M-ARY FSK

M-ary Frequency Shift Keying (M-FSK) is a digital modulation technique

that uses multiple frequencies to represent data. It's an extension of
Frequency Shift Keying (FSK), where instead of just two frequencies, it
employs a set of M frequencies. Each of these M frequencies represents a
unique symbol, allowing for the transmission of more information per
symbol compared to binary FSK.

Working Principle of M-ary FSK:

1. M distinct symbols (e.g., 00, 01, 10, 11 for M=4) are represented using M different
carrier frequencies.
2. At every symbol time, one of the M frequencies is selected to transmit based on the
input bits.
3. Non-coherent or coherent detection methods can be used at the receiver to
demodulate the signal.

Transmitter Working of M-ary FSK:

 Input binary data is grouped into k-bit blocks.

 Each k-bit block represents one of M symbols (where M = 2^k).
 Each symbol is assigned a unique frequency f0,f1,...,fM−1.
 The transmitter selects the frequency corresponding to the symbol.
 A sine wave at that frequency is generated and transmitted for one symbol period T.
 This process repeats for each symbol.

Receiver Working of M-ary FSK:

 The receiver captures the incoming signal.

  It uses M bandpass filters (non-coherent) or correlators (coherent), each tuned to a
different frequency.
 The filter or correlator that gives the highest output indicates the transmitted
frequency.
 The detected frequency is mapped back to its corresponding symbol.
 The symbol is then converted back to the original k-bit group.

Applications of M-ary FSK:

 Wireless sensor networks

 Satellite and telemetry systems
 Low-power RF communication (e.g., RFID)
 Modems and data links in noisy environments

Advantages of M-ary FSK:

 Supports higher data rates than binary FSK

 Good noise immunity, especially with non-coherent detection
 Simple hardware implementation for small M
 Effective in low signal-to-noise ratio (SNR) environments

What is Delta Modulation?

 A digital modulation technique used to encode analog signals.

 Converts analog signal into a binary stream representing changes (deltas), not
absolute values.
  Only 1-bit is used per sample:
o 1 if the signal increased
o 0 if the signal decreased

Delta modulation (DM) is a method of analog-to-digital and digital-to-analog conversion

used primarily for speech transmission, where quality is not a top priority

🔹 Working Principle (Step-by-Step)

 The analog signal is sampled at regular intervals.

 Each sample is compared with the previous estimate of the signal.

 If the signal increases, output is 1 (increase estimate by Δ).

If the signal decreases, output is 0 (decrease estimate by Δ).

 This produces a binary stream of 1s and 0s .

 At the receiver, an accumulator or intergrator is used on each bit to reconstruct the

analog signal.

Transmitter Working of Delta Modulation:

 The input analog signal is first sampled at uniform time intervals.

 The current sample is compared with the previous estimate of the signal by using a
comparator.
 If the current sample is higher, the modulator outputs 1, and the estimate is increased
by a fixed step size Δ.
 If the current sample is lower, the modulator outputs 0, and the estimate is decreased
by Δ.
 The difference (delta) is not sent—only a single bit (1 or 0) indicating the direction
of change is transmitted.
 The process continues for each sample, producing a 1-bit binary stream.

✅ Receiver Working of Delta Modulation:

 The receiver gets the binary stream (1s and 0s).

 It uses an accumulator or integrator to reconstruct the signal.
 For each bit received:
o If it is 1, the estimate is increased by Δ.
o If it is 0, the estimate is decreased by Δ.
 A 1-bit DAC converts this sequence of steps into an approximate analog signal by
adding or subtracting step size to update the estimate
 The output is a reconstructed version of the original analog signal, but with a
staircase shape.
Advantages

 Simple implementation (1-bit logic)

 Requires low bandwidth
 Suitable for real-time voice transmission
 Less complex than PCM (Pulse Code Modulation)

🔹 Disadvantages

 Slope Overload Distortion: if signal changes too fast

 Granular Noise: if step size is too large during slow signal change
 Requires high sampling rate to avoid distortion.

