“I have no special talents. I am only passionately curious.”

– Albert Einstein

Questions on communication systems at the level expected from you are generally simple and interesting. Today we will discuss three questions (MCQ) on communication systems which appeared in Kerala Engineering Entrance 2007 question paper.

(1) The time variations of signals are given as in (A), (B) and (C).

Point out true statement from the following

(a) A, B and C are analog signals

(b) A and B are analog, but C is digital signal

(c) A and C are digital, but B is analog signal

(d) A and C are analog, but B is digital signal

(e) A, B and C are digital signals (e)

Digital signals will have two discrete levels only, corresponding to the zero level and one level as shown in graph (B). Analog signals have various instantaneous values, as shown in graphs (A) and (C). The correct option therefore is (d).

(2) A photo detector used to detect the wave length of 1700 nm, has energy gap of about

(a) 0.073 eV

(b) 1.2 eV

(c) 7.3 eV

(d) 1.16 eV

(e) 0.73 eV

The energy gap of a photo detector should be equal to or less than the energy of the photon which it is intended to detect. The product of the energy of a photon in *electron volt* and the wave length in *Angstrom* is 12400. Therefore, the energy of the 1700 nm photon is 12400/ 17000 electron volt = 0.73 eV. So, the correct option is (e). Option (a) is not suitable since a semiconductor with a gap as low as 0.073 eV (if at all available) will be unreliable due to the breaking of bonds by thermal excitation.

[You can use the expression *hc/λ* for calculating the energy of the photon in joule and then convert it into electron volts by dividing it by the electronic charge of 1.6×10^{–19 }joule. But it will be more difficult and time consuming].

(3) The optical fibres have an inner core of refractive index n_{1} and a cladding of refractive index n_{2} such that

(a) n_{1} = n_{2} (b) n_{1} ≤ n_{2} (c) n_{1} < n_{2} (d) n_{1} > n_{2} (e) n_{1} ≥ n_{2}

The cladding has to be *rarer *than the core so as to totally reflect the beam of light entering the optical fibre. So, the correct option is (d).