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.