🔹 Applications

 Voice encoding in telephony

 Low-bandwidth audio transmission
 Military and wireless communication systems
Pulse Amplitude Modulation (PAM)

Definition

 Pulse Amplitude Modulation (PAM) is a modulation technique in which the amplitude of

each pulse is varied according to the instantaneous value of the analog message signal.
 The pulses are equally spaced in time, and only their amplitudes change with respect to the
signal.
 It is a basic form of pulse modulation and serves as a foundation for other techniques like
PCM.
 PAM can be of two types: single polarity (all positive pulses) and double polarity (positive
and negative pulses).

Generation of PAM Signal

 The analog message signal is first applied to a sampler circuit, which takes periodic
samples of the signal.
 The sampling process must satisfy the Nyquist sampling theorem, i.e., the sampling
frequency should be at least twice the highest frequency in the message signal.
 These sampled values are then used to modulate the amplitude of a series of short-
duration pulses generated by a pulse generator.
 Each pulse occurs at a fixed time interval but has an amplitude that matches the
sampled value of the analog signal at that instant.
 The resulting waveform is a train of pulses with varying amplitudes, where each
pulse represents a snapshot of the original signal at specific time intervals.
 The PAM signal can then be transmitted through a communication channel for further
processing or conversion into a digital format like PCM.

Demodulation of PAM Signal

 The demodulator receives the PAM waveform consisting of amplitude-varying pulses.

 These pulses are passed through a low-pass filter (LPF) to remove high-frequency
components, particularly those introduced by the sharp pulse edges.
 The LPF reconstructs a continuous-time analog signal by joining the tops of the
amplitude-modulated pulses.
 This process is often aided by a sample-and-hold circuit which maintains each
sampled value until the next sample arrives, aiding in better signal reconstruction.
 The output of the filter approximates the original analog signal that was initially
sampled and modulated.
 The accuracy of demodulation depends on how well the sampling and filtering are
performed and whether noise has distorted the signal during transmission.
Overall Working

 The analog signal is sampled, and each sample is converted into a pulse with an amplitude
matching the sample value.
 This pulse train is transmitted over the channel.
 At the receiver, the pulses are filtered to reconstruct the original signal.
 The system ensures information is preserved in the amplitude of discrete pulses, enabling
analog-to-pulse conversion.
Generation and Detection of Binary FSK (BFSK) Signal

Generation of Binary FSK Signal

 Binary Frequency Shift Keying (BFSK) is a digital modulation technique where two different
carrier frequencies are used to represent binary 1 and 0.
 A high frequency is assigned to binary 1 (called f1) and a low frequency is assigned to
binary 0 (called f2).
 The amplitude of the signal remains constant; only the frequency changes according to the
binary data.
 A binary data stream is fed into a FSK modulator.
 The modulator consists of:
o Two oscillators: one generating f1 and another generating f2.
o A digital switch or multiplexer to choose between the two frequencies based on the
binary input.
 When the input bit is 1, the switch selects oscillator output at frequency f1, and when the
input is 0, it selects frequency f2.
 The output is a continuous waveform with alternating frequencies based on the bit pattern,
forming the BFSK signal.

Detection of Binary FSK Signal

 At the receiver, the BFSK signal is passed into a demodulator that detects which frequency is
present during each bit interval.
 There are two main detection methods:
1. Coherent Detection:
 The receiver uses reference oscillators synchronized with the transmitter's
f1 and f2.
 Two correlators or matched filters detect the presence of either f1 or f2
during each bit interval.
 The output with the higher correlation determines whether the bit was 1 or
0.
 2. Non-Coherent Detection:
 No phase synchronization is required.
 The received signal is passed through two bandpass filters, one tuned to f1
and another to f2.
 Each filter output is then fed into an envelope detector to measure signal
strength.
 The stronger output determines whether the bit was 1 or 0.
 The recovered binary data is then reconstructed based on the frequency detected in each
interval.

