RF modules are partially finished circuits that can be incorporated into larger designs. They include receivers, transmitters, and transceivers. RF modules use several different modulation methods and radio techniques. On-off key (OOK) modulation turns a signal on or off. Amplitude modulation (AM) causes the baseband signal to vary the amplitude or height of the carrier wave to create the desired information content. Frequency modulation (FM) causes the instantaneous frequency of a sine wave carrier to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal. Amplitude shift key (ASK) is a modulation method that transmits data by varying the amplitude of the transmitted signal. Frequency shift key (FSK) is a digital modulation scheme that uses two or more output frequencies. Phase shift key (PSK) varies the phase of transmitted digital signals in accordance to the baseband data signal. Radio techniques limit localized interference and noise. Direct sequence spread spectrum techniques spread signals over a large band by multiplexing the signal with a code or signature that modulates each bit. Frequency-hopping spread spectrum techniques move signals through a narrow set of channels in a sequential, cyclical, and determined pattern.