Experiment No.
1: To Study AM Modulator and
Determine the Modulation Index
Aim:
To study the working of an Amplitude Modulation (AM) modulator circuit and determine the
modulation index.
Theory:
Amplitude Modulation (AM) is a modulation technique in which the amplitude of a carrier
signal is varied in proportion to the amplitude of the modulating signal. The modulation index
(m) quantifies the extent of modulation and is calculated using:
Modulation Index (m) = (Maximum amplitude of modulated signal - Minimum amplitude
of modulated signal) / (Maximum amplitude of modulated signal + Minimum amplitude
of modulated signal)
= – / +
Carrier Signal: A high-frequency sine wave used to carry the information.
Modulating Signal: A low-frequency signal (e.g., audio) that contains the information.
Modulated Signal: The resultant signal after combining the carrier and modulating
signals.
When m<1, the signal is under-modulated; when m=1, it is fully modulated; and when m>1, it
is over-modulated, leading to distortion.
Apparatus Required:
1. Function Generator
2. Oscilloscope
3. Resistors: 2.2 kΩ
4. Capacitor: 3-40 pF
5. Inductor: 10 µH
6. Diode: IN4148
7. Breadboard
8. Connecting wires
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Circuit Diagram:
Working Principle:
1. Carrier Signal Generation: The function generator produces a high-frequency
sinusoidal signal (carrier wave) within the range of 1-100 MHz.
2. Modulating Signal Input: The audio input signal (e.g., from a microphone or audio
source) is applied to the circuit.
3. Diode Mixing: The carrier signal and modulating signal are combined using a diode.
The diode acts as a non-linear element, creating harmonics and mixing products.
4. Filter Circuit (LC Tank): The LC tank circuit (inductor and capacitor) acts as a
bandpass filter, selecting the desired AM signal frequency.
5. AM Modulated Output: The output of the LC tank circuit is the AM modulated signal,
which carries the information from the audio signal on the carrier wave.
Experimental Procedure:
1. Circuit Setup:
Connect the function generator to the input of the circuit.
Connect the oscilloscope to the output of the circuit.
Assemble the circuit on the breadboard as shown in the diagram.
2. Carrier Signal Generation:
Set the function generator to produce a sinusoidal wave at a frequency within
the range of 1-100 MHz.
Adjust the amplitude of the carrier signal to a suitable level.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
3. Modulating Signal Input:
Apply an audio signal (e.g., from a microphone or audio source) to the
circuit.
4. Observation:
Observe the output waveform on the oscilloscope.
Note the amplitude variations of the carrier wave in response to the audio signal.
Measure the frequency of the carrier wave and the modulated signal.
Measure the maximum and minimum amplitudes of the modulated wave on the
oscilloscope.
5. Calculation of Modulation Index:
Calculate the modulation index (m) using the formula:
= – / +
6. Data Collection:
Record the values of the carrier frequency, modulating signal frequency, and
modulation index.
Repeat the experiment for different carrier frequencies and modulation indices.
Experimental Table:
Results and Discussion:
Present the collected data in a tabular format.
Plot the graphs of the carrier signal, modulating signal, and AM modulated signal.
Analyze the effect of different carrier frequencies and modulation indices on the AM
signal.
Compare the experimental values of the modulation index with the theoretical values
(if available).
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Discuss the limitations of the circuit and possible improvements.
Observations:
The oscilloscope should display the AM modulated signal with amplitude variations
corresponding to the audio signal.
The frequency of the AM modulated signal should be equal to the carrier frequency.
The modulation index can be calculated from the peak-to-peak amplitude of the
modulating signal and the peak amplitude of the carrier.
Discussion:
The circuit provides a basic understanding of AM modulation.
Limitations include potential distortion due to non-linearity of the diode and the
simplicity of the filter circuit.
Possible improvements include using a more sophisticated filter circuit, incorporating
a buffer stage, and using a more efficient diode.
Safety Precautions:
Ensure the power supply is properly grounded.
Avoid touching the circuit while it is powered on.
Use appropriate safety measures when working with high frequencies.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Experiment No. 2: To Study AM Demodulator
Aim:
To study the working of an AM demodulator circuit and observe how the original modulating
signal is recovered from the amplitude-modulated (AM) signal.
Theory:
Amplitude demodulation is the process of extracting the original modulating signal
(information signal) from an amplitude-modulated (AM) wave. A commonly used demodulator
circuit for AM is the envelope detector, which consists of a diode, a capacitor, and a resistor.
The envelope detector works as follows:
1. The diode rectifies the incoming AM signal (allowing only positive cycles).
2. The capacitor charges and discharges to follow the envelope of the modulated signal.
3. The resistor determines the discharge rate and ensures smooth recovery of the original
signal.
The demodulated signal is observed at the output of the circuit.