Summary

 BFSK uses two frequencies to represent binary 1 and 0.

 The modulator selects the appropriate frequency based on the input bit stream.
 The demodulator detects the frequency during each bit period to recover the original binary
data.
 BFSK is more noise-resistant than ASK, but requires more bandwidth.
Difference Between BPSK and BFSK

Aspect BPSK (Binary Phase Shift Keying) BFSK (Binary Frequency Shift Keying)

Modulation Type Phase modulation Frequency modulation

Modulated
Phase of the carrier Frequency of the carrier
Parameter

Signal
Representation

Waveform Phase shifts by 180∘180^\circ180∘ for bit

Frequency changes for bit change
Difference change

Bandwidth High (typically 2× bit rate due to

Low (equal to bit rate)
Requirement frequency separation)

Error Slightly worse than BPSK in coherent

Better (less error at same SNR)
Performance systems

Power Efficiency More power-efficient Less power-efficient

Complexity of Requires coherent detection (needs phase Can work with non-coherent
Receiver synchronization) detection (simpler, less accurate)

Spectral Lower due to wide spacing of

High
Efficiency frequencies

Moderate; improved with coherent

Noise Immunity High (especially in coherent systems)
detection

Application Satellite communication, CDMA, deep- Low-power radio (e.g. Bluetooth,

Examples space links RFID), early modems

📝 Conclusion

 BPSK is more power- and bandwidth-efficient, but needs accurate phase synchronization.
 BFSK is simpler and robust against some channel distortions, but less efficient.
Quadrature Amplitude Modulation (QAM)

Definition

 Quadrature Amplitude Modulation (QAM) is a digital modulation technique that combines

both Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK).
 In QAM, data is transmitted by varying both the amplitude and the phase of a carrier signal.
 It uses two carrier waves that are 90° out of phase (called in-phase (I) and quadrature (Q)
components).
 The combination of different amplitude levels and phase shifts allows multiple bits to be
transmitted per symbol.
 For example, 16-QAM transmits 4 bits per symbol, 64-QAM transmits 6 bits per symbol, and
so on.

Generation of QAM Signal

 A digital bit stream is first grouped into blocks (e.g., 4 bits per symbol for 16-QAM).
 These blocks are mapped to symbols using a constellation diagram that defines specific
amplitude and phase combinations.
 Each symbol is split into two components:
o The in-phase (I) component modulates a cosine carrier wave.
o The quadrature (Q) component modulates a sine carrier wave (90° phase shifted).
 The two modulated waves are then summed to produce the final QAM signal:

s(t)=I(t)⋅cos⁡(2πfct)+Q(t)⋅sin⁡(2πfct)s(t) = I(t) \cdot \cos(2\pi f_c t) + Q(t) \cdot \sin(2\pi f_c

t)s(t)=I(t)⋅cos(2πfct)+Q(t)⋅sin(2πfct)

 The result is a single waveform whose amplitude and phase vary according to the input bits.

Demodulation of QAM Signal

  At the receiver, the QAM signal is passed into coherent detectors synchronized with the
original carrier wave.
 The signal is split into two parts and multiplied with:
o a cosine wave to extract the in-phase (I) component
o a sine wave to extract the quadrature (Q) component
 These components are then passed through low-pass filters to remove high-frequency
terms.
 The resulting I and Q signals are compared with the reference constellation map to
determine the original bits.
 A symbol-to-bit converter then reconstructs the original binary sequence.

Overall Working

 QAM transmits information using two modulated carrier signals that are phase-shifted by
90°.
 The amplitude and phase combinations allow multiple bits to be carried in each symbol.
 The receiver extracts the I and Q components, compares them to the reference
constellation, and recovers the transmitted data.
 QAM provides higher data rates but requires better signal-to-noise ratio for accurate
detection.

Applications of QAM

 Used in digital television broadcasting (e.g., DVB-C cable systems).

