LAB MANUAL EXPERIMENT NO. 9

Transcription

1 LAB MANUAL EXPERIMENT NO. 9 Aim of the Experiment: 1. Measure the characteristics of a Directional Coupler. 2. Use of the Directional Coupler and Ratio Meter to construct a Scalar Network Analyzer for measuring the S-parameters of any unknown two-port device-under-test (DUT). Requirement: The users have to install the LabIEW Run time Engine on the computer to run the.exe file for performing the virtual experiment. The Run Time Engine can be downloaded free of cost from the following link: Pre-requisite Knowledge: a) Scattering Parameters of microwave devices Theory: A. Directional Coupler Fundamentals: Directional Coupler (DC) is a passive, reciprocal, four-port device, where one port is isolated from the input port. Some commonly used schematics for the DC are shown in Figure 9.1 below. (a) (b) Figure 9.1 (a) Schematic from [1]; (b) Schematic used in this experiment The three characteristic parameters of a DC are defined as follows: Coupling C P1 10 log( ) P3 Directivit y D P3 10 log( ) P 4

2 P1 Isolation I 10 log( ) P4 Here P i denotes the power measured (in linear scale) at the i th (i=1, 2, 3 and 4) port of the DC. Since practically the powers at different ports are measured in dbm using power meters, we can use the following relations for calculation of coupling (C), directivity (D) and isolation (I) in db scale: C ( P1 P3 D ( P3 P4 I ( P1 P4 C( D( The directivity of a coupler is the measure of the coupler s ability to isolate forward and backward waves, as is the isolation [1]. An ideal DC would have infinite directivity and isolation. But practically if the port-2 of the DC is not terminated by matched load, it will lead to some finite reflection. In that case, the voltage at port-4 can be written as: 4 I C ( 1 L 1 ) ( C D C L e j ) Here, C and D are the coupling and directivity in the linear scale, when the ratios are calculated in voltage scale, instead of the power scale. Γ L is the reflection coefficient at port-4: L l L e 2 j l The quantities β, l and λ are the propagation constant, length traversed by the wave and the operating wavelength respectively, which yield the value θ, the delay in the phase. So the power measured at port-4 is given by: j 2 P P( C D C e B. Scalar Network Analyzer (SNA) using DC: 4 1 L ) The DC can be used as a Scalar Network Analyzer (SNA), which can measure the amplitudes (not phase like ector Network Analyzer i.e. NA) of the reflection and transmission parameters of a DUT (device-undertest), provided the coupling and directivity are known. The schematic diagram for the measurement of S 11, which is a measure of the reflection coefficient, is shown in Figure 9.2 below: Figure 9.2 Arrangement for Measurement of S 11 ( of the DUT

3 Let P i (dbm) be the input power provided by the RF source. If C P and I P be the coupling and isolation of the DC in linear power scale, then the power incident on the DUT (in linear scale) is given as: P x P i ( CPPi I P i P ) (1) Let P y (dbm) be the power reflected from the DUT. If the power meter connected at port-4 shows a reading of P z (dbm) then we can write: P P C( (2) y z Then the S 11 ( of the DUT is given as: S11( Py Px (3) For measuring S 21 (, which corresponds to the transmission coefficient of the DUT, we make an arrangement according to the schematic diagram shown in Figure 9.3. If P o (dbm) be the output power of the DUT as measured by the power meter we can obtain S 21 ( from the following equation: S21( Po Px (4) Figure 9.3 Arrangement for Measurement of S 21 ( of the DUT Finally we can use a ratio meter to obtain the S 11 ( and S 21 ( simultaneously using the schematic in Figure 9.4 below. Figure 9.4 Schematic diagram showing usage of ratio meter to calculate S 11 and S 21 (in of the DUT

4 Here the scattering parameters S 11 ( and S 21 ( can be measured according to the formulae below: S11( Pb Pa S21( Pc Pa C( In this case we assume that the power flowing into the DUT is equal to the input power from the RF Source. Procedure: A. Characteristics of the Directional Coupler: To start the virtual experiment the user has to download the file directionalcoupler.exe from the website and run it. A GUI (graphical user interface) as shown in Figure 9.5 will appear: Figure 9.5 Screenshot from the virtual experiment of directional coupler The provision for changing values of the coupling (, directivity (, and magnitude of reflection coefficient of the load ( are provided in the top left side of the GUI. The default input power provided at port-1 is 0 dbm. We can connect RF power source at port-2 also (since directional coupler is reciprocal device) but not at ports 3 and 4. For ports 2, 3 and 4, we can either terminate them with matched load or connect the power meter to measure the power that is coming out of the ports. It should be noted that we cannot connect the power-meter at more than one-port simultaneously.

