation to 10dB. 3. Set the network analyzer to perform a power sweep from -23 to +5dBm at 2 GHz by selecting Sweep-Sweep Type and choosing Power Sweep as the Sweep Type. 4. Perform a 2-port THRU Response calibration through the Calibration Wizard. Choose the 85052D 3.5mm Calibration Kit. b) Set up the Amplifier: 5. The amplifier we are using in this class needs to be biased by a 12Vdc source. Do not push the output on/off button at this time. You ll need to connect the +25V and ground/com of the power supply to the +12 and GND connections of the amplifier. 6. Connect port 1 of the PNA to the RFin of the amplifier and port 2 of the PNA to RFout of the amplifier. 7. Now turn on the output of the power supply by pushing the Output on/off button and look at the PNA output. Right click using the mouse on the PNA screen and select AUTO SCALE. Your PNA image should look similar to Figure

4 Figure 2: P out vs P in c) Measure your P1dB point: 8. View the log magnitude of S21 on the PNA. Set two markers to measure the 1dB gain compression input power. You can add multiple markers by selecting Marker-Select Marker choosing the marker you want to select. Use one marker to indicate the max Pout and the other marker to find the input power that gives a -1dB output power. Record this 1dB gain compression Pout and Pin. IV. ADS Simulation Setup using Harmonic Balance Harmonic Balance (HB) is a frequency-domain analysis technique for simulating nonlinear circuits and systems. The key advantage of Harmonic balance simulation is that is allows simulation of circuits with multiple input frequencies. Thus, when simulating non-linear behavior in circuits we often use the Harmonic Balance technique as it includes intermodulation frequencies, harmonics, and frequency conversion between harmonics. HB is capable of simulating the harmonics produced by the DUT and those created by the signal source (stimulus). We will design a simple PA in this part of the lab and measure its P1dB and TOI points. 1) Create new work-space in ADS and call it PA_Ex_wrk. Add the DemoKit_Non_Linear library. 9-4

5 2) Create new schematic and attach the FET_curve_tracer template as shown in the Figure 3 below: Figure 3: Attach FET DC Analysis Template 3) Add the FET1 Demo transistor to the schematic. To move the FET info click on the transistor, press F5 from keyboard and move the info to desired position on schematic. 4) Add the TechInc button from the side pallete under DemoKit_Non_Linear. Your schematic should now look like Figure 4. Figure 4: FET DC Analysis Schematic 9-5

6 5) Simulate the workspace and bias your FET at the DC operating point shown in Figure 5. Figure 5: FET DC Analysis Output 6) Save the Data Display shown above in Figure 5 as FET_cuves.dds. 7) Create a new schematic, name it Amplifer and recreate the schematic shown below in Figure 6. Figure 5: FET Amplifier Schematic 9-6

7 8) Create a symbol for the Amplifier designed in Step 7 by clicking on Window Symbol and use the Systems-Amps&Mixers Template. Your symbol should like the one shown below in Figure 6. Figure 6: FET Amplifier Symbol 9) Create a new schematic and name it HB_1tone. Place a HB Simulator in the schematic from the side pallet by navigating to Simulation-HB. Make the center frequency 7GHz, set order = 3 and choose RF_pwr as the sweep parameter. Additionally, place a P_1Tone component from Sources-Freq Domain followed by the Amplifier symbol created in step 8 by navigating to Insert Component Component Libraray Double-click on Amplifier_wrk. Your schematic at the end of this step should like the one shown in Figure 7. Figure 7: 1-Tone Swept Power Harmonic Balance Simulation Schematic 9-7

8 10) Mark the 1dB compression point on the data display. Your data display should look like the Figure 8 as shown below: Figure 8: 1-Tone Swept Power Harmonic Balance Simulation Result 9-8

9 11) Finally, we will setup the two-tone Harmonic Balance simulation to derive the TOI points for the amplifier designed. Create the schematic shown below in Figure 9 to simulate the two-tone behavior of the amplifier. Figure 9: 2-Tone Swept Power Harmonic Balance Simulation Schematic 12) Your final data display for the TOI simulations should look like Figure 10. Do the results match the data-sheet? Figure 10: 2-TOI Simulation Results 9-9

10 TA Theory and Conclusion Questions Theory: 1. What is the physical significance of P1dB? Why is it a useful metric to characterize non-linear behavior of RF circuits? 2. Describe the procedure to calculate TOI points of an amplifier. Derive the general equations for IIP3, OIP3 and show the physical interpretation of TOI points. How and why are these points useful to characterize non-linear behavior of circuits? 3. What is Harmonic Balance? How is it different than traditional SPICE simulation? List some advantages for using Harmonic Balance for studying RF circuits/systems and mention some common potential problems that could arise when using this simulation engine. 4. What is PAE? Describe why it is a useful figure-of-merit for Power-Amplifiers? Conclusion: 1. Compare your measured S21, S11, and S22 to the datasheet as shown in the table below, are they close? What may cause the discrepancies? Frequency S21 S11 S22 Measured Datasheet Measured Datasheet Measured Datasheet 1GHz 2GHz 3GHz 4GHz 5GHz 6GHz 2. Why are we not able to simulate the P1dB point from the S-parameter data measured on the VNA using Harmonic Balance simulation? 3. Are the TOI points found using Circuit-Envelop simulation in ADS valid for the PA used in this lab? If not, then what could be some potential flaws with the results? 4. Can S-parameters help characterize non-linear behavior in amplifiers? If not, what potential shortcomings do you see in the S-parameter formulism? 9-10