Frequency Measurement in Microwave Engineering .pptx

Frequency Measurement in Microwave Engineering

1. Microwave Measurement At low frequency , some eqpt are interfaced to the microwave signals when the signal carries voltage and current in wires. At lower frquencies , the measuring and testing eqpts used for microwave measurements are not simple. Each setup requires the careful understanding regarding the impact of test equipt on the cct which is to be measured and system under test. LO 11.2.7

2. Microwave Measurement Test eqpt Microwave test eqpts are of five types. Slotted line carriage Tunable detector Power meter Spectrum analyser VSWR meter LO 11.2.7

3. Power measurement Measurement of microwave power plays an important role in microwave engineering. It determine the output powers of generators and decides the functioning of transmitting and receiving system . Microwave power defined same as power at low frequencies i.e it is the product of voltage , rms current and power factor. Pav = Irms.Vrms.Cos Φ Measurement of very low power (< 1 mW ) Measurement of low power (< 10 mW ) Measurement of high power ( 10 mW to 10W) LO 11.2.7

4. Frequency Measurement Microwave frequency is measured by using a commercially available spectrum analyser and cavity wavemeter . The frequency also can be computed from measured guide wavw length in a voltage standing wave pattern along a short circuited line by using slotted line .there are two techniques' to measure the microwave frequency . Mechanical technique Electronic technique LO 11.2.7

5. Attenuation measurement Attenuation measurement defined as α = 10 log 10 (Pi/Po) Pi= Input power to the device Po= Output power from the device Attenuation measurement can be performed by two methods. ( i ) Direct or power ratio measuring method. (ii) RF substitution method. LO 11.2.7

6. VSWR MEASURMENTS Voltage Standing Wave Ratio (VSWR) VSWR : 1.30 Reflection Coefficient : 13% (0.13) VSWR : 1.20 Reflection Coefficient : 9.1% (0.091) VSWR : 1.10 Reflection Coefficient : 4.7% (0.047) VSWR : 1.08 Reflection Coefficient : 3.8% (0.038) VSWR : 1.06 Reflection Coefficient : 2.9% (0.029) VSWR = 1 + (Reflection Coefficient) 1 - (Reflection Coefficient) Reflected Signal Incident Signal LO 11.2.7

7. STANDING WAVE RATIO-SWR SWR = Standing Wave Ratio SWR is the ratio of the maximum and minimum values of a standing wave. It can be expressed in terms of the Forward and Reverse voltages or currents It is usually based on voltages thus Voltage Standing Wave Ratio - VSWR SWR = V MAX / V MIN SWR = (V FORWARD + V REVERSE ) / (V FORWARD -V REVERSE ) : 1 LO 11.2.7

8. Impedance Matching Method used to measure the impedance at microwave frequency are Slotted line method Reflectometer method LO 11.2.7

9. Return Losses Return losses Return Loss is an alternative expression for match based on the ratio of forward and reflected power and is expressed in dB Return Loss, dB =10.Log ( P FORWARD / P REVERSE ) or 20.Log (V FORWARD / V REVERSE ) For info, it is related to SWR: Return Loss, dB = 20.Log ( (SWR-1) / (SWR+1) ) LO 11.2.7

10. Return losses Return losses The loss/attenuation calculations are composed of three main contributions Propagation losses (due to earths atmosphere and terrain) Branching losses (comes from using hardware used to deliver the transmitter/receiver output from/to the antenna Miscellaneous losses(unpredictable and sporadic in character like fog, moving object crossing the path, poor eqpt installation and less than antenna alignment LO 11.2.7

11. Phase Shifter AM Gen Isolator H Plane Tee Adjustable phase shifter Isolator Network of phase shifter measured H Plane Tee Crystal Detector CRO LO 11.2.7

12. Antenna Measurement To determine the actual performance of ant and for the purpose obtaining design data measurements are made. Radiation pattern measurement. Beam width measurement. Gain measurement. LO 11.2.7