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 R_A (Fig. 2a) so that the phase difference between v_o1 and v_i 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 v_i (which is a 1700-Hz sine wave), should not exceed 1 V.