Electronic Design

Phase Shifter Has Eight Outputs

Here's a cellular phase-shifter circuit that's a key portion of an eight-level PSK (phase-shift keying) transmitter operating at the standard frequency of 1700 Hz. Moreover, by connecting accordingly calculated resistive dividers at some outputs and using buffers, it's possible to obtain a set of signals for QAM (quadrature amplitude modulation).

The circuit consists of eight cascaded identical cells, each cell being a dc-controlled active phase shifter (Fig. 1). Because the dc control is common for all shifters, the circuit is adjusted by trimming RA (Fig. 2a) so that the phase difference between vo1 and vi is zero. As a result, each shifter will introduce a phase difference of exactly Π/4. The eight signals for PSK are available at the op amps' outputs (Fig. 2b).

Phase accuracy is acceptable for 1%-tolerance resistors and 5%-tolerance 100-nF capacitors. Also, the amplitude of vi (which is a 1700-Hz sine wave), should not exceed 1 V.

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