(1) A device that produces modulation; that is, a device that varies the amplitude, frequency, or phase of an ac signal [11] [12]. (2) A circuit used in servosystems to convert a dc signal to an ac signal. The output ac signal is a sine wave at the frequency of the ac reference voltage. The amplitude of the output is directly related to the amplitude of the dc input. The circuit's function is opposite to that of a DEMODULATOR [15]. (3) In radar, it produces a high-voltage pulse that turns the transmitter on and off [18].