Q17 of 26 Page 1

(a) State three important factors showing the need for translating a low frequency signal into a high frequency wave before transmission.

(b) Draw a sketch of a sinusoidal carrier wave along with a modulating signal and show how these are superimposed to obtain the resultant amplitude modulated wave.


a) The low frequency signals need to be translated into high frequency waves before transmission. Three reasons for this are


i. For efficient transmission and receiving the signals, antennas must have lengths equal to quarter wavelength of the signal. Setting up vertical antennas of such big size is practically impossible.


ii. The energy radiated from an antenna is practically zero. The power radiated at audio frequency is quite small; hence, transmission occurs in loss.


iii. The various information signals transmitted at low frequency get mixed with noise and hence cannot be distinguished.


(b)



More from this chapter

All 26 →
15

Light of intensity ‘I’ and frequency ‘v’ is incident on a photosensitive surface and causes photoelectric emission. What will be the effect on anode current when (i) the intensity of light is gradually increased, (ii) the frequency of incident radiation is increased, and (iii) the anode potential is increased? In each case, all other factors remain the same.

Explain, giving justification in each case.


 16

When is a transistor said to be in active state? Draw a circuit diagram of a p-n-p transistor and explain how it works as a transistor amplifier. Write clearly, why in the case of a transistor (i) the base is thin and lightly doped and (ii) the emitter is heavily doped.

 18

You are given three circuit elements X, Y and Z. When the element X is connected across an a.c. source of a given voltage, the current and the voltage are in the same phase. When the element Y is connected in series with X across the source, voltage is ahead of the current in phase by π/4. But the current is ahead of the voltage in phase by π/4 when Z is connected in series with X across the source. Identify the circuit elements X, Y and Z. When all the three elements are connected in series across the same source, determine the impedance of the circuit. Draw a plot of the current versus the frequency of applied source and mention the significance of this plot.

 19

Write symbolically the nuclear ∈+decay process of C116. Is the decayed product X an isotope or isobar of ?Given the mass values m ( ) = 11.011434 u and m (X) = 11.009305 u. Estimate the Q-value in this process.