5 Figure 9.6 Snapshots from different parts of the GUI showing the results when input power is 0 dbm and power meter is connected at different ports Figure 9.6 shows the power (in dbm) at ports 2, 3 and 4 for default values of input power, coupling factor, directivity and reflection coefficient at the load. We can observe that since some finite load reflection coefficient (30 is present, the isolation (= db as calculated from definition) is not equal to the sum of coupling factor (=20 and directivity (=30. It can be found that the isolation becomes 50 db when the reflection coefficient of the load is made 80 db. B. Scalar Network Analyzer application: For this virtual experiment the users have to run the file sna.exe. A GUI (as shown in Figure 9.7) will pop-up. The directional coupler parameters, coupling ( and directivity ( can be specified initially. The default values for these parameters are 20 db and 30 db respectively. Figure 9.7 Schematic diagram for virtual experiment with Scalar Network Analyzer

6 After that, there is provision for selecting a DUT. By default, three DUTs with fixed parameters are specified in the application: (a) low-pass filter, (b) band-pass filter and (c) high-pass filter. All of them work in the default frequency range 1 to 5 GHz. The user can also manually provide the values of start frequency, stop frequency and frequency increment (all in GHz). Accordingly the slider-control of frequency values will change its scale. The user can enter the operating frequency value and measure the power at different ports of the directional coupler for a given input power. The termination-condition at the ports 2, 3 and 4 for S 11 ( and S 21 ( measurement can be done by following the theory given in the previous section of this manual. Figure 9.8(a) Figure 9.8(b) (a) Selecting the DUT text file option and browsing for the file (b) Format for writing the DUT text Along with the default DUTs, the user can also select.txt files characterizing any arbitrary DUT by specifying the magnitude of scattering parameters (S 11 and S 21 ) over a given frequency range. The format for writing the.txt files is shown in Figure 9.8(b). The left-most column contains the frequency values in GHz. The second and third columns have the values of S 11 and S 21 respectively in linear scale. The start, stop and increment frequencies cannot be specified manually here, since they are automatically specified according to the.txt file. After all the measurements with the power-meter are complete and the user presses the stop button the screen changes to that shown in Figure 9.9, where the ratio meter configuration (like Figure 9.4) comes into the picture. The variation of S 11 and S 21 of the device in db within the specified frequency range is shown in two network-analyzer-like panels.

7 Figure 9.9 Screenshot of the GUI which comes after pressing the stop button; The Ratio meter configuration as in Figure 9.4 can be seen here. C. Example: Characterization of the Band-Pass Filter using SNA: In this section we are going to show how the DC and power meter can be used for characterizing a DUT. We have taken the example of a band pass filter, one default DUT. Step-1: The Coupling ( and Directivity ( values are kept at 20 db and 30 db (default values). Step-2: The frequency settings are kept at: Start Frequency=1 GHz. Stop Frequency=5 GHz, Frequency Increment=0.1 GHz. Step-3: For calculating the S 11 (, the DC and power-meter are kept according to the settings shown in Figure The input power is kept at 0 dbm. The frequency can be varied via slider-control or by manually entering the values. The power at port-4 for different frequencies has to be noted down. Since the coupling and directivity values are known beforehand, the power input to the DUT (P x ) can be computed from equation-(1). The power reflected from the DUT (P y ) can be found out from equation-(2). Then using equation-(3) we can calculate the S 11 (. In Figure 9.11 (a), (b) and (c), we show the power-meter readings for three frequency values (1.5 GHz, 2.6 GHz and 4 GHz). The measured power is very low (almost -48 dbm) at 2.6 GHz which is inside the passband of the filter. Step-4: In Figure 9.12, the arrangement of DC and power meter for measurement of S 21 ( is shown. From the reading of the power meter, we can get the value of P o and then calculate S 21 ( from equation-(4). Like the previous step, in figures 9.13 (a), (b) and (c), the power meter readings for 1.5 GHz, 2.6 GHz and 4 GHz are

8 shown. The power coming out of the DUT is dbm for the frequency 2.6 GHz, which falls inside the filter pass-band. Step-5: When the stop button is pressed, the ratio meter configuration is shown and the plot of S 11 and S 21 over the frequency range 1 to 5 GHz is displayed (Figure 9.14). The users can tally their results with these plots. Figure 9.10 Settings of the DC and Power Meter for Measuring S 11 ( Figure 9.11(a) Figure 9.11(b) Figure 9.11(c) Measuring S 11 : Readings of the power-meter at port-4 (port-2 and port-3 being terminated with matched load) for three frequency values (a) 1.5 GHz (b) 2.6 GHz (c) 4 GHz Similar experiments can be carried out for low-pass filter and high pass filters. The users can also input the S- parameters of other two-port devices like amplifier or attenuator and do the measurements.

9 Figure 9.12 Settings of the DC and Power Meter for Measuring S 21 ( Figure 9.13(a) Figure 9.13(b) Figure 9.13(c) Measuring S 21 : Readings of the power-meter at output port of DUT (port-3 and port-4 being terminated with matched load) for three frequency values (a) 1.5 GHz (b) 2.6 GHz (c) 4 GHz Figure 9.14(a) Figure 9.14(b) REFERENCES: ariation of S 11 ( and S 21 ( with frequency for the DUT (Band Pass Filter) 1. Microwave Engineering, Third Edition, D M Pozar. 2. Principles of Microwave Measurement, J. H. Bryant.