Amplitude Modulation
Amplitude Modulation:
In amplitude modulation the information is stored in the variation in amplitude of the signal. Modulation of a signal is done to reduce the noise and transmit the signal, and we demodulate the signal at the receiver.
SSB – It is single side band. In this signal only one of the sidebands is present.
DSB- It is the double sideband. The fig shown below is DSB. It we suppress its carrier the signal is called DSBSC that is double side band suppressed carrier
Amplitude modulator-
Amplitude modulator is a device that modulates the signal there are two types of AM modulators low level and high level.
Low level – The RF oscillator is stabilized in order to maintain the carrier frequency deviation within a prescribed limit. The carrier frequency is equal to the transmitter frequency.
1. Diode modulator –
During negative portion no signal passes and during positive portion the current is the series of pulses which are proportional to the amplitude of modulating signal then it goes to the tuned circuit which creates a negative half cycle for every positive pulse.
2. Transistor modulator –
It has a mixing network, a transistor and a LC tuned circuit. The EB junction acts as a diode and modulation and amplification also occur in transistor and tuned circuit generates missing half cycle.
3. Differential Amplifier –
A constant current transistor is used in the circuit. Two transistors form a differential pair and one is a constant current source. The output voltage can be taken between the two collectors, producing a balanced or differential, output. No special biasing circuits are needed as the correct value of collector current is supplied directly by the constant-current source.
High level – It varies the voltage and power in the final RF amplifier stage of the transmitter it has high efficiency and high quality performance.
1. Collector modulator –
This circuit shows the output stage of the transistor it is a high power RF class C amplifier. It conducts for only a portion of the positive half cycles of the carrier the is applied to the transistor thus the collector current is in form of current pulses which are supplied to tuned circuit.
2. Series modulator –
It consists of an emitter follower which is in series with the supply voltage and the collector voltage changes with the variations amplified audio modulating signal. This series modulator replaces the transformer with an emitter follower. The modulating signal is to be applied to the emitter follower. The emitter follower appears in series with the collector supply voltage. This causes the amplified audio modulating signal to vary the collector supply voltage to the class C amplifier.
Demodulators-
These circuits are present are present in the receivers and they recover the original signal and the information in the signal.
1. Diode detector – The AM signal is coupled using transformer and it is given to a half wave rectifier and filtering of that is done by the use of a capacitor. As it recovers the envelope of the AM signal it is also called as envelope detector
2. Synchronous detector – It has an internal clock to on and off the AM signal and the rectifier is same that of diode detector. The clock has the same phase and frequency as that of the carrier.
Generation of SSB-
Filter method- The output of the balanced modulator is a double-sideband signal. An SSB signal is produced by passing the DSB signal through a highly selective bandpass filter that selects either the upper or lower sideband.
Phasing method- The phasing method of SSB generation uses a phase-shift technique that cancels out one of the sidebands. It uses two balanced modulators, which effectively eliminate the carrier. The carrier oscillator is applied directly to the upper balanced modulator. This phase shifting action causes cancellation of side band.
DSB and SSB demodulation-
The carrier must be generated back. A balanced modulator is used it is called as a product detector as it generates the modulating signal. The difference is the modulating signal and higher is filtered out by the use of a low pass filter.
Balanced modulators- A balanced modulator is used to generate the DSB signal and it suppresses the carrier leaving only the sum and difference frequencies at the output.
1. Lattice modulator- It is also called as a diode ring modulator. The carrier signal is used at the source of forward and reverse bias for the diodes and the diodes act as switches that connect the modulating signal at the secondary to the primary.
2. IC balanced modulators- IC 1496/1596 are used in balanced modulators the carrier frequency can be up to 100MHz and the suppression of 50 to 65dB can be achieved.
In amplitude modulation the information is stored in the variation in amplitude of the signal. Modulation of a signal is done to reduce the noise and transmit the signal, and we demodulate the signal at the receiver.
SSB – It is single side band. In this signal only one of the sidebands is present.
DSB- It is the double sideband. The fig shown below is DSB. It we suppress its carrier the signal is called DSBSC that is double side band suppressed carrier
Amplitude modulator-
Amplitude modulator is a device that modulates the signal there are two types of AM modulators low level and high level.
Low level – The RF oscillator is stabilized in order to maintain the carrier frequency deviation within a prescribed limit. The carrier frequency is equal to the transmitter frequency.
1. Diode modulator –
During negative portion no signal passes and during positive portion the current is the series of pulses which are proportional to the amplitude of modulating signal then it goes to the tuned circuit which creates a negative half cycle for every positive pulse.
2. Transistor modulator –
It has a mixing network, a transistor and a LC tuned circuit. The EB junction acts as a diode and modulation and amplification also occur in transistor and tuned circuit generates missing half cycle.
3. Differential Amplifier –
A constant current transistor is used in the circuit. Two transistors form a differential pair and one is a constant current source. The output voltage can be taken between the two collectors, producing a balanced or differential, output. No special biasing circuits are needed as the correct value of collector current is supplied directly by the constant-current source.
High level – It varies the voltage and power in the final RF amplifier stage of the transmitter it has high efficiency and high quality performance.
1. Collector modulator –
This circuit shows the output stage of the transistor it is a high power RF class C amplifier. It conducts for only a portion of the positive half cycles of the carrier the is applied to the transistor thus the collector current is in form of current pulses which are supplied to tuned circuit.
2. Series modulator –
It consists of an emitter follower which is in series with the supply voltage and the collector voltage changes with the variations amplified audio modulating signal. This series modulator replaces the transformer with an emitter follower. The modulating signal is to be applied to the emitter follower. The emitter follower appears in series with the collector supply voltage. This causes the amplified audio modulating signal to vary the collector supply voltage to the class C amplifier.
Demodulators-
These circuits are present are present in the receivers and they recover the original signal and the information in the signal.
1. Diode detector – The AM signal is coupled using transformer and it is given to a half wave rectifier and filtering of that is done by the use of a capacitor. As it recovers the envelope of the AM signal it is also called as envelope detector
2. Synchronous detector – It has an internal clock to on and off the AM signal and the rectifier is same that of diode detector. The clock has the same phase and frequency as that of the carrier.
Generation of SSB-
Filter method- The output of the balanced modulator is a double-sideband signal. An SSB signal is produced by passing the DSB signal through a highly selective bandpass filter that selects either the upper or lower sideband.
Phasing method- The phasing method of SSB generation uses a phase-shift technique that cancels out one of the sidebands. It uses two balanced modulators, which effectively eliminate the carrier. The carrier oscillator is applied directly to the upper balanced modulator. This phase shifting action causes cancellation of side band.
DSB and SSB demodulation-
The carrier must be generated back. A balanced modulator is used it is called as a product detector as it generates the modulating signal. The difference is the modulating signal and higher is filtered out by the use of a low pass filter.
Balanced modulators- A balanced modulator is used to generate the DSB signal and it suppresses the carrier leaving only the sum and difference frequencies at the output.
1. Lattice modulator- It is also called as a diode ring modulator. The carrier signal is used at the source of forward and reverse bias for the diodes and the diodes act as switches that connect the modulating signal at the secondary to the primary.
2. IC balanced modulators- IC 1496/1596 are used in balanced modulators the carrier frequency can be up to 100MHz and the suppression of 50 to 65dB can be achieved.
Comments
Post a Comment