Electronic Design

QPSK Demodulator Is A Good Match For Low-Cost Modulator

This demodulator is suitable for the quadrature phase-shift keying (QPSK) modulator previously published by the same author, "Novel Low-Cost QPSK Modulator Needs No Adjustments" (electronic design, Sept. 16, 2002, p. 92). Also based on common CMOS logic, it requires no adjustments. These characteristics make it appropriate for low-cost applications, such as factory communications.

The typical approach to QPSK demodulation decomposes the modulated input signal via two multipliers driven by a fixed-frequency oscillator whose two outputs are 90° apart (Fig. 1). This operation also produces terms at twice the rate of the oscillator. So, two low-pass filters (LPFs) are added to eliminate them. Finally, a parallel-to-serial converter translates the dibits to bits, thus producing the desired PCM signal.

An alternative demodulator that's suitable for the aforementioned QPSK modulator generates four modified carrier phases (0°, 90°, 180°, and 270°) obtained in the modulator by rotating the angles of the QPSK constellation. Then, a simple XNOR operation between each phase and the modulated signal will produce a logic "1" only if both coincide (Fig. 2). If the received phase is 0° (related to dibit 00), all XNORs produce a logic "0." If the phase is 90° (dibit 10), only the upper branch gives a "1." This situation is inverted for 270° (dibit 01). In this case, the logic "1" is produced at the lower branch. But if the phase is 180° (dibit 11), a logic "1" is added to both branches. The result is the demodulated PCM signal.

In a practical circuit, the phase-references generator produces inputs to the XNOR gates (Fig. 3). Also, the input, called ERROR (UART), adapts the initial phase reference in accordance with the circuit, "Automatic Phase Detector For Digital Communications" (electronic design, Sept. 2, 1997, p. 168). This circuit forms a closed loop between the PCM signal and ERROR (UART) that provides the necessary synchronization for coherent detection. Like the modulator, this circuit runs for a carrier frequency of fm/8. It has been tested at 2 MHz (with basic CMOS logic) and at 20 MHz (with HCMOS).

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