Frequency (Fm) and Phase (Pm) Modulations - Free download as PDF File .pdf), Text BASIC DEFINITIONS Relationship between the angle and frequency of a sinusoidal FREQUENCY MODULATION Goal: Determine the spectrum and. The main goal of modulation today is to squeeze as much data into the least Amplitude Shift Keying (ASK) and Frequency Shift Keying (FSK) . levels or the smaller the difference between the amplitude levels, the greater. Objectives Know the relationship of carrier frequency, modulation frequency and modulation index to To modulate the signal just means to systematically vary one of the three parameters of the signal: amplitude, frequency or phase.
FREQUENCY AND PHASE MODULATION (ANGLE MODULATION) - ppt video online download
As the amplitude of FM carrier is constant, the noise interference is minimum. The amplitude of FM carrier is constant and is independent of depth of modulation.
Hence transmitter power remains constant in FM whereas it varies in AM. As against the limitation of depth of modulation in AM, in FM depth of modulation can be increased to any value, without causing any distortion.
BDG xx 21 4. Because of guard bands provided in FM, adjacent channel interference is very less. Radius of propagation is limited as FM uses space waves with line of sight.
So it is possible to operate many independent transmitters on the same frequency with minimum interference. FM equipments are more complex and hence costly. Area covered by FM is limited, to line of sight area but AM coverage area is large. FM radio was developed in the United states mainly by Edwin Armstrong in the s. Modulating differences In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted.
Importance It is used in both analog and digital communication and telemetry It is used in both analog and digital communication and telemetry Frequency Range AM radio ranges from to KHz OR Up to Bits per second. FM radio ranges in a higher spectrum from 88 to MHz. OR to bits per second. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz.
Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is kHz.
Transmitter and receiver are more complex as variation of modulating signal has to be converted and detected from corresponding variation in frequencies.
Noise AM is more susceptible to noise because noise affects amplitude, which is where information is "stored" in an AM signal. FM is less susceptible to noise because information in an FM signal is transmitted through varying the frequency, and not the amplitude. FM PM 1 The max frequency deviation depends on amplitude of modulating signal and its frequency The max phase deviation depends on amplitude of modulating signal 2 Frequency of the carrier is modulated by modulating signal.
Phase of the carrier is modulated by modulating signal. Modulation index remains same if modulating signal frequency is change. Modulators — Carrier frequency can be generated by LC oscillator. By varying the values of L or C of tank circuit, carrier frequency can be changed. When these components are used with LC tank circuits, we are able to vary frequency of oscillator by changing the reactance of L or C.
Indirect FM — Modulation is obtained by phase modulation of the carrier An instantaneous phase of the carrier is directly proportional to the amplitude of the modulating signal. Direct FM- The frequency of carrier is varied directly by modulating signal An instantaneous frequency variation is directly proportional to the amplitude of the modulating signal.
Frequency modulation using Varactor Diode — There exists small junction capacitance in the reverse biased condition of all the diodes. The varactor diodes are designed to optimise this characteristic.
As the reverse bias across varactor diode is varied, its junction capacitance changes. These changes are linear and wide 1 to pF BDG xx 30 Frequency modulation using varactor diode — All diodes show small junction capacitance in the reverse biased condition. High frequency stability as crystal oscillator is isolated from modulator.
FREQUENCY AND PHASE MODULATION (ANGLE MODULATION)
To avoid distortion, the amplitude of modulating signal is to be kept small. The varactor diode must have non linear characteristics of capacitance vs. Use — This method is used for low index narrow band FM generation. If such a device is placed across the tank circuit of the L-C oscillator, then FM will be produced when the reactance of the device is varied by the modulating voltage.
At the carrier frequency, the oscillator inductance is tuned by its own capacitance in parallel with the average reactance to the variable reactance device. Due to FET characteristics, linear relationship between modulating voltage and transconductance can be achieved. Disadvantages - Frequency stability is poor as lumped components are used. Use — This method is used for low modulation index application. BDG xx 34 Indirect FM — Phase modulation is used to achieve frequency modulation in the indirect method, It is necessary to integrate the modulating signal prior to applying it to the phase modulator, This transmitter is widely used in VHF and UHF radio telephone equipment.
The crystal oscillator is isolated from modulator, so frequency stability is very good. Because of nonlinear capacitance Vs. Amplitude of modulating signal should be kept small to avoid distortion.
Use —Used for narrow band low index FM. Indirect FM Transmitters — Produces the FM whose phase deviation is directly proportional to modulating signal amplitude. For any analog modulator type, there are two inputs and one output. The two inputs are modulating signal i. The output is referred as modulated waveform.
Amplitude Modulation AM Amplitude Modulation AM is the modulation technique in which carrier amplitude varies based on analog baseband information signal to be transmitted using wireless device. AM broadcast signals are mainly propagated by ground waves during the day and by sky waves at night time. Advantages, disadvantages and applications of AM Different amplitude modulation techniques have their own advantages and disadvantages as mentioned below: It has lower power consumption and it is simple technique of modulation.
But it is complex in detection at AM receiver. It is used in analog TV transmission systems to transmit color information. It is used for efficient management of spectrum. But generation of SSB modulation is difficult and it is complex in detection at receiver. It is used for 2-way radio FDM. But demodulation system is complex.