Question

QUESTION 2 (25 MARKS) In FM transmission circuits, explain the purpose and function of: a. i. Pre-emphasis and de-emphasis circuits. (C01: P01 - 3 Marks) ii. Limiter circuit (C01: P01 - 3 Marks) b. A sinusoidal carrier wave of frequency fe = 110 MHz is modulated by a modulating signal m(t). The characteristics of the modulating signal is illustrated in Figure Q2(b) and the Frequency Deviation Constant of the FM Modulator, ks = 10Hz/volt. Determine the minimum bandwidth required for transmission of the FM signal. 6/10 (CO2: PO2 - 4 marks) IV m 2X10 sec Figure Q2(b)

Answer 1

Ta 9. Pre emphasis circult :- At the transmitter, the modulating signal is Paxped through a simple network which ampli- Hies the high frequency, components more than the low frequency components. The smiplest form of a high pass filter. At higher than cutoff frequency (to compo- nents amplifies. The output amplitude increas with frequecy at rate 6dB/ octave. This pre emphasis circuit increases the energy content of the higher frequency signals so that they will send to become stronger than the high frequency nofse components, DE- Emphasis! De-emphasis means attenuating thoxe frequencies by the amount by which they are boosted. Pre emphasis is done in at the transmitter and de emphasis is done in the receiver, main purpose of these to improve the signal to ncelpe ratio, Hpest). Hadeld) = 1 CS Scanned with CamScanner

mw. Hpelt) Pre emphasis filter FM BPF mical Ines Yu-) Detector Deemphant six circuit Harlt) FM Pre-emphaped output R Era = OJBL De-emphasis DEM Audio out I Odg CS Scanned with CamScanner

Cal limiter circuit: A limiter is a circult that allows signals below a&becified input power as level to paxt unaffee ted while attenuating the peaks of strongen signal that enceed this threshold, → Leimiling is a type of dynamic range compression clipping is an extreme version of limiting. limiters are compon as a safety device in broadcast application to prevent sudden volum Peako forom Occuning. Limiters are also goed af Protective features in some components of amplia fiers, to prevent unwanted distortion. y limiter circuit as Half wave rectifier. -> Vo + Vo V8 t CS Scanned with CamScanner

ط &c=110MHz m(t) = sin umat This 2x10 sec tms 5kHz mit)= sin 27.5ooot doi dt Frequeney Deviation constant K6= 10 Hz /valt Sit)= Ac Cos (27: 110x108+ añKb Jmedt) Oi= Wet + da ko Jm t de Wetaalme = L9 Awe animit) kb 66 - (ax Im (4)1.Kb) lax 3) 1.107 At=10 But = 16 Bb= L Deviation ration | B= 2000 Bandwidth required for transmission of = 2x5X103 (Qovot! Im signal BW=20.01 MHz ON=ma Rh) 107 / bm Ablom 10/5000 - CamScanner