Electronic Design

Versatile Shift-Keying Generator Uses Current-Feedback Amplifiers

Digital information often must travel through analog channels in applications like satellite communication, digital cellular phones, digital microwave communication, and others. This transmission is made possible by the sinusoidal signal-modulation techniques of ASK, FSK, PSK, and QAM.

By limiting the amplitude, frequency, and phase between predetermined levels, ASK, FSK, and PSK can respectively be generated. The growing demand for transmission capacity has triggered the development of various bandwidth-reduction methods. Quadrature amplitude modulation (QAM) is a hybrid between ASK and PSK that can reduce bandwidth usage to about one-eighth of the original requirement.

The versatile shift-keying generator presented in this design idea is built using current-feedback amplifiers (CFAs). The design is centered around a quadrature oscillator. A number of digitally controlled switches (MOS-implemented) generate all four modulation signals: ASK, PSK, FSK, and QAM. The use of CFAs makes the design workable at much higher frequencies than conventional op-amp-based designs.

The sinusoidal oscillations are characterized by the differential equation:

d2y(t)/dt2 + 4π2 f2 y(t) = 0

Upon solving, y(t) is obtained as either sin(ωt) or cos(ωt), depending on the value of y(0). For y(0) =1, y(t) = cos(2πf t), with the frequency of oscillation given by f = 1/2πRC.

The design shown in Figure 1 uses AD844-type ICs (U1-U6). U1, U2, and U3 form the quadrature oscillator. U4 provides the necessary amplitude gain required for ASK. U5 and U6 are configured as inverters to provide a phase shift of 180°. ASK, FSK, and PSK are obtained by controlling S3 (ASK), S4 (PSK), S5, and S6(FSK). By controlling the switches S1, S2, S3, and S4 in a sequential manner, quaternary or four-level PSK (QPSK) with a single amplitude level (4QAM) can be generated.

For the experimental results shown in Figure 2, standard 1-kΩ resistances with 5% tolerances and 1-µF capacitors are used. Figure 2(a) depicts simultaneous amplitude, frequency, and phase-shift keying. Figure 2(b) represents quadrature PSK (QPSK).

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