 Widely applied in modems (e.g., DSL, cable modems).
 Employed in Wi-Fi standards like IEEE 802.11 (e.g., 16-QAM, 64-QAM, 256-
QAM).
 Used in cellular communication (e.g., LTE, 4G, 5G systems).
 Applied in optical fiber communication for high data rate transmission.
 Essential in software-defined radios and broadband wireless systems.

✅ Advantages of QAM

 High spectral efficiency: Transmits multiple bits per symbol (e.g., 64-QAM = 6
bits/symbol).
 Better bandwidth utilization compared to ASK, FSK, or PSK.
  Adaptability: Modulation order (e.g., 16, 64, 256-QAM) can be varied based on
channel quality.
 Effective for high-speed data transmission in both wired and wireless systems.
 Combines amplitude and phase variations, offering a trade-off between noise
resistance and data rate.

Special Features of QAM

 Combination of Amplitude and Phase Modulation:

QAM modulates both amplitude and phase of the carrier signal, unlike PSK or ASK
which use only one.
 High Data Rate Capability:
Each symbol in QAM can represent multiple bits (e.g., 4 bits in 16-QAM, 6 bits in 64-
QAM), enabling faster transmission.
 Constellation Diagram Representation:
QAM uses a 2D constellation diagram with unique amplitude-phase pairs, making it
easy to visualize and decode.
 Variable Modulation Order:
Supports different modulation orders (like 16-QAM, 64-QAM, 256-QAM), allowing
flexible trade-off between data rate and noise performance.
 Efficient Use of Bandwidth:
QAM offers higher spectral efficiency, making it ideal for bandwidth-limited
applications.
 Adaptive Modulation:
In modern systems like 4G/5G, QAM can adapt its order based on channel
conditions, improving reliability and performance.
 Used in Both Wireless and Wired Communication:
Suitable for various media, including optical fibers, radio, TV, Wi-Fi, and cellular
networks.

Phase Shift Keying (PSK)

Definition

 Phase Shift Keying (PSK) is a digital modulation technique in which the phase of the
carrier signal is varied according to the digital data.
 The amplitude and frequency remain constant, only the phase changes.
 Each distinct phase represents a unique bit pattern.
 Common types:
o BPSK (Binary PSK) – 2 phases → 1 bit/symbol
o QPSK (Quadrature PSK) – 4 phases → 2 bits/symbol
o 8-PSK, 16-PSK, etc., for higher data rates.

Generation of PSK Signal

  A binary input bit stream is first grouped (e.g., 2 bits for QPSK).
 The bit group is mapped to a specific phase value (e.g., 0°, 90°, 180°, 270° for
QPSK).
 The carrier signal is phase-shifted accordingly using a phase modulator.
 The modulator changes the phase of the carrier signal without changing its
amplitude.
 The output is a PSK-modulated signal where each symbol duration carries phase-
modified carrier.

Demodulation of PSK Signal

 The received PSK signal is passed into a coherent demodulator.

 The demodulator uses a locally generated reference carrier that is synchronized in
phase with the transmitter.
 A phase detector compares the incoming signal's phase with the reference carrier.
 The result is then mapped back to the original bit sequence based on the detected
phase.
 In non-coherent PSK, synchronization is not required but performance is slightly
lower.

Overall Working

 PSK encodes digital data by shifting the phase of the carrier.

 At the transmitter, phase changes are introduced based on input bits.
 At the receiver, these phase changes are detected and converted back to the original
binary data.
 PSK is more bandwidth-efficient and noise-resistant than ASK.

✅ Applications of PSK

 Used in Wi-Fi (IEEE 802.11) and Bluetooth communication.

 Common in satellite and space communication.
 Applied in modems and RFID systems.
 Used in digital cellular systems like 2G and 3G.

✅ Advantages of PSK

 Better noise immunity compared to ASK.

 Efficient bandwidth usage.
 Suitable for long-distance communication.
 Constant amplitude signal → less power fluctuation and simpler amplification.
Pulse Width Modulation (PWM)

Definition

 PWM is a pulse modulation technique where the width (duration) of each pulse is varied in
proportion to the amplitude of the modulating (analog) signal.
 The amplitude and position of the pulse remain constant.
 PWM is used to convert analog signals into pulse signals and is widely applied in control
systems and communication.

