Question

a. In FM transmission circuits, explain the purpose and function of: i. Pre-emphasis and de-emphasis circuits. (CO1: PO1 - 3 Marks) ii. Limiter circuit. (CO1: 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, kf = 107 Hz/volt. Determine the minimum bandwidth required for transmission of the FM signal. (CO2: PO2 - 4 marks) m(t) 1 V 2 X 10 sec Figure Q2(b)

Answer 1

a)

Noise usually will have higher amplitude and higher frequency components. This high frequency noise will lead to frequency distortion when its amplitude is higher than the components present in the modulating signal. To overcome this situation most of the FM circuits use techniques known as Pre-emphasis at the transmitter and De-emphasis at the receiver.

Pre-emphasis

The Pre-emphasis helps amplify high frequency signal components such that they will have magnitude higher than noise components. This lead to improvement in the Signal to Noise Ratio i.e. SNR.

As shown in the figure, it consists of R and C components such that t = R1*C where in t should be about 75 µs . This circuit will have lower frequency cutoff at 2123 Hz. As mentioned in the curve, all the frequencies higher than 2123 is amplified at the rate of 6dB/octave.

7.5F 6dB/octave Frequency Modulator Pre-emphasized FM Output R1 (10K) R2 (1K) 3dB OdB 23,343 Hz Pre-emphasis circuit 2123 Hz Pre-emphasis curve

Pre-emphasis circuit also has upper cutoff frequency from where signal enhancement will be flatten.
Upper cutoff frequency, Fu = R1+(R2/(2*pi*R1*R2*C))
Due to this characteristics, it is simple High Pass Filter with amplification.

De-emphasis

The De-emphasis does exactly reverse of the Pre-emphasis counterpart. It is used at the receiver part. It helps bring pre-amplified signal back to the normal amplitude level. It is a simple Low Pass Filter with time constant of about 75 µs .

EM FM Demodulator Audio Out OdB -3dB IN 2123 Hz De-emphasis circuit De-emphasis curve

De-emphasis circuit will have cutoff frequency of about 2123 Hz. To bring the signal back to normal level, all the frequency components above 2123 Hz is attenuated at 6dB/octave.

As explained, pre-emphasis operation performed at the transmitter is compensated by de-emphasis operation at the receiver. Both these modules help increase amplitude of high frequency signal while in transmission. Hence the same can not be masked or obliterate by the noise.

Ref : https://www.rfwireless-world.com/Terminology/Pre-emphasis-vs-De-emphasis.html

Limiter circuit

The purpose of the limiter is to provide a constant level of signal to the FM demodulator, thus reducing the effect of signal level changes in the output. For instance, if two or more signals are received at the same time, a high performance limiter stage can greatly reduce the effect of the weaker signals on the output. This is commonly referred to as the FM capture effect. The limiter also reduces the effect of noise spikes.

b)

(b) 110 MHz m(t) IM-modulated Constant Kg - 10 Hz /var Frequency Deviation Minimum bandwidth required for Transmission m(t) im 2x10 " seci - -4 5kHz tm = 2x 10 fm 2x109 Upk-pk = 2 IV Am t Modulated S m(a) dd S = A cos (watt 275 kq signal FM fm (t) Smit) ( mara Kf Am (0) ان Constant p = 2U K f = Ⓡ000 phase 3 fm 5*10 2 2 (p+1) from Bandwidth = 2 (2000+1) 5k 3 2 (10) 2 x 10% (10 +5) 5x1 +5X10 03 ) 1 6 20.0 1 x 10 Bandwidth = 20.01 MHZ