FM transmission flash cards

- superimposing the intelligence signal on a high-frequency carrier so that its phase angle or frequency is altered as a function of the intelligence amplitude

angle modulation

two types of angle modulation  

frequency modulation and phase modulation

this type of modulator provides an indirect method of modulating provides an indirect method of modulating a stable crystal oscillator, through the use of a phase modulator stage, without the need for the cumbersome AFC circuity. the advantage of indirect FM modulation is its improved ability to stabilize the frequency. but the disadvantage is that they have insufficient frequency deviation. the armstrong modulator was also considered the first working FM system

armstrong modulation

used to remove or clip, external noise picked up during transmission signals. the use of a detector circuit is also an option for performing the same function

limiter

how much the carrier frequency will deviate for a given modulating input voltage level

deviation constant (k)

amount of carrier frequency increase or decrease around its reference value. this type of deviation is determined by the amplitude of the intelligence signal

frequency deviation

measure of the extent to which a carrier is varied by the intelligence

modulation index

high-level mathematical technique for determining the exact bandwidth of an FM signal

bessel functions

s a commonly used term in both television and FM broadcasting. it is the result of dividing the maximum possible frequency deviation by the maximum input frequency

deviation ratio (DR)

the process of increasing the relative strength of the high frequency components of the audio signal before it is fed to the modulator of an FM transmitter

pre-emphasis

the process of an FM receiver that reduces the amplitude of high-frequency audio signals down to their original values to counteract the effect of the pre-emphasis network in the transmitter

de-emphasis

a system that uses direct generation with automatic frequency control (AFC) that helps control carrier drift in FM transmission

crosby system

capacitor microphone fm generator: when no sounds is entered into the microphone, the capacitor is at A

rest (or constant value)

capacitor microphone fm generator: the rate of the capacitance change is equal to the A of the sound striking the mic, and the amount of capacitance change is proportional to the B of the sound waves

frequency, amplitude

capacitor microphone fm generator: the variance of the capacitance will have a direct affect on the oscillators A

frequency

capacitor microphone fm generator: the intelligence frequency of the imposing sound waves determines the A of frequency (or deviation) change WHILE the intelligence amplitude of the impinging sound waves determines the B of frequency (or deviation) change

rate, amount

capacitor microphone fm generator: the capacitor varied by the A of the individuals voice. this in turn causes the capacitance to go B around its center value.

amplitude and frequency, up and down

the greatest advantage FM over AM, is its ability to suppress A. this is due to the fact that, in FM, the intelligence is not carrier by B,

noise, amplitude changes but instead by frequency changes.

another advantage of indirect FM modulation over direct modulation, is its A (not noise)

improved frequency stability

although fm signals are better at suppressing A than am signals, they are still not completely immune to A, for there will still be slight variations of A due to the B.

noise, limiter and detector stages

we have the pre-emphasis and de-emphasis network; for they are designed to provide a near

constant noise reduction between high and low frequency intelligence signals

a method used to approximate the bandwidth of an FM signal

formula: BW = 2($\delta$max + FImax)

carsons rule

in standard broadcast FM, the maximum intelligence modulating frequency is A and the maximum allowed deviation is B(meaning full volume on the reciever end) above and below the carrier

15khz, 75khz

in narrow broadcast FM, the maximum modulating frequency is A (range B, used for voice transmission) and the maximum allowed deviation is C above and below the carrier.

3khz, 100hz - 3khz. 10khz

narrow bands are usually found in

taxicabs, police, aircraft, and other

three practical methods of direct FM generation

varactor diode, reactance modulator, voltage controlled oscillator

a diode with a small internal capacitance that varies as a function of its reverse bias voltage. in other words, the intelligence signal alters the capacitance of the diode to shift the resonant frequency of a tank circuit

varactor diode

an amplifier designed so that its input impedance has a reactance that varies as a function of the amplitude of the applied input voltage

reactance moduator

oscillator designed so that its output voltage varies as a function of the amplitude of the applied input voltage

voltage controlled oscillator

stage in a FM receiver that creates an output level that varies as a function of its input frequency; extracts the intelligence signal

discriminator

negative feedback control system in FM receivers used to achieve stability of the local oscillator

automatic frequency control

undesired radiation of the local oscillator signal through a receivers antenna

local oscillator reradiation

undesired mixing of two signals in a receiver resulting in an output frequency component equal to that of the desired signal

intermodulation distortion

decibel difference between the largest tolerable receiver input level and its sensitivity

dynamic range

minimum input RF signal to a receiver that is required to produce a specific audio signal at its output. this in turn will produce a specific amount of quieting on the receivers end

sensitivity

the minimum FM receiver input signal that begins the limiting process

quieting voltage

another term for quieting voltage

threshold voltage/limiting knee voltage voltage

closed loop control system that uses negative feedback to maintain constant output frequency

phase-locked loop

circuit that provides an output proportional to the phase difference of two inputs

phase comparator

another name for phase comparator

phase detector

when the phase comparator of a PLL generates a signal that forces the VCO to equal the input frequency

captured state

a PLL iin the captured state

locked

frequency at which the PLL runs with the input signal removed

free running frequency

total gain of all the internal blocks inside the PLL

loop gain

range of the frequencies in which the PLL will remain locked

hold-in range

an additional channel of multiplexed information authorized by the FCC for stereo FM radio stations to feed services to selected customers

subsidiary communication authorization

a form of distortion (or noise) caused by a non-square law device (also known as a detector circuit), that results in audible noise through the speakers

cross modulation

a circuit designed to respond only to frequency changes of the input signals, and as a result amplitude variations (or limiting), will have no effect of the signal

ratio detector