SUMMARY

Now that you have completed this chapter, a short review of what you have learned is in order. The following review will refresh your memory of angle modulation and pulse modulation.

FREQUENCY-SHIFT KEYING (fsk) is similar to cw keying in amplitude modulation and is a form of angle modulation. The carrier frequency is changed between two discrete values by the opening and closing of a key.

In FREQUENCY MODULATION (fm) the instantaneous frequency of the radio-frequency wave is varied in accordance with the modulating signal; the amplitude of the radio-frequency wave is kept constant.

MODULATION INDEX is the ratio of the maximum frequency difference between the modulated and the unmodulated carrier, or between the deviation frequency and the modulation frequency.

The number of SIGNIFICANT SIDEBANDS and the modulating frequency will determine the bandwidth of the fm wave. The number of significant sidebands can be determined from the modulation index.

The REACTANCE-TUBE MODULATOR is frequency modulated by using a reactance tube in shunt with the tank circuit.

The SEMICONDUCTOR-REACTANCE MODULATOR is used to frequency modulate low-power semiconductor transmitters.

The MULTIVIBRATOR MODULATOR uses an astable multivibrator with a modulating voltage inserted in series with the base return of the multivibrator transistors.

The VARACTOR FM MODULATOR uses a VARACTOR. This is a specially designed diode that has a certain amount of capacitance between the junction that can be controlled by reverse biasing.

In PHASE MODULATION the carrier's phase is caused to shift at the rate of the modulating audio. The amount of phase shift is controlled by the amplitude of the modulating wave.

A BASIC PHASE MODULATOR may be a single tube in series with a capacitor to form a phase-shift network. As the impedance of the tube changes, the phase of the output shifts.

PHASE-SHIFT KEYING (psk) is similar to cw and fsk. It consists of phase reversals of the carrier frequency as modulating signal data elements open and close the modulator key.

PULSE MODULATION is modulation in which we allow oscillations to occur for a given period of time only during selected intervals.

PULSE-REPETITION TIME (prt) is the specific time period between each group of rf pulses.

PULSE-REPETITION FREQUENCY (prf) is found by dividing the pulse repetition time into 1. This defines how often the groups of pulses occur.

PULSE WIDTH (pw) or PULSE DURATION (pd) is the time that a pulse is occurring.

REST TIME (rt) is the time referred to as nonpulse time.

PEAK POWER is the maximum power during a pulse.

AVERAGE POWER equals the peak power averaged over one complete cycle.

DUTY CYCLE is the ratio of working time to total time, or the ratio of actual transmit time to transmit time plus rest time, for intermittently operated devices.

The SPARK-GAP MODULATOR consists of a circuit for storing energy, a circuit for rapidly discharging the storage circuit, a pulse transformer, and a power source.

The THYRATRON MODULATOR is an electronic switch which requires a positive trigger of only 150 volts. The trigger must rise at the rate of 100 volts per microsecond to fire or cause the modulator to conduct.

In communications PULSE-MODULATION SYSTEMS, the modulating wave must be SAMPLED at 2.5 times the highest modulating frequency to ensure accuracy.

PULSE-AMPLITUDE MODULATION (pam) is modulation in which the amplitude of each pulse is controlled by the instantaneous amplitude of the modulation signal at the time of each pulse.

PULSE-DURATION MODULATION (pdm) or PULSE-WIDTH MODULATION (pwm) are both designations for a type of modulation. The width of each pulse in a train is made proportional to the instantaneous value of the modulating signal at the instant of the pulse.

PULSE-POSITION MODULATION (ppm) has the advantage of requiring constant transmitter power. The amplitude and width of the pulses are kept constant. At the same time, the position of each pulse, in relation to the position of a recurrent reference pulse, is varied by each instantaneous sampled value of the modulating wave.

PULSE-FREQUENCY MODULATION (pfm) is a method of pulse modulation in which the modulating wave is used to frequency modulate a pulse-generating circuit.

PULSE-CODE MODULATION (pcm) refers to a system in which the standard value of a quantized wave is indicated by a series of coded pulses that give the modulating wave's value at the instant of the sample.