In frequency modulation, the frequency of a carrier is changed in response to the amplitude of the modulating signal. The information signal may be an analog signal such as an audio signal or it may be a digital signal. The FM signals are generated by varying the frequency of a carrier signal according to the variations of a modulation signal. The modulation signal may be an analog or a digital signal which is generated by a message transducer, such as a microphone, or a binary device, such as a microprocessor. Generally, the modulation signal is applied to a frequency modulator which produces the FM signal. The FM signal may be processed in accordance with the requirements of a particular application to recover the modulation signal. When digital signals frequency modulate a carrier, the resulting modulation is known as frequency-shift keying. A bilevel digital information signal takes one of two states depending upon the state of digital information signal. Since frequency modulation is the time differential of phase modulation, a phase modulator circuit can be converted to a frequency modulator circuit by including an integrator circuit in the signal path to its modulation signal input. Transmission systems for transmitting digital symbols modulated on a carrier according to a constellation having a predetermined number of phase states are used for a large number of applications. Examples of such applications are mobile radio, radio relay systems, and broadcast systems like microwave video distribution systems.

Frequency modulators normally comprise an oscillator responsive to an amplitude variable signal for producing a corresponding frequency variable signal. Frequency modulation circuits require VCO output signals to be modulated with a defined modulation deviation in order to transmit a given amount of information. In most of today's advanced communication devices frequency modulation is accomplished by modulating the frequency of a voltage controlled oscillator. This is usually accomplished by varying the resonant frequency of oscillation in a tank circuit which is used to generate a carrier signal. To modulate the frequency being generated by the frequency resonator tank circuit, a modulation signal is routed via resistor to inductor, capacitor and modulation varactor. The varactor will change its capacitance in proportion to an applied modulation signal resulting in a slight variation in the capacitance of the tank circuit. This change in capacitance causes the oscillator frequency to change in response to the modulation signal, resulting in the frequency modulation of the carrier signal. Dependent upon the impedance of the modulation capacitor, modulation amplitude and component characteristics, the circuit represents a modulation dependent impedance and provides a modulation impedance signal to the resonator tank circuit. For a mobile radio communication use, an output frequency of the VCO is changed according to control of a channel selection and change of the VCO output frequency is performed by changing a frequency-dividing factor of the programmable frequency divider. A radio frequency (RF) modulator design that is commonly-used in radio frequency circuits is a reflective shunt diode modulator. Such a modulator consists of a transmission line which is connected to ground at intervals by PIN diodes.

A typical RF modulator may operate using frequency modulation at the lower end of the FM broadcast band. The frequency may be selectable between several standard FM channels to avoid using a channel that is also being used by a local radio station transmitter. RF oscillators and modulators for use in home and commercial appliances typically must meet certain requirements in power and frequency output. For example, appliances such as video games which may be operated with conventional television receivers, must comply with FCC regulations on harmonic surpession and maximum output voltage to minimize spurious signal propagation. Video signal receivers such as direct broadcast satellite receivers and video cassette recorders (VCRs) provide video output signals as radio frequency (RF) modulated signals to a television receiver. In order to modulate the output of a radio frequency source, a modulator or attenuator is usually connected in series with the source. Generally, the modulator is set to be linear over a predetermined range of frequencies and the amount of attenuation produced by the modulator is proportional to some input signal or voltage. Unlike amplitude modulation techniques (AM) where the amplitude of a carrier signal is modulated by the information signal, in frequency modulation (FM) and phase modulation (PM), the frequency and phase of the carrier signal is respectively modulated. FM and PM are usually referred to collectively as angle modulation techniques.