Generation of PWM Signal

  The modulating signal (analog input) is compared with a sawtooth or triangular waveform
using a comparator circuit.
 When the analog signal voltage is greater than the sawtooth, the output is high (logic 1).
 When it is less, the output is low (logic 0).
 The result is a pulse waveform with varying widths, directly related to the amplitude of the
analog signal at each instance.

Demodulation of PWM Signal

 The PWM signal is passed through a low-pass filter (LPF).

 The filter removes the high-frequency components and retrieves the original analog signal.
 In some circuits, a monostable multivibrator or integrator is used before the LPF to smooth
out the signal.
 The output closely follows the original input signal’s amplitude variation.

Overall Working

 PWM converts analog signals to digital pulses with variable width.

 These pulses carry information about the instantaneous amplitude of the analog signal.
 The pulse train can be transmitted efficiently, and the analog waveform is recovered using a
filter at the receiver.

✅ Applications of PWM

 Used in DC motor speed control and servo motors.

 Widely used in power electronics (e.g., inverters, switching regulators).
 Found in audio amplifiers and signal generation.
 Used in LED dimming and brightness control.
 Applied in communication systems for analog signal transmission.

✅ Advantages of PWM

 High efficiency in power delivery systems.

 Noise immunity due to digital nature.
 Simple generation and demodulation techniques.
 Allows precise control of power to devices (motors, LEDs, etc.).
 Less power loss in switching devices due to on/off operation.
Pulse Position Modulation (PPM)

Definition

 Pulse Position Modulation is a pulse modulation technique where the position (timing) of
each pulse is varied in proportion to the amplitude of the modulating signal.
 The amplitude and width of the pulses remain constant.
 PPM encodes information by shifting the position of the pulse within a prescribed time slot.

Generation of PPM Signal

 The analog input signal is first converted into PWM (Pulse Width Modulated) signal.
 Then, PWM is converted to PPM using a monostable multivibrator.
 The monostable circuit triggers a pulse at the trailing edge of the PWM pulse.
 The position of this pulse in time depends on the width of the PWM pulse, which itself
depends on the amplitude of the modulating signal.
 Thus, the position of the output pulse represents the instantaneous amplitude of the analog
signal.

Demodulation of PPM Signal

  The PPM signal is first passed into a synchronizing circuit to establish time slots.
 The position of the pulse within each time slot is then measured using a timing circuit.
 This timing information is converted into a voltage level, representing the original analog
signal.
 A low-pass filter is often used to smooth out the recovered signal.

Overall Working

 PPM converts analog signal information into pulses with constant amplitude and width, but
varying time positions.
 The varying pulse position carries the analog signal’s amplitude information.
 The receiver measures the timing difference to reconstruct the original signal.

✅ Applications of PPM

 Used in optical communication systems (e.g., IR remote controls).

 Found in space communication due to noise immunity.
 Used in radio-controlled (RC) systems for encoding control signals.
 Applied in ultrasound and radar systems.

✅ Advantages of PPM

 Better noise immunity than PAM and PWM.

 Constant amplitude pulses reduce power loss and make it suitable for optical transmission.
 Efficient bandwidth usage compared to PWM.
 Simple and reliable in systems where synchronization is available.

PAM vs PWM vs PPM

 PAM (Pulse
PWM (Pulse Width PPM (Pulse Position
Feature Amplitude
Modulation) Modulation)
Modulation)

Amplitude of pulse Width (duration) of Position (timing) of

Definition
varies with signal pulse varies pulse varies
Pulse Amplitude Varies Constant Constant
Pulse Width Constant Varies Constant
Pulse Position Fixed Fixed Varies
Generation Requires PWM as
Simple Moderate
Complexity intermediate step
Uses integrator and Requires synchronizer
Demodulation Uses a low-pass filter
filter and timing detector
Noise Sensitivity High Moderate Low
Synchronization Not critical Not critical Critical
Bandwidth High (due to narrow
Less Moderate
Requirement pulses)
Power Efficiency Low High High
Motor control, power Optical, IR, wireless,
Applications Audio systems, DSL
electronics aerospace
Transmission Type Analog Analog or Digital Digital

✅ Summary:

 PAM is simplest but highly noise-prone.

