# Digital Communications BasicsRF Cafe Quiz #23

All RF Cafe quizzes would make perfect fodder in employment interviews for technicians or engineers - particularly those who are fresh out of school or are relatively new to the work world. Come to think of it, they would make equally excellent study material for the same persons who are going to be interviewed for a job.

Note: Some material based on books have quoted passages.

This quiz challenges your knowledge of digital communications basics.

1.  What is meant by binary frequency shift keying
(BFSK)?

a)  Shifting two frequencies to key a microphone
b)  Using two distinctly different frequencies to represent digital 0s and 1s
c)  Shifting digital information at a binary frequency
d)  Half of quadrature shift keying

2.  The constellation to the right indicates what type of modulation?

a)  QPSK
b)  OQPSK
c)  MSK
d)  Orion

3.  What is the primary benefit of offset QPSK (O-QPSK) over QPSK?

a)  Greater efficiency
b)  Easier to code
c)  Constant envelope power
d)  No benefit

4.  What is the phase shift for the QPSK modulator block diagram to the right?

a)  0°
b)  45°
c)  90°
d)  180°

5.  What is the image to the right an example of?

a)  Tropospheric waveguiding
b)  A VCO output
c)   A Spirograph drawing
d)  An eye pattern

6.  What is the purpose of the lowpass filter (LPF) in the QPSK modulator shown in Question 4?

a)  Prevents DC from entering the mixers
b)  Limits the bandwidth of the transmitted signal
c)  Adds a DC component to the signal
d)  Serves no purpose

7.  What is the spectrum to the right an example of?

b)  Octary Shift Keying (OSK)
c)  8-PSK

8.  Which of the following items affects the bit error rate (BER) of a received signal?

a)  Oscillator phase noise
b)  Baseband filtering
c)  Carrier suppression
d)  All the above

9.  Which of these issues presents a problem for zero-IF (direct conversion) receivers?

a)  LO leakage creating a DC offset voltage
b)  Mixer flicker noise is included in the baseband signal
c)  2nd-order intercept products
d)  All the above

10. What is the Z-transform of the Dirac delta function, δ[n] ?

a)  1
b)
c)
d)  ∞