Short Questions and Answers on Radio Receivers
Q.1. How can we improve the image rejection capability?
Ans. Image rejection depends on the front end selectivity of the receiver i.e. the selectivity of the RF circuit. The use of RF amplifier improves the image frequency rejection.
Q.2. What is the value of Standard intermediate frequency for AM radio?
Ans. 455 kHz.
Q.3. What is double spotting?
Ans. Double spotting means the same station gets picked up at two different nearby points on the receiver dial.
Q.4. Are the IFs for MW and SW bands different?
Ans. No, IF is same for both.
Q.5. What is the effect of increase in intermediate frequency on the image frequency rejection?
Ans. Image frequency rejection improves with increase in IF.
Q.6. What are the drawbacks of a TRF receiver?
Ans.
(i) Instability
(ii) Variation in bandwidth with change in carrier frequency.
(iii) Low sensitivity
(iv) Poor adjacent channel rejection (poor selectivity).
Q.7. Define selectivity.
Ans. It is the ability of a receiver to reject unwanted signals.
Q.8. Which frequency bands are used in the commercial AM radio receivers?
Ans.
(i) Medium wave-band (MW): 530 kHz to 1650 kHz.
(ii) Short wave band (SW): 3 MHz to 30 MHz
Q.9. A circuit that blocks the audio until a signal is received is called as a …………. circuit.
Ans. Squelch.
Q.10. If sensitivity of three receivers are 10 mV, 15 mV and 6 mV respectively at 1000 kHz which one is the most sensitive?
Ans. The one with a sensitivity of 6 mV.
Q.11. What are the standard values of m, fm and output power for selectivity measurement?
Ans. Same as those for sensitivity measurement (m = 30%, fm = 400 Hz, Pout = 50 mW)
Q.12. The ……. circuit in superheterodyne receiver compensates for a wide range of input signal levels.
Ans. AGC circuit.
Q.13. What is the position of volume control in sensitivity measurement?
Ans. Volume control should be full.
Q.14. Why is the local oscillator frequency always higher than the signal frequency?
Ans. So as to ensure that the ratio Cmax/Cmin of the variable ganged capacitor can be practically obtainable.
Q.15. Why is dummy antenna used in sensitivity measurement?
Ans. To couple maximum power to the radio receiver from Signal generator.
Q.16. Why is delayed AGC preferred over simple AGC?
Ans. Because in delayed AGC, the weak input signals do not get suppressed, as the gain reduction does not take place for weak signals.
Q.17. What are the values of “m” and ” fm” in sensitivity measurement?
Ans. m = 30%, fm = 400 Hz.
Q.18. What is the use of BFO (beat frequency oscillator) in the communication receiver?
Ans. BFO is used for receiving the coded messages in Morse code. It makes the dots, dashes and spaces in the Morse code audible to the operator.
Q.19. What is the standard output power in sensitivity measurement?
Ans. 50 mW.
Q.20. If the same station is getting tuned at 1600 kHz and 700 kHz, then, what is the value of IF?
Ans. The difference between the two points (frequencies) at which the same station is received, is always 2 IF. Therefore, 2 IF = (1600 – 700) = 900 kHz. Thus, IF = 450 kHz.
Q.21. Which blocks of the AM receiver affect the sensitivity?
Ans. The RF amplifier and IF amplifiers decide the sensitivity of a receiver.
Q.22. What is the cause of diagonal clipping in a diode detector?
Ans. The cause is slow RC time constant
Q.23. Define sensitivity.
Ans. It is the ability of the receiver to amplify weak signals, it is measured in mV or in dB below 1V.
Q.24. Which blocks decide the level of selectivity?
Ans. The frequency response of IF amplifiers decide the selectivity. The responses of mixer and RF amplifier also play a small but significant role.
Q.25. Cascading tuned circuits cause the selectivity to………….
Ans. Increase.
Q.26. Define fidelity. Which block determines the fidelity?
Ans. It is the ability of ‚a receiver to reproduce all the modulating frequencies equally. The AF amplifier determines fidelity of a receiver.
Q.27. Images are caused by the lack of………..at the mixer input.
Ans. Front end selectivity.