 PWM improves noise resistance and power efficiency.
 PPM provides best noise immunity but requires precise timing synchronization.

Modulation

Modulation is the process by which a characteristic of a high-frequency carrier wave—such

as its amplitude, frequency, or phase—is varied in accordance with the instantaneous
amplitude of the low-frequency message (information) signal. The carrier wave is a
sinusoidal wave of much higher frequency than the message signal.

Mathematically, if the message signal is m(t)m(t)m(t) and the carrier wave is

c(t)=Accos⁡(2πfct)c(t) = A_c \cos(2 \pi f_c t)c(t)=Accos(2πfct), modulation produces a signal
s(t)s(t)s(t) whose amplitude, frequency, or phase depends on m(t)m(t)m(t).
✅ Need for Modulation

1. To reduce the antenna size

o Without modulation, transmitting low-frequency signals would require very large
antennas (hundreds of meters long).
o Modulation increases frequency, allowing the use of practical-sized antennas.
2. To enable multiplexing (multiple signal transmission)
o Modulation allows multiple signals to be transmitted over the same channel without
interference (frequency division multiplexing).
3. To improve signal range and coverage
o High-frequency carrier waves can travel longer distances and better penetrate
obstacles.
4. To reduce noise and interference
o Modulated signals are less affected by noise than baseband (unmodulated) signals.
5. To match channel bandwidth
o Communication channels often have fixed bandwidth. Modulation allows efficient
use of available bandwidth.

Types of Modulation

Modulation techniques are categorized based on how the carrier wave is altered to transmit
information:

1. Analog Modulation

In analog modulation, a continuous carrier wave is varied in proportion to the instantaneous

amplitude of the analog message signal.

 Amplitude Modulation (AM):

Carrier amplitude varies with the message signal; frequency and phase remain
constant.
Example: AM radio.
 Frequency Modulation (FM):
Carrier frequency varies according to the message signal amplitude; amplitude and
phase remain constant.
Example: FM radio.
 Phase Modulation (PM):
Carrier phase is varied in direct proportion to the message signal amplitude; amplitude
and frequency remain constant.
Example: Used in telemetry and angle modulation systems.
2. Digital Modulation

Digital modulation encodes digital data by changing the carrier wave’s amplitude, frequency,
or phase in discrete steps.

 Amplitude Shift Keying (ASK):

Carrier amplitude switches between discrete levels representing bits (0 and 1).
Example: Optical communications.
 Frequency Shift Keying (FSK):
Carrier frequency shifts between discrete frequencies to represent digital bits.
Example: Caller ID systems.
 Phase Shift Keying (PSK):
Carrier phase shifts between discrete angles to encode digital bits.
o BPSK (Binary PSK): Two phases (0°, 180°) represent 1 bit per symbol.
o QPSK (Quadrature PSK): Four phases represent 2 bits per symbol,
increasing data rate.
Example: Wi-Fi, Bluetooth.
 Quadrature Amplitude Modulation (QAM):
Combines amplitude and phase changes for higher bit rates, representing multiple bits
per symbol.
Example: Digital TV, cable modems.

3. Pulse Modulation

Pulse modulation converts the message signal into pulses with varying properties:

 Pulse Amplitude Modulation (PAM): Pulse amplitude varies according to the

message.
 Pulse Width Modulation (PWM): Pulse width (duration) varies with message
amplitude.
 Pulse Position Modulation (PPM): Pulse position shifts in time according to
message amplitude.

