TERMS DEFINITION
FM and PM Two forms of angle modulation.
Angle modulation
It was first introduced in the year 1931 as an alternative to amplitude modulation.
Major E. H.
Armstrong
He developed the first successful FM radio system in 1936, and in July 1939, the first regularly scheduled broad-casting of FM signals began in Alphine, New Jersey. Also developed the superheterodyne receiver.
Angle Modulation
A modulation that results whenever the phase angle () of a sinusoidal wave is varied with respect to time.
Angle Modulated wave is expressed mathematically as:
= angle modulated w ave =Peak carrier amplitude (volts) = carrier radian frequency (angular velocity)
= instantaneous phase deviation (radian)
Direct Frequency Modulation
( FM )
Varying the frequency of a constant-amplitude carrier directly
proportional to the amplitude of the modulating signal at a rate equal to the frequency of the modulating signal.
Direct Phase Modulation
( PM )
Varying the phase of a constant-amplitude carrier directly proportional to the amplitude of the modulating signal at a rate equal to the
frequency of the modulating signal.
ixliii its unmodulated value.
Carrier Rests Frequency
The original unmodulated carrier frequency in the resultant angle-modulated waveform. respect to its reference phase.
Instantaneous the first time derivative of the instantaneous phase deviation.
Instantaneous Frequency
= rad/sec
The precise frequency of the carrier at a given instant of time and is defined as the first time derivative of the instantaneous phase.
Phase Modulation
It is define as angle modulation in which the instantaneous deviation is proportional to the amplitude of the modulating signal voltage and the instantaneous frequency deviation is proportional to the slope or first derivative of the modulating signal.
For a modulating signal the phase and frequency modulation are Phase modulation = = K
Frequency modulation = rad/sec
Deviation Sensitivities
Are the output–versus- input transfer functions for the modulators, which give the relationship between what output parameter changes in respect to specified changes in the input signal.
Peak Phase Deviation
It is the radian for the phase modulated carrier.
Index of Modulation Peak Phase Deviation is sometimes called?
Frequency Deviation
The change in frequency that occurs in the carrier when it is acted on by a modulating-signal frequency.
Carrier Swing The peak-to-peak frequency deviation (2f) is sometimes called?
ixliv
Percent Modulation
The ratio of the frequency deviation actually produced to the maximum frequency deviation allowed by law stated in percent form.
Percent modulation is modulation =
Phase Modulator
A circuit in which the carrier is varied in such a way that its instantaneous phase is proportional to the modulating signal.
Rest Frequency
The unmodulated carrier is a single frequency sinusoidal and is commonly called?
Frequency Modulator
A circuit which the carrier is varied in such a way that its instantaneous phase is proportional to the integral of the modulating signal.
Frequency deviator Frequency modulator is often called?
Four commonly used equivalences are as follows:
PM modulator = differentiator followed by an FM modulator
PM demodulator = FM demodulator followed by an integrator
FM modulator = integrator followed by a PM modulator
FM demodulator = PM demodulator followed by a differentiator
Bessel Function
It is the first kind for several values of modulation index provides the number of side frequency pairs and their corresponding magnitude.
First Carrier Null The carrier component goes to zero is called?
Second Carrier Null The carrier component once again disappears is called?
1%
A side frequency is not considered significant unless it has an amplitude equal to or greater than ____ of the unmodulated carrier amplitude.
Low-index Case The modulation index is less than 1.
Medium Index Modulation indices greater than 1 and less than 10.
High-index Case The modulation index is greater than 10.
Narrowband FM Low-index FM systems are sometimes called?
Carson’s Rule
B =2( )Hz A rule which is an approximation and gives transmission bandwidths that are slightly narrower than the bandwidths. It defines a bandwidth
ixlv frequency deviation divided by the maximum modulating-signal frequency.
Deviation Ratio DR =
DR = Deviation Ratio (unit less)
= maximum peak frequency deviation (hertz) = maximum modulating signal frequency (hertz)
20 MHz
The FCC has assigned the commercial FM broadcast service a _______
band of frequencies that extends from 88 MHz to 108 MHZ.
200 kHz The 20–MHz band is divided into 100, ________ wide channels beginning at 88.1 MHz.
75 kHz ;
15 kHz
To provide high-quality, reliable music, the maximum frequency deviation allowed is _______ with a maximum modulating-signal frequency of _______.
Adjacent Channel Interference
The highest side frequencies from one channel are allowed to spill over into adjacent channels, producing an interference known as?
200 kHz
It is the wide of the guard band is usually on either side of each assigned channel.
The average power in the unmodulated carrier is
= carrier pow er (w atts) = peak unmodulated carrier voltage (volts)
R = load resistance (ohms)
FM Noise Triangle
The noise voltage at the output of an FM demodulator increases linearly with frequency. This is called?
Pre-emphasis
The high-frequency modulating signals are emphasized or boosted in amplitude in the transmitter prior to performing modulation.
De-emphasis
The reciprocal of pre-emphasis that restores the original amplitude-versus-frequency characteristics to the information signals.
Pre-emphasis Network
A circuit that provides a constant increase in the amplitude of the modulating signal with an increase in frequency.
Break Frequency
It occurs at the frequency where and equal R.
ixlvi
Direct FM
It is a angle modulation in which the frequency of the carrier is varied directly by the modulating signal.
Direct FM
Three common methods for producing direct frequency modulation.
Varactor Diode
Direct frequency modulator used for low-index Applications, such as two-way mobile radio. accurate, and directly proportional to the input modulating signal.
Direct PM
Angle modulation in which the frequency of the carrier is deviated indirectly by the modulating signal.
Varactor Diode and Transistor
Direct PM Modulator
Two common methods for producing direct phase modulation.
Frequency Up-Conversion
The process of up-converting the frequency of the modulated carrier after modulation has been performed.
Heterodyning and Frequency Multiplication
Two basic methods of performing frequency up-conversion.
Heterodyne Method
An up-conversion method where a low-frequency modulated carrier can either be up- or down- converted to a different location in the frequency spectrum without changing its modulation properties.
Multiplication Method
An up-conversion method where the modulation properties of a carrier can be increased at the same time that the carrier frequency is up-converted.
ixlvii Direct FM
Transmitters
Transmitters that produce an output waveform in which the frequency deviation is directly proportional to the modulating signal.
Automatic Frequency Control
( AFC )
A circuit that compares the frequency of he non crystal carrier oscillator to a crystal reference oscillator and then produces a correction voltage proportional to the difference between the frequencies.
Frequency Discriminator
A frequency-selective device whose output voltage is proportional to the difference between the input frequency and its resonant frequency.
Wipe Off When you remove the modulation from the FM wave is called?
dc Correction Voltage
A voltage added to the modulating signal to automatically adjust the master oscillator’s center frequency to compensate for the low-frequency drift.
Wideband FM Transmitter
A transmitter that uses a phase-locked loop to achieve crystal stability from a VCO master oscillator and at the same time, generate a high index, wideband FM output signal.
FM Modulator It is preceded by a differentiator generates a PM waveform.
Indirect FM Transmitters
Transmitters that produce an output waveform in which phase deviation is directly proportional to the modulating signal.
PM Modulator It is preceded by a integrator produces an FM waveform.
Noise Immunity Probably the most significant advantage of angle modulation over amplitude modulation transmission.
Capture Effect
It allows a receiver to differentiate between two signal received with the same frequency.
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