Frequency Modulation & Amplitude Modulation, Overview
Frequency Modulation and Amplitude Modulation, FM and AM modulation– In this article we will study in detail about what is Modulation and Demodulation. We will also study in detail about the two types of the modulation Frequency Modulation and Amplitude Modulation. We will also study about the different procedures, circuit diagram, and many more things. Without any further delay, let’s get started!!!
What is modulation?
The Modulation is a technique used for the encoding of information from a message source in a way that is suitable for transmission. This is done by altering the properties of a wave. Through superimposing a message (Video, Voice, or other sort of data) on to a high frequency signal known as a carrier wave. Different video voice and other data can be transmitted.
The modulation process a parameter of the carrier wave (such as amplitude, frequency and phase) is changed in accordance with the modulating signal also called message signal. The change acts as a code for data transmission.
The modulated signal is then transmitted by the transmitter.
Receiver demodulates the received message signal and gets the original information signal back.
What is demodulation?
It is the process of extracting an original message signal from the modulated wave is known as demodulation. This circuit which demodulates the modulated wave is called as the demodulator. This AM demodulation is done using a low pass filter which can filter out the high frequency carrier from the AM wave in a way that only the envelope of this carrier wave is visible at the output of the filter
The Low frequency signals like a voice signal can’t be transmitted to a longer distance with acceptable noise. That’s why we take the help of a high frequency signal. The high amplitude of this high frequency signal is manipulated in proportion to our low frequency signal that is to be transmitted. It is also known as amplitude modulation. By receiving the modulated signal at the receiver demodulation is performed to extract the original signal.
The modulation means the changing of Properties of the carrier signal according to the Properties of the modulated signal.
Types of Modulation
- Amplitude modulation
- Frequency modulation.
The Amplitude modulation means the changing of Properties of the carrier signal according to the Properties of the modulated signal.
The Frequency modulation means the changing of Properties of the carrier signal according to the Properties of the modulated signal.
- First collect the required material on to the workstation.
- Make a Connection of Carrier wave and a message wave (to be modulated) from the signal generator.
- Now Mount C828 transistor on the bread board.
- Put these Waves to the base of C828 transistor through a resistor as shown in diagram.
- Take a wire and take an output to be observed from collector which is also connected to the positive probe of the oscilloscope to observe the result.
- Now connect an inductor, capacitor to the collector end of the transistor which is also connected to the 5v positive Vcc.
- We Take the Ground of transistor emitter, oscilloscope, function generator and connect it to the ground of 5v supply.
- The circuit is ready.
SNAP OF RESULT CIRCUIT:-
We have learned that
- Magnitude of carrier signal varies according to the magnitude of message signal.
- The Modulation is dependent on message signal amplitude
- The Message Signal on the CRO after modulation is observed.
Study and analyze the single sideband in amplitude modulation
The Sideband is a band of frequencies, containing power which are the lower and higher frequencies of the carrier frequency. The both sidebands contain the same information. Following Representation of amplitude modulated wave in the frequency domain is as shown in the following figure.
The Both sidebands in the image contain the same information. In the transmission of such a signal which contains a carrier along with two sidebands can be termed as Double Sideband Full Carrier system DSB-FC. The plotted as shown in the following figure.
This transmission is inefficient. The Two thirds of the power is being wasted in the carrier which carries no information.
Now when the carrier is suppressed and the power saved is distributed to the two sidebands this process is called as Double Sideband Suppressed Carrier system DSBSC. The plot is shown in the following figure.
We get an idea that as the 2 sidebands carry the same information twice why we can’t suppress one sideband. Yes it is possible to do so.
This process of suppressing one of the sidebands along with the carrier and this transmitting of a single sideband is known Single Sideband Suppressed Carrier system SSB-SC or SSB. The plot is shown in the following figure.
SSB-SC or SSB which transfer a single sideband has high power as the power allotted for both the carrier and second sideband is utilized in transmitting this Single Sideband.
So this modulation done using this SSB technique is known as SSB Modulation.
Advantages of Sideband Modulation
These are the advantages of SSB modulation
- The BANDWIDTH or spectrum space occupied is lesser than AM and DSB signals.
- The transmission of more number of signals is allowed.
- The Power is saved.
- We can be transmit High power signal.
- We have Less amount of noise is present.
- The Signal fading is less likely to occur.
Disadvantages of Sideband Modulation
The SSB modulation disadvantages are
- The Generation and detection of SSB signal is a complex process.
- The Quality of the signal gets affected unless the SSB transmitter and the receiver have an excellent frequency stability.
Applications of Sideband Modulation
The SSB modulation applications are
- Power saving requirements and low bandwidth requirements.
- The Land, air and in maritime mobile communications.
- Used in point to point communications.
- 1st of all take all the required material.
- Connect the circuit as per circuit diagram.
