Unit 3 - 02 FM-Transmitter & Receiver- and Noise.pdf

FM Transmitter

• • • •

FM Broadcast Range 88 to 108 MHz. Channel Separation 200 KHz Frequency Deviation ∆ = 75 KHz FM Broadcast
Monophonic
Stereophonic ( Left and Right Channel)

FM Stereo Transmitter • • • • •

Signals L + R and L – R are Pre-Emphasized. Input Audio Signal (L-R)’ is Converted to NBFM by DSBSC Modulator. Uses Armstrong Method of FM Generation Carrier Used for this Modulation is 38 KHz. This carrier is Obtained from Pilot Source of 19 KHz via Frequency Doubler. Signal (L+ R)’ is used Directly

FM Transmitter Composite Baseband Signal for Final Wideband FM Modulation is

Pilot is Kept at 19 KHz as on its both side there is no signals up to 4 KHz.

FM receiver • FM receiver is similar to the superheterodyne layout • Only Difference is Envelope Detector is replaced by Limiter – Discriminator and De-Emphasis Circuits • RF is 88 to 108 MHz, IF is 10 MHz. RF

mixer

LO

IF

limiter

AF power amp

Discriminator

deemphasis

Receiver components: RF amplifier • AM may skip RF amp but FM requires it • FM receivers are called upon to work with weak signals (~1 (~1µV V or less as compared to 30 µ for AM) µV • An RF section is needed to bring up the signal to at least 10 to 20 µV before mixing

Limiter • A limiter is a circuit whose output is constant for all input amplitudes above a threshold • Limiter’s function in an FM receiver is to remove unwanted amplitude variations of the FM signal

Limiter

Limiting and sensitivity • A limiter needs about 1V of signal, called quieting or threshold voltage, to begin limiting • When enough signal arrives at the receiver to start limiting action, the set quiets, i.e. background noise disappears • Sensitivity is the min. RF signal to produce a specified level of quieting.

Sensitivity example • An FM receiver provides a voltage gain of 200,000(106dB) prior to its limiter. The limiter’s quieting voltage is 200 mV. What is the receiver’s sensitivity? 200 mV/200,000= 1µV->sensitivity of receiver

Discriminator • The heart of FM is this relationship fi(t)=fc+kfm(t)

• What we need is a device that linearly follows inst. frequency f is at the IF frequency carrier

Of 10.7 MHz (Fixed for all channels)

Disc.output

-75 KHz

+75 KHz fcarrier

Deviation limits

f

Examples of discriminators • Slope detector - simple LC tank circuit operated at its most linear response curve output

fc

fo

f

Phase-Locked Loop • PLL’s are increasingly used as FM demodulators and appear at IF output fin

Phase comparator

Error signal

Lowpass filter

Output proportional to Difference between fin and fvco

Control signal:constant When fin=fvco

fvco

VCO

VCO input

Zero crossing detector FM

Hard

limiter

Zero Crossing detector

Multivibrator

Averaging circuit

Output

FM input

Hard limiter

ZC detector

multiV Averaging circuit

more frequent ZC’s means higher inst freq in turn means Larger message amplitudes

FM Receiver • After Limiter and Discriminator, Signal is Passed through various Filter Circuits. • Output of Filters is De-Emphasized and Audio Signals obtained as L and R separate channels of Stereophonic Receiver. • Pilot Freq. Signal after extracted from Filter Used as Synchronous Detector Carrier for DSBSC Modulated Waves.

Noise In Communication System A. External Noise:

B. Internal Noise:

(1)Atmospheric Noise

(1) Thermal Agitation Noise

(2)Extraterrestrial Noise • Solar Noise • Cosmic Noise

(2) Shot Noise (3) Transit Time Noise

(3) Industrial Noise

Noise Calculations

(1)

Addition of Noise Due to Several Sources

(2)

Addition of Noise Due to Several Amplifier

NOISE IN ANALOG MODULATION AMPLITUDE MODULATION

Channel model •Distortionless •Additive White Gaussian Noise (AWGN) Flat noise spectrum:white noise

No/2

Noise power=hatched area -W

W

Receiver Model • The objective here is to establish a relationship between input and output SNR of an AM receiver Modulated signal s(t) BPF

detector

filter

output BT=2W

Noise n(t)

-fc

fc

Establishing a reference SNR • Define “channel” SNR measured at receiver input.

=

The output of the product modulator is

Therefore, any reduction in input SNR is linearly reflected in the output.

• Following a similar approach,

(SNR )o (SNR )c

2

ka P =