Circuit for demodulating PSK modulated signal by differential-defection

DETAILED DESCRIPTION It is therefore an object of this invention to provide an improved demodulator circuit in which a PSK modulated signal is converted in frequency into a quasi-base band signal and this quasi-base band signal is then differential-detected by a differential-detector circuit and a phase error compensator circuit which are simple in construction.
According to the invention, there is provided a circuit for demodulating a PSK modulated signal by differential-detection, comprising: a pair of mixers for mixing the PSK modulated signal with a pair of locally oscillated signals of different phases to perform both frequency conversion to a quasi-base band and complex separation; an analog to digital converter for converting the outputs of the mixers into a digital complex signal; a phase angle processor for processing phase angles from the converted digital complex signal; a differential-detector for outputting differential phase angle data between successive symbols outputted from the phase angle processor; a phase error compensator for compensating a phase error contained in the differential phase angle data; and a decision circuit for outputting a demodulated digital signal from the compensated differential phase angle data.
With this demodulator, partly since the digital complex signal converted into a quasi-base band is once processed into a phase angle signal, and partly since differential detection and phase-error compensation take place by utilizing this phase angle signal it is possible to simplify the circuit components including multiplier or the like with respect to the complicate conventional circuits.
The manner in which a quadru-phase PSK is differential-detected will now be described by way of illustrated example.
On the transmitting side, the quadru-phase PSK is modulated by string data that are differential-encoded by a gray code.
On the receiving side, demodulation is performed by detecting how many degrees the phase angle of N-th symbol has angularly moved through from the (N-1)th symbol between two successive symbols