Microwave Engineering Multiple Choice Questions on “Radiometer Systems”.
1. __________ system obtains information about a target by transmitting a signal and receiving the echo from the target.
A. Radar
B. Sonar
C. Radiometer
D. None of the mentioned
Answer: A
Clarification: Radar stands for radio detection and ranging. A radar system obtains information about a target by transmitting a signal and receiving the echo from the target. They are also called as active remote sensing systems.
2. Radiometry is a technique or the principle on which radar works.
A. True
B. False
Answer: B
Clarification: Radiometry is a passive technique which develops information about a target solely from the microwave portion of the blackbody radiation that it either emits directly or reflects from the surrounding bodies.
3. According to Planck’s radiation law, a body can radiate energy to the medium surrounding it under all conditions invariably.
A. True
B. False
Answer: B
Clarification: A body in thermal equilibrium only can radiate energy according to Planck’s radiation law. The radiating body has to be maintained in thermal equilibrium. In the microwave region this reduces to P=kTB, where k is the Boltzmann’s constant.
4. A major challenge in designing a radiometer is:
A. Design complexity
B. High cost
C. Requirement of highly sensitive receivers
D. None of the mentioned
Answer: C
Clarification: The basic problem with building a radiometer is to build a receiver that can distinguish between the desired external radiometric noise and the inherent noise of the receiver. This application thus requires that are highly efficient in detecting the required signal.
5. The receiver model of a total power radiometer is based on the:
A. AM receiver
B. FM receiver
C. Super heterodyne receiver
D. None of the mentioned
Answer: C
Clarification: The front end of a receiver is a standard super heterodyne circuit consisting of an RF amplifier, a mixer/ local oscillator, and an IF stage. These stages are the same as that used in a super heterodyne receiver.
6. The system bandwidth of a total power radiometer is determined by the:
A. RF amplifier
B. Local oscillator
C. IF filter
D. IF amplifier
Answer: C
Clarification: The system bandwidth of a total power radiometer is determined by the IF filter section present in the receiver circuit of the radiometer. The upper and lower cutoff frequency of the IF filter specify the system bandwidth.
7. The integrator circuit after the detector in the receiver circuit is used to smooth out the short term variations in the signal power.
A. True
B. False
Answer: B
Clarification: The integrator used is essentially a low pass filter with a fixed cutoff frequency, and serves to smooth out short term variations in the noise power.
8. The dominant factor affecting the accuracy of the total power radiometer is the variation of ___________
A. Gain in the overall system
B. The feedback circuit
C. Efficiency of the system
D. None of the mentioned
Answer: A
Clarification: The dominant factor affecting the accuracy of the total power radiometer is the variation of the gain of the overall system. Since such variations have relatively a longer time constant, it is possible to eliminate this error by repeatedly calibrating the device.
9. A Dicke radiometer is identical to the total power radiometer in all aspects except that they have different receiving antenna.
A. True
B. False
Answer: B
Clarification: In a Dicke radiometer, the input is periodically switched between the antenna and a variable power noise source. This switch is called Dicke switch. Repeatedly calibrating the device is the principle behind the operation of Dicke radiometer.
10. A typical radiometer would measure the brightness temperature over the range of about:
A. 50-300 K
B. 100-200 K
C. 400-500 K
D. None of the mentioned
Answer: A
Clarification: Typical radiometer would measure the brightness temperature over the range of about 50-300K. This then implies that the reference noise source would have to cover the same range, which is difficult to achieve practically.
,