Mathematical Explanation:
The AM signal is represented as:
( ) = [1 + ⋅ ( )] ⋅ (2 )
Where:
: Modulation index
( ): Modulating signal
: Carrier frequency
The demodulator recovers ( ), the original modulating signal.
Apparatus Required:
1. Function Generator
2. Oscilloscope
3. Resistors: 10 kΩ
4. Capacitor: 0.01 µF
5. Diode: 1N34
6. Breadboard
7. Connecting wires
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Circuit Diagram:
Working Principle:
1. AM Signal Input: The AM modulated signal from the previous experiment or an
external source is applied to the input of the circuit.
2. Diode Detection: The diode acts as a non-linear element, allowing current to flow only
in one direction. This results in the rectification of the AM wave, producing a half-wave
rectified waveform.
3. RC Filter: The RC filter (resistor and capacitor) smooths the rectified waveform,
removing the high-frequency components (carrier signal) and leaving behind the low-
frequency audio signal.
4. Recovered Audio Signal: The output of the RC filter is the recovered audio signal,
which can be observed on the oscilloscope.
Experimental Procedure:
1. Circuit Setup:
Connect the function generator to the input of the circuit.
Connect the oscilloscope to the output of the circuit.
Assemble the circuit on the breadboard as shown in the diagram.
2. AM Signal Input:
Apply the AM modulated signal from the previous experiment or an external
source to the input of the circuit.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
3. Observation:
Observe the output waveform on the oscilloscope.
Note the presence of the recovered audio signal.
Adjust the values of the resistor and capacitor to optimize the demodulation
process.
4. Data Collection:
Record the values of the resistor and capacitor used in the circuit.
Note the quality of the recovered audio signal (e.g., distortion, noise).
Experimental Table:
Results and Discussion:
The envelope detector successfully recovered the original modulating signal from the
AM wave.
The amplitude of the recovered signal depended on the RC time constant.
Discussion Points:
1. A higher value of the RC time constant resulted in smoother recovery but
delayed response.
2. A lower RC time constant led to faster response but increased ripple.
3. Accurate demodulation required a balance between smoothness and
responsiveness in the RC network.
Conclusion
The experiment demonstrated the working of an AM demodulator using an envelope
detector. The original modulating signal was successfully extracted, verifying the
principle of amplitude demodulation.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Experiment No. 3: To Study FM Modulation
Aim:
To study the process of Frequency Modulation (FM) and observe the variation of the carrier
frequency with respect to the instantaneous amplitude of the modulating signal.
Theory:
Frequency Modulation (FM) is a technique in which the frequency of the carrier wave is varied
in proportion to the instantaneous amplitude of the modulating signal, while the amplitude of
the carrier remains constant. This technique is widely used for high-fidelity audio transmissions
and other communication systems due to its resilience to noise and interference.
Carrier Frequency (fc): The central frequency of the unmodulated carrier wave.
Frequency Deviation (Δf): The maximum shift of the carrier frequency from its central
frequency due to modulation.
Modulation Index (β): The ratio of the frequency deviation to the modulating signal
frequency:
β = Δf / fm
Bandwidth: Calculated using Carson's Rule:
BW = 2(Δf + fm)
FM offers superior noise immunity and fidelity compared to Amplitude Modulation (AM),
making it suitable for applications like FM radio broadcasting, TV sound signals, and telemetry.
Apparatus Required:
1. Function Generator
2. Oscilloscope
3. Resistors (values will depend on the specific FM modulator circuit used)
4. Capacitors (values will depend on the specific FM modulator circuit used)
5. Inductors (values will depend on the specific FM modulator circuit used)
6. Active components (transistors, operational amplifiers, etc.) for the FM modulator
circuit
7. Breadboard
8. Connecting wires
Circuit Diagram:
The specific circuit diagram will depend on the chosen FM modulator topology. Some common
approaches include:
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Varactor Diode-Based FM Modulator: This circuit utilizes the voltage-dependent
capacitance of a varactor diode to vary the resonant frequency of an LC tank circuit.
Voltage-Controlled Oscillator (VCO) Based FM Modulator: A VCO is an electronic
circuit that generates an output signal whose frequency is directly proportional to an
applied input voltage. The modulating signal is used to control the input voltage of the
VCO, thus modulating the frequency of the output signal.
Working Principle:
Varactor Diode-Based FM Modulator:
1. Carrier Signal Generation: An LC tank circuit generates the carrier signal.
2. Modulating Signal Input: The modulating signal is applied to the varactor diode.
3. Capacitance Variation: The capacitance of the varactor diode changes in response to
the modulating signal.
4. Frequency Modulation: The change in capacitance alters the resonant frequency of
the LC tank circuit, resulting in frequency modulation of the carrier signal.
VCO-Based FM Modulator:
1. Modulating Signal Input: The modulating signal is applied to the input of the VCO.
2. Frequency Control: The VCO generates an output signal whose frequency is directly
proportional to the input voltage. The modulating signal controls this input voltage,
modulating the output frequency.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Experimental Procedure:
1. Circuit Setup:
Assemble the chosen FM modulator circuit on the breadboard.
