This article discusses a constant carrier output modulation format called Nyquist quadrature frequency shift keying (NQFSK) that presents a perfect quadriphase eye pattern at the transