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DIRECTIONAL COUPLER Objective: To characterize the stripline based directional coupler. Equipment: Power Meter, Source(VCO), DC power supply, matched terminations Theory: A directional coupler is a hybrid wave guide joint which couples power in to an auxiliary arm in one direction. It is a four-port device and contains the following characteristics. 1 2 Directional 4 3 Coupler Characteristics: If a power is fed into port(1), the power is coupled only to ports(2) and (3). i.e., power flows in the forward direction of the auxiliary arm port(3) but no power couples to port(4), i.e., in backward direction. Similarly power fed in (2) coupled into ports(1) and (4) and not in port(3). All the four ports are matched, i.e., if three of them are terminated in matched loads, the forth port is automatically terminated in matched load. The phase difference between the two output ports is 900. The characteristics of directional couplers are expressed in two parameters namely directivity, isolation and coupling defined as follows. Directivity(D) of directional coupler is defined as the ratio, expressed in decibles of the power output in the coupled auxiliary arm to the power flowing in uncoupled auxiliary arm. Theoretically, D should be infinite because no power should couple to arm 4. D = 10log10(P2/P4) db Coupling factor or Coupling is defined as the ratio, expressed in decibles, of the power (entering) at input of the main line to the power coupled at the output in the auxiliary arm. C = 10log10(P3/P1) db The isolation of the directional coupler is between the ports 1 and ports 4. Since the port 4 is also called isolated port. I = 10log10(P1/P4) db The directional coupler which is used in this experiment is that broad side coupled striplines based one. Striplines are transmissions lines like waveguides but these reduces the devices manufacturing sizes to considerable range. This is the one advantages of using striplines for making devices. The directional coupler is a –3 dB coupler, i.e., the coupled power is less by 3 dB compared to input power. This is dual directional coupler, means it contains two directional couplers of coupling –8.34 dB each. These two are joined tandem to get –3 dB coupling. The two coupled striplines are fabricated on either side of the dielectric material of dielectric constant 2.94 as shown in the figure below. BLOCK DIAGRAM: Directional Power +5V VCO Coupler Meter Bias voltage 1V Procedure: 1. Connect the VCO to the power meter.(Care must be taken while connecting sensor to device). 2. Apply the voltages VD and VT(=1V) to the VCO. 3. Measure the power (P1) at the output of VCO using power meter. 4. Connect the Directional coupler to the VCO output and measure the power available at the port 3 and terminate the other two ports with the matched termination. Let this power to be P2. 5. Then calculate the coupling coefficient using formula C = 10log10(P3/P1) db 6. Measure the power at port2 and then the power at port4 for same input port In each case the other two ports should be terminated with matched load. 7. Calculate the directivity using the formula D = 10log10(P3/P4) db 8. Measure the power at the port 4 with other two ports terminated with loads. Let it be P4. 9. Then calculate the isolation between the ports 1 and 4 using the formula I = 10log10(P4/P1) db Precautions: 1. Care must be taken while connecting the SMA Connector. 3. Sensor of power meter should not rotate.
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