Connect the function generator to provide the modulating signal.
Connect the oscilloscope to observe the output FM signal.
2. Signal Generation:
Apply a sinusoidal modulating signal from the function generator.
3. Observation:
Observe the output FM signal on the oscilloscope.
Note the frequency variations of the carrier wave in response to the modulating
signal.
Measure the frequency deviation (Δf) of the FM signal.
4. Data Collection:
Record the frequency of the carrier wave, the frequency of the modulating
signal, and the frequency deviation (Δf).
Repeat the experiment for different modulating signal frequencies and
amplitudes.
Results and Discussion:
1. Observation: The FM signal's frequency shifts proportionally to the instantaneous
amplitude of the modulating signal. The amplitude of the FM signal remains constant.
2. Calculation: Using the experimental data, the modulation index (β) and bandwidth
were calculated.
3. Discussion:
o FM provides a higher signal-to-noise ratio (SNR) compared to AM.
o The observed frequency deviation matches the expected theoretical value.
o Higher modulating signal amplitude results in a greater frequency deviation.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Discussion:
The experiment provides a basic understanding of FM modulation principles.
Limitations may include circuit non-linearities, component tolerances, and the accuracy
of measurements.
Potential improvements could involve using more precise components, implementing
feedback mechanisms for better stability, and using software tools for signal analysis
and processing.
Safety Precautions:
Ensure the power supply is properly grounded.
Avoid touching the circuit while it is powered on.
Use appropriate safety measures when working with high frequencies.
Conclusion
The experiment successfully demonstrated the principles of Frequency Modulation. The
characteristics of the FM signal, such as frequency deviation, modulation index, and
bandwidth, were observed and calculated. FM proves to be an efficient technique for noise-
immune communication.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Experiment No. 4: To Study AM Receiver Using BC547
Transistor
Aim:
To study the working of an AM (Amplitude Modulation) receiver circuit using a BC547
transistor.
Theory:
An AM receiver is used to demodulate amplitude-modulated signals. The circuit consists of
basic components including a diode for demodulation, a transistor for amplification, and
passive components for tuning and filtering. The BC547 transistor is commonly used for
amplification due to its high gain and low power consumption. The receiver converts
electromagnetic waves from an antenna into an audio signal that can be heard through
headphones or a speaker.
The key stages in an AM receiver are:
1. Tuning Circuit: Filters the desired AM signal using an LC circuit.
2. Demodulation: The OA91 diode rectifies the AM signal to extract the audio signal.
3. Amplification: The BC547 transistor amplifies the demodulated audio signal for
driving headphones.
Apparatus Required:
1. BC547 transistor
2. OA91 diode
3. Long wire antenna
4. Coil with 80 turns (inductor)
5. Variable capacitor (365 pF)
6. Resistors (820kΩ, 22Ω)
7. Capacitors (100nF, 10nF, 4.7nF)
8. 3V battery or power source
9. Headphones
10. Breadboard and connecting wires
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
Circuit Diagram:
Working Principle:
1. Antenna: The long wire antenna captures the AM signals from the air.
2. Tuning Circuit: The LC circuit (coil and variable capacitor) resonates at the desired
AM carrier frequency, filtering out other signals.
3. Demodulation: The OA91 diode rectifies the AM signal, extracting the modulated
audio signal.
4. Amplification: The BC547 transistor amplifies the weak audio signal for audible
output through headphones.
5. Audio Output: The demodulated and amplified signal is sent to headphones, where the
original audio can be heard.
Experimental Procedure:
1. Connect the circuit components as shown in the diagram on a breadboard.
2. Attach a long wire antenna to capture AM signals.
3. Adjust the variable capacitor to tune the circuit to a specific AM station.
4. Power the circuit using a 3V battery.
5. Listen to the audio output through the headphones.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
6. Note the quality of the audio signal received and adjust the tuning for optimal reception.
Experimental Table:
Results and Discussion:
The AM receiver successfully tuned and demodulated AM signals using the BC547
transistor for amplification.
Clear audio output was achieved when the circuit was properly tuned to the desired AM
frequency.
The variable capacitor played a crucial role in selecting the desired frequency, and the
BC547 provided sufficient amplification for headphone output.
Discussion Points:
1. The tuning range of the circuit depends on the LC combination used.
2. Environmental noise and interference may affect audio quality.
3. Further improvements can include using a more sensitive transistor or adding a second
amplification stage for better sound clarity.
Conclusion:
The experiment demonstrated the working of an AM receiver circuit using a BC547
transistor. It effectively tuned, demodulated, and amplified AM signals, providing
audible output through headphones. This simple and efficient design highlights the
principles of AM signal reception and demodulation.
Prepared by: Dr. Md. Firoz Ahmed-2, Associate Professor, Dept. of ICE, R.U