Summary Table

Modulation
Parameter Varied Analog/Digital Example Applications
Type
AM Amplitude Analog AM Radio
FM Frequency Analog FM Radio
PM Phase Analog Telemetry
ASK Amplitude (Discrete levels) Digital Optical Communications
FSK Frequency (Discrete values) Digital Caller ID
PSK Phase (Discrete shifts) Digital Wi-Fi, Bluetooth
Digital TV, Cable
QAM Amplitude & Phase Digital
Modems
 Modulation
Parameter Varied Analog/Digital Example Applications
Type
PAM, PWM, Pulse amplitude, width,
Pulse Modulation Digital Data Transmission
PPM position

Working of AM Superheterodyne Receiver

🔹 1. Introduction

The Superheterodyne Receiver is a type of radio receiver that converts all incoming radio
frequencies to a fixed intermediate frequency (IF), making amplification and filtering more efficient
and stable.

It was invented by Edwin Armstrong and is widely used in AM, FM, and other communication
systems due to its selectivity and sensitivity.

🔹 2. Basic Block Diagram

arduino
CopyEdit
Incoming RF → RF Amplifier → Mixer → IF Amplifier → Detector → Audio
Amplifier → Loudspeaker
↑
Local Oscillator

🔹 3. Block-by-Block Working

📍 a) Antenna

 Captures the incoming modulated AM signal (e.g., at 1 MHz).

 Passes it to the RF amplifier.

📍 b) RF Amplifier

 Amplifies the weak received signal.

 Improves the signal-to-noise ratio (SNR).
 Selects the desired frequency band.

📍 c) Local Oscillator (LO)

 Generates a frequency that is offset from the received signal.

 Typically:

fLO=fRF+fIFf_{LO} = f_{RF} + f_{IF}fLO=fRF+fIF

 e.g., if fRF=1 MHzf_{RF} = 1 \text{ MHz}fRF=1 MHz and fIF=455 kHzf_{IF} = 455 \text{ kHz}fIF
=455 kHz, then
fLO=1.455 MHzf_{LO} = 1.455 \text{ MHz}fLO=1.455 MHz

📍 d) Mixer

 Mixes RF signal and LO signal to produce:

fIF=∣fLO−fRF∣f_{\text{IF}} = |f_{LO} - f_{RF}|fIF=∣fLO−fRF∣

 Output includes:
o fRFf_{RF}fRF
o fLOf_{LO}fLO
o Intermediate frequency (IF) =455= 455=455 kHz

📍 e) IF Amplifier

 Amplifies the fixed intermediate frequency (e.g., 455 kHz).

 High selectivity and stable gain due to fixed frequency.
 Most of the receiver's gain and filtering is applied here.

📍 f) Detector / Demodulator

 Extracts the original audio signal from the modulated carrier.

 For AM, typically an envelope detector is used.

📍 g) Audio Amplifier

 Amplifies the demodulated audio signal to drive a loudspeaker or headphones.

🔹 4. Intermediate Frequency (IF)

 Standard IF for AM: 455 kHz

 Chosen to balance:
o Good selectivity
o Adequate image frequency rejection
o Ease of filtering

🔹 5. Advantages of Superheterodyne Receiver

 High selectivity due to fixed IF filtering

 Better sensitivity with gain concentrated at IF
 Easier to design high-quality filters at a single frequency
 Superior performance over wide frequency ranges
📝 Conclusion

The AM Superheterodyne Receiver efficiently converts varying RF signals to a fixed IF, simplifying
design and improving performance. It is the standard architecture used in almost all modern AM
radios due to its reliability and high quality of reception.

Narrowband FM Detection using Foster-Seeley Discriminator

🔹 1. Introduction

The Foster-Seeley discriminator is a frequency demodulation circuit used to detect Narrowband

Frequency Modulated (NBFM) signals. It is a phase-shift discriminator that converts frequency
variations of the FM signal into corresponding amplitude variations, which are then demodulated.

