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The basis of phase modulator modulation:

Radio frequency signal consists of oscillating carrier in the form of sine wave, which is the basis of signal. The instantaneous amplitude follows this curve in a positive and then negative direction, returning to the starting point after one full cycle - it follows the curve of the sine wave. A sine wave can also be represented by a movement around a point in the circle, and the phase at any given point is the Angle between the starting point and the point on the waveform. The phase also advances over time, so points on the waveform can be said to have a phase difference between them. Phase modulation works by modulating the phase of the signal, that is, changing the rate at which the point moves around the circle. If modulation is not applied, this changes the phase of the signal. In other words, the rotation speed around the circle is modulated with respect to the mean. For this, it is necessary to change the frequency of the signal in a short period of time. In other words, when phase modulation is applied to the signal, there is a frequency change, and vice versa. Phase and frequency are inextricably linked because phase is an integral of frequency.

PURPOSE:To increase input-output isolation of the whole phase modulator by providing a zero setting circuit which makes a signal voltage from a driving circuit zero when a carrier cut-off signal is applied.CONSTITUTION:Diode switch circuit 10 is a five-stage low-pass filter type switch circuit using PIN diodes 12 and 14; when voltages differing in direction are applied to diodes 12 and 14 from control circuit 16, the switch turns on and when forward voltages are applied, diodes 12 and 14 conduct to form a short circuit, thereby cutting off a carrier. Control circuit 16 applies the forward voltages to diodes 12 and 14 applied with a carrier cut-off signal. Driving circuit 30, when the carrier cut-off signal is applied via line 40, controls forcibly a voltage signal to be supplied two- phase PSK modulator 20 to zero.