Short Questions and Answers on Digital Modulation Techniques


Q.1. State various CW modulation systems.

DRex Electronics

Ans. ASK, FSK, PSK, QPSK, MSK, M-ary PSK etc.

Q.2. Represent ASK mathematically.


Q.3. State the bandwidth requirement of ASK system.

Ans. Maximum Bandwidth

Bmax = 2 fHz.

Q.4. In BASK the information is transmitted via ………………..

Ans. Amplitude switching of the carrier.

Q.5. In a CW modulation system, the input signal is ……………….. and transmitted signal is ………………..

Ans. Digital, Analog (sinusoidal).

Q.6. In a BPSK system, the information is transmitted via ………………..

Ans. Phase change of the carrier.

Q.7. The phase difference between the two possible transmitted signals in BPSK is ………………..

Ans. 180° or π radians.

Q.8. Express the BPSK signal mathematically.


Q.9. What does √2Ps represent?

Ans.  √2Ps  is equal to amplitude A of the carrier.

Q.10. The noise immunity of BPSK system is …………………. than that of the BASK system.

Ans. Better.

Q.11. What type of receiver is used for the BPSK detection?

Ans. Synchronous detector.

Q.12. What is its speciality?

Ans. The received BPSK signal is multiplied with the locally generated synchronous carrier.

Q.13. What is the advantage of synchronous detection?

Ans. Error probability is reduced.

Q.14. What is the disadvantage of the BPSK receiver?

Ans. The receiver is complicated, because it requires a synchronous carrier to be generated locally.

Q.15. What is the role of bit synchronizer in the BPSK receiver?

Ans. The bit synchronizer will open and close the switched connected in the integrator circuit. It ensures that the integration will take place exactly over one bit duration.

Q.16. What is the maximum Bandwidth of BPSK system?

Ans. B.W. = 2 fb Hz.

Q.17. State the value of Euclidean distance for BPSK.


Q.18. State important disadvantages of DPSK.

Ans. (i) Noise in one bit interval causes error in two bit positions.

(ii) Error rate in DPSK is higher than that of BPSK.

(iii) The effect of noise is higher in DPSK than BPSK.

Q.19. State true or false: Errors occur in pairs in DPSK.

Ans. True

Q.20. QPSK is a ………………..

(a) Two level modulation (b) Multilevel modulation (c) None of them

Ans. Multilevel modulation

Q.21. The bit rate is ……………….. than baud rate in QPSK.

(a) Half (b) Double (c) Same

Ans. Double

Q.22. What are advantages of QPSK system?

Ans. (i) BW is small.

(ii) Noise immunity is high.

(iii) Bit rate doubles for same baud rate.

Q.23. What is the main advantage of M-ary PSK?

Ans. The main advantage of M-ary PSK is the reduction in the channel bandwidth decreases with increase in number of messages (M).

Q.24. Compare error probability of M-ary PSK and QPSK.

Ans. Probability of error increases as M is increased.

Q.25. Represent the M-ary PSK signal mathematically.


Q.26. State the expression for Euclidean distance for M-ary PSK.


Q.27. What is the Bandwidth of M-ary PSK?

Ans. BW = 2 f/N, Where N= Number of bits per message.