 Best suited for NBFM (modulation index β<1\beta < 1β<1)

 Provides good linearity for small deviations
 Operates around a fixed intermediate frequency (IF)

🔹 2. Circuit Diagram Description

The key components are:

 Input Transformer with center-tapped secondary

 Two Diodes (D1, D2)
 Two Capacitors and Two Load Resistors
 Tank Circuit (tuned to IF)
🔹 3. Working Principle

 The FM signal is passed to the resonant transformer circuit, tuned to the center frequency.
 The voltage across the secondary of the transformer is phase-shifted based on the
frequency deviation from the center frequency.
 The two diodes (D1 and D2) rectify the signals.
 The difference between the voltages across the two halves of the transformer gives a DC
voltage that varies with the frequency of the input.

🔹 4. Operation

At Center Frequency:

 The voltages at both diodes are equal.

 Output = 0 volts (no audio signal)

When Frequency Increases:

 Phase of the signal shifts.

 Diode D1 conducts more → output becomes positive.

When Frequency Decreases:

 Phase shifts in the opposite direction.

 Diode D2 conducts more → output becomes negative.

→ The output is a voltage proportional to frequency deviation, which corresponds to the original
message signal.
🔹 5. Advantages

 Simple circuit
 High sensitivity for small deviations (ideal for NBFM)
 Good linearity around center frequency

🔹 6. Limitations

 Not suitable for wideband FM

 Requires precise tuning
 Sensitive to amplitude variations (often needs a limiter stage before it)

✅ Conclusion

The Foster-Seeley Discriminator is a classic and reliable method for Narrowband FM demodulation,
converting frequency shifts into proportional voltage changes with a symmetric and balanced
detection approach.

Narrowband FM (NBFM) vs Wideband FM (WBFM)

🔹 1. Definitions

✅ Narrowband FM (NBFM):

 Frequency modulation with a small frequency deviation (Δf\Delta fΔf) and low modulation
index (β<1\beta < 1β<1).
 Bandwidth is close to twice the message frequency.
 Used in voice communication like two-way radios, walkie-talkies.

✅ Wideband FM (WBFM):

 Frequency modulation with a large frequency deviation and higher modulation index (β>1\
beta > 1β>1, often β>5\beta > 5β>5).
 Bandwidth is much larger and follows Carson’s Rule.
 Used in FM broadcasting, where high fidelity is required.

🔹 2. Generation Methods

🔸 NBFM Generation:

 Can be generated using direct modulation (voltage-controlled oscillator)

 Or by generating phase modulation (PM) with an integrator + phase modulator
🔸 WBFM Generation:

 Usually generated by:

1. Direct method: Using a reactance modulator with a crystal oscillator.
2. Indirect method (Armstrong method): Generating NBFM, then using frequency
multipliers to achieve large deviation.

🔹 3. Mathematical Formulas

 Modulation Index β=Δffm\beta = \frac{\Delta f}{f_m}β=fmΔf

Where:

 Δf\Delta fΔf = frequency deviation

 fmf_mfm = maximum modulating frequency
 Bandwidth using Carson’s Rule:

BW=2(Δf+fm)BW = 2(\Delta f + f_m)BW=2(Δf+fm)

🔹 4. Differences:

Feature Narrowband FM (NBFM) Wideband FM (WBFM)

Modulation Index β\betaβ β<1\beta < 1β<1 β>1\beta > 1β>1

Frequency Deviation Δf\Delta

Small (typically < 5 kHz) Large (typically ≥ 75 kHz)
fΔf

≈ 2(Δf+fm)2(\Delta f + f_m)2(Δf+fm
Bandwidth ≈ 2fm2f_m2fm
)

Simple generation and

Complexity Complex generation and detection
detection

Noise Immunity Lower Higher

Application Voice, mobile radio FM radio broadcasting

Audio Quality Moderate High-fidelity audio

Example Walkie-talkie FM radio (88–108 MHz)

Conclusion

 NBFM is bandwidth-efficient and simple, suitable for low-quality voice communication.

 WBFM offers high fidelity and noise resistance, ideal for broadcasting applications.