- Connect the dual trace CRO probe to.
- Connect the single side band modulation kit to the circuit.
- Observe the wave response.
CIRCUIT DIAGRAM: –
WAVE FORM: –
We learnt and study about single side band in amplitude modulation. We observed the response of single side band. They are utilized in radio communications Television telemetry radar communications and in military communications such as amateur radio etc.
To study and analyze the double-sideband modulation and demodulation
The process of Amplitude Modulation the modulated wave consists of the carrier wave and 2 sidebands. In the modulated wave has the information only in the sidebands. The Sideband is nothing but a band of frequencies containing power which are the lower and higher frequencies of the carrier frequency.
So the transmission is inefficient. When the two thirds of the power is being wasted in the carrier which carries no information.
the carrier is suppressed and saved power is distributed to the two sidebands then a process is called as Double Sideband Suppressed Carrier system DSBSC.
Double-sideband suppressed-carrier transmission (DSB-SC) is transmission in which frequencies produced by amplitude modulation (AM) are symmetrically spaced above and the carrier frequency below and carrier level is reduced to the lowest practical level ideally being completely suppressed.
DSBSC modulation unlike in AM the wave carrier is not transmitted so much of the power is distributed between the side bands which implies an increase of the cover in DSBSC compared to AM for the same power use
DSBSC transmission is a special case of double-sideband reduced carrier transmission. This is used for radio data systems. The mode is frequently used in Amateur radio voice communications especially on High Frequency bands.
- 1st of all take all the required material.
- Connect the circuit as per circuit diagram on breadboard.
- Connect the connecting leads according to circuit diagram on trainer.
- Give power supply to trainer and circuit on breadboard.
- Apply modulating and carrier signals to circuit.
- Connect the CRO probe to output.
- Analyze output.
- Trace these waveforms on tracing paper.
CIRCUIT DIAGRAM: –
WAVE FORM: –
Let us consider the baseband modulating signal,
x(t) = Ax cos (2πfxt).
and carrier signal
c(t) = Ac cos (2πfct)
Now the mathematical representation of the signal at the output of the product modulator is given as
s(t) = x(t).c(t)
Maximum frequency is fc + fx
Minimum frequency is fc – fx
We know that
The Bandwidth is given as
BW = fmax – fmin
and BW = fc + fx – (fc – fx)
BW = 2fx
So at the output, the DSBSC wave contains a signal whose frequency is twice the frequency of the baseband signal.
We performed double sideband modulation and demodulation. Observed the response of for modulation and demodulation of DSB.
- The Bandwidth or spectrum space occupied is lesser than AM and DSB signals.
- More number of signals of Transmission is allowed.
- Total Power is saved.
- High power signal can be transmitted.
- Some amount of noise is present.
- The Signal fading is less likely to occur.
To study frequency modulation and demodulation
The modulation of the frequency of the carrier signal is altered in proportion to the message signal while phase and amplitude are kept constant is called frequency modulation.
The Modulation mechanisms can also be digital or analogue. The analogue modulation scheme has an input wave that changes like a sine wave continuously but it will be a bit more complicated when it comes to digital. In the voice sample is considered at some rate then compressed into a bit. In turn it is made into a specific type of wave that is superimposed on the carrier.
The frequency modulation the amplitude of the carrier wave is kept constant but now its frequency is varied in accordance with the amplitude of the audio frequency signal also Phase of the carrier wave is also kept constant. In instantaneous frequency of the resulting frequency modulated signal equals,
ωi = ωc + Kf f(t)
Kf represents the frequency sensitivity of the modulator.
Frequency modulation is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.
Generate a frequency message signal the net frequency of the radio carrier is changed in line with the amplitude of the incoming audio signal. Now the audio signal is modulated onto the radio frequency carrier the new radio frequency signal moves up and down in frequency. So the amount by which the signal moves up and down is important. It is called the deviation and normally quoted as the number of kilohertz deviation. Signal may have a deviation of plus and minus 3 kHz, i.e. ±3 kHz. So the carrier is made to move up and down by 3 kHz.
In there is a variety of different methods that can be used to generate frequency modulated signals.
The simple form of FM demodulation and relies on the selectivity of the receiver itself to provide the demodulation. Which is not particularly effective and this is not used except when the receiver does not have an FM capability.
The form of FM detection has very many limitations the curve of selectivity in the radio will not be at all linear and distortion will arise and the receiver will be sensitive to amplitude variations etc.
Varactor diode oscillator:
The method simply requires the use of a Varactor diode placed within the tuned circuit of an oscillator circuit. It is even possible to use a Varactor diode within a crystal oscillator circuit. When crystal oscillators are used the signal needs to be multiplied in frequency and only narrow band FM is attainable.
