Avionics MCQs – Communication Receivers
1 - Question
Which of the following are the two main characteristics of receivers?
a) Selectivity and sensitivity
b) Gain and attenuation
c) Modulation and demodulation
d) Multiplexing and demultiplexing
Explanation: A communication receiver must be able to identify and select the desired signal from thousands of others present in the frequency spectrum (selectivity) and to provide sufficient amplification to recover the modulating signal (sensitivity). A receiver with good selectivity will isolate the desired signal in the RF spectrum and eliminate or at least greatly attenuate all other signals.
2 - Question
What circuit provides the initial selectivity in a receiver?
d) LC tuned circuits
Explanation: Initial selectivity in a receiver is normally obtained by using LC tuned circuits. By carefully controlling the Q of the resonant circuit, we can set the desired selectivity.
3 - Question
The ability to pick up week signals by a receiver is called _________
Explanation: A communication receiver’s sensitivity, or ability to pick up weak signals, is mainly a function of overall gain, the factor by which an input signal is multiplied to produce the output signal. In general, the higher the gain of a receiver, the better its sensitivity.
4 - Question
What is the input signal level that is approximately equal to the average internally generated noise value called?
Explanation: The MDS or Minimum Discernible Signal is the input signal level that is approximately equal to the average internally generated noise value. This noise value is called the noise floor of the receiver. MDS is the amount of signal that would produce the same audio power output as the noise floor signal. The MDS is usually expressed in dBm.
5 - Question
Analog signals : Signal to noise ratio:: Digital signals _________
a) Signal Attenuation
b) Bit error rate
c) Error rate
d) Digital noise ratio
Explanation: For analog signals, the signal-to-noise ratio is the main consideration in analog signals. For digital signal transmission, the bit error rate (BER) is the main consideration. BER is the number of errors made in the transmission of many serial data bits.
6 - Question
A sensitive and selective receiver can be made using only amplifiers, selective filters, and a demodulator.
Explanation: A sensitive and selective receiver can be made using only amplifiers, selective filters and a demodulator. This is called a tuned radio frequency or TRF receiver. Early radios used this design. However, such a receiver does not usually deliver the kind of performance expected in modern communications applications.
7 - Question
What type of receivers converts all incoming frequency into a lower frequency for better selectivity and sensitivity?
a) Analogous receivers
b) Superheterodyne receivers
c) Digital receiver
d) Crystal receiver
Explanation: Superheterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers and filters is used to provide a fixed level of sensitivity and selectivity. Most of the gain and selectivity in a superheterodyne receiver are obtained in the IF amplifiers.
8 - Question
What is the output of a mixer circuit?
a) Sum of frequencies
b) Difference of frequencies
c) Sum and difference of frequencies
d) Multiplication of frequencies
Explanation: The key circuit is the mixer, which acts as a simple amplitude modulator to produce sum and difference frequencies. The incoming signal is mixed with a local oscillator signal to produce this conversion.
9 - Question
What is connected between the antenna and the mixer to avoid oscillator radiation?
d) Power amplifier
Explanation: LNA or low noise amplifier is placed in between the antenna and the mixer. The oscillator signal is relatively strong and some of it can leak through and appear at the input of the mixer. The RF amplifier between the mixer and the antenna isolates the two, significantly reducing any local oscillator radiation.
10 - Question
Which of the following is not an advantage of using RF amplifiers in the initial stage of signal selection?
a) Low noise
b) Reduced oscillator radiation
c) Better selectivity
d) Less power consumption
Explanation: The use of RF amplifiers in the initial stage of signal selection reduces the effect of noise.RF amplifiers improve sensitivity, because of the extra gain; improve selectivity, because of the added tuned circuits; and improve the S/N ratio. Further, spurious signals are more effectively rejected, minimizing unwanted signal generation in the mixer.
11 - Question
The LNA of a transducer has failed during the transmission of a signal. Which component’s failure has caused this?
b) RF amplifier
Explanation: A duplexer is a device which acts as a switch and stops the high power transmission signals from entering into the receiver, which is tuned to handle very low power for better sensitivity. When the duplexer fails, the high power signals enter the receiver where the Low noise amplifier is damaged.
12 - Question
What type of oscillators are used to input a wide range of frequencies to the IF amplifier?
a) LC oscillators
b) Crystal oscillators
c) Variable frequency oscillators
d) Diode oscillators
Explanation: The local oscillator is usually a variable frequency oscillator or a frequency synthesizer so that its frequency can be adjusted over a relatively wide range. As the local-oscillator frequency is changed, the mixer translates a wide range of input frequencies to the fixed IF.