Q.28. Two receivers A and B are available with the bandwidths of their AF amplifiers equal to 8 kHz and 15 kHz. Which one has better fidelity?
Ans. B, i.e. the one with 15 kHz bandwidth.
Q.29. The key conceptual circuit in superheterodyne receiver is……..
Ans. Mixer circuit.
Q.30. What is the image of 1000 kHz Signal if IF = 470 kHz?
Ans. Image frequency = fs + 2 IF = 1000 + (2 x 470) = 1960 kHz.
Q.31. If f1 and f2 are the frequencies of signals applied to a mixer, then, what are the frequencies present at the mixer output?
Ans. Assuming f1 > f2, the frequencies at mixer output are: f1, f2, (f1 – f2) and (f1 + f2).
Q.32. What type of AGC is used in the simple commercially available radio receivers?
Ans. Simple AGC.
Q.33. The selectivity of a superheterodyne receiver……………with increase in signal frequency.
Ans. Remains constant
Q.34. Decreasing Q of a resonant circuit causes the bandwidth to …………….
Ans. Increase.
Q.35. Why the IF of a receiver should not be too high?
Ans. Because, then, it results in poor selectivity and hence poor adjacent channel rejection. This is due to difficulties in obtaining a high Q at high intermediate frequency.
Q.36. Why the IF of a receiver should not be too low?
Ans. Because then the image rejection is poor and selectivity is too sharp cutting the sidebands.
Q.37. The dc AGC voltage is derived from a………..circuit
Ans. From the output of the detector circuit.
Q.38. Define detection or demodulation.
Ans. It is the process of recovering back the original modulating signal from the AM wave.
Q.39. AGC voltage controls the gain of…………
Ans. RF amplifiers and IF amplifiers.
Q.40. How to choose the RC time constant of the filter in a diode detector?
Ans. The RC time constant should be slow enough to keep the RF ripple as small as possible but it should be fast enough to enable the detector circuit to follow the fastest modulation variations.
Q.41. For a dual conversion superheterodyne, if the input frequency is 50 MHz and LO frequencies are 60 MHz and 11 MHz, then the two IFs are………..and …………
Ans. 10 MHz and 1 MHz.
Q.42. Why does negative peak clipping take place?
Ans. Due to the fact that modulation index at the detector output is higher than at its input, there is a possibility of overmodulation. This overmodulation leads to diagonal clipping.
Q.43. The main cause of image interference is……..
Ans. Poor front end selectivity.
Q.44. What is the function of AGC?
Ans. To keep the receiver output constant by automatic adjustment of overall receiver gain.
Q.45. What is delayed AGC? Is the delay introduced in time domain?
Ans. The delay is not introduced in the time domain. Here, delay means that the AGC bias is not applied until the input signal strength reaches a predetermmed level.
Q.46. Where is the delayed AGC used?
Ans. In the communication receivers.
Q.47. The sensitivity of a receiver depends on the receiver’s overall…….
Ans. Gain.
Q.48. What is double conversion? Why is it used?
Ans. Double conversion means the process of mixing (heterodyning) is carried out twice. This will produce two IFs. The first IF is high so as to ensure better image rejection, and the second one is low to get the advantages such as good adjacent channel rejection and high selectivity.
Q.49. What is the application of communication receiver?
Ans. Communication receiver is not meant to receive the entertainment signals but it has been designed for reception of signals used in communications.
Q.50. A large input signal causes the gain of the receiver to be………..by the AGC.
Ans. Reduced.
Q.51. What is the function of squelch or muting circuit?
Ans. The squelch circuit is used to cut off the first audio amplifier when the input signal is absent. This will ensure that the noise is not allowed to pass through to the loud speaker.
Q.52. If the Q of a tuned circuit is 250 at the resonant frequency of 500 kHz. Then, bandwidth is……..
Ans. BW = 2 kHz.
Q.53. Tuning a superhet receiver is done by varying the frequency of its…….
Ans. Local oscillator.
Q.54. Most of the gain and selectivity in a superhet receiver is obtained in the-…….
Ans. IF amputier.
Q.55. In the double conversion receiver, there are two…………circuits.
Ans. Mixer.