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Constellation diagram is a graphical representation of a signal in the complex coordinate plane. From the representation, the various class of digitally phase modulated signal such as BPSK, QPSK, 8PSK, 16QAM and 64QAM can be recognized. Constellation diagram has been used for modulation classification in spectrum monitoring, surveillance, cognitive-radio, and electronic warfare. Conventionally signal constellation diagram is generated using an IQ demodulator. In this paper, we present a new algorithm to generate the signal constellation diagram using the cross time-frequency distribution (XTFD). From the resulting cross time-frequency representation (XTFR), signal parameters such as the carrier frequency, instantaneous energy, instantaneous information bearing phase (IIBP) and symbol rate can be estimated directly. The estimated parameters are then used to generate the constellation diagram. The advantages of the proposed method are that it does not need the signal to be demodulated and it is more robust to noise. Thus, the resulting constellation diagram generated using the XTFD has outperformed the conventional IQ demodulator method for minimum SNR of 3dB.