Phase locked loop:
The Phase locked loops provide an excellent method of generating frequency modulation. As it is often necessary to manage the constraints within the loop carefully but done once it provides an excellent solution.
In radio that is designed to receive frequency modulated signals there is some form of FM demodulator or detector.
The circuit takes in frequency modulated RF signals and it bring the modulation from the signal to output only the modulation that had been applied at the transmitter.
To be able to demodulate FM it is necessary for the radio receiver to convert the frequency variations into voltage variations
This is frequency to voltage converter. As the carrier frequency deviates to the lower end of the different frequency range over which it deviates a lower voltage may be produced it deviates higher in frequency, a higher voltage is produced.
• First collect all the required material.
• Connect circuit diagram as per circuit diagram on bread board.
• Connect power supply to the circuit.
• Connect input signal (Carrier signal and Modulating signal) through signal generator.
• Turn ON power supply and function generator.
• Connect oscilloscope at output to analyze the modulated wave.
• Observe the effect upon the carrier signal.
• Set CRO second channel with modulating signal and adjust time base for single steady waveforms.
• Expanding the trace with X-magnifier and observe the deviation in signal following the modulating signal polarities.
By observing modulation index = Δf/ fm
FM spectrum is likely to amplitude modulation where carrier component depends upon the FM
Bandwidth = (2Δf + fm).
The carrier signal’s frequency is changing according to the amplitude of message signal.
We observed that by varying the frequency of message signal the modulation varies. Message signal is observed at oscilloscope after demodulation.
Also by varying the amplitude of modulated signal calculate the different value of modulation index and we observe the modulated signal behavior.
De-tuned Resonant Circuit
The simplest form of demodulator which works but does have a few drawbacks. The parallel tuned circuit is deliberately detuned so the incoming carrier occurs approximately halfway up the left hand slope of the response.
Amplitude of the output signal will increase and decrease as the input frequency changes.
Example if the frequency of the incoming signal were to increase the operating point would move towards the right on the diagram. Which would cause an increase in the amplitude of the output signal.
The FM signal will therefore result in an amplitude modulated signal at the output.
FM input is applied to the base of the transistor and the collector there is the detuned resonant circuit that we have met earlier. it also includes the loading effect caused by the second winding which performs as a transmitter then the signal at the collector of the transistor includes an amplitude modulated component which is passed through the diode detector. Diode conducts every time the input signal applied to its anode is more positive than or the voltage on the top plate of the capacitor.
When the voltage falls below the capacitor voltage the diode decreases to conduct and the voltage across the capacitor leaks away until the next time the input signal is able to switch it on again.
Output is passed to the Low Pass Filter then from AC Amplifier Block. Unwanted DC component is removed and the low pass filter removes the ripple at the IF frequency.
Phase Locked Loop Detector
This is demodulator that employs a phase comparator circuit. It is a good demodulator and the advantage that it is available as a self contained integrated circuit so there is no setting up is required. We plug it in and it works. For reasons it is often used in commercial broadcast receivers. This has low levels of distortion and is almost immune from external noise signals and will provides very low levels of distortion.
DC output voltage from the output of the low pass filter as shown in Fig controls the frequency of this oscillator. And now this DC voltage keeps the oscillator running at the same frequency as the original input signal and it is 90° out of phase. if we did then why not just add a phase shifting circuit at the input to give the 90 phase shift? Now the answer can be seen by imagining what happens when the input frequency changes as it would with a FM signal. If the input frequency increases and the VCO decreases frequency is made to follow it. By doing this the input control voltage must increase and decrease. These changes in the DC voltage level form the demodulated signal. Then signal passes through a Low Pass Filter to smooth the output then pass through an AC amplifier.
The Working of the ratio detector centers on a frequency sensitive phase shift network with a transformer and diodes that are effectively in series with one another. So when a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and capacitor C3 charges up as a result.
The Transformer enables the circuit to detect changes in the frequency of the incoming signal. It has 3 windings. So the primary and secondary windings act in the normal way to produce a signal at the output. The 3rd winding is un-tuned and the coupling between the primary and third winding is very tight which means that the phasing between signals in these two windings is the same.
Primary and Secondary windings are tuned and lightly coupled. Now this means that there is a phase difference of 90 degrees between the signals in these windings at the center frequency. Now if the signal moves away from the center frequency the phase difference will change. And in turn the phase difference b/w the secondary and 3rd windings also different. When the voltage occurs it will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1 and R2. So as a result this causes a current to flow in the 3rd winding and the modulation to appear at the output.
These capacitors C1 and C2 filter any remaining RF signal which may appear across the resistors. These capacitor C4 and R3 also act as filters ensuring no RF reaches the audio section of the receive
With this my article on the Frequency Modulation and Amplitude Modulation comes to an end. I hope you have learnt something new from this article. Don’t forget to Subscribe to my Website and YouTube channel “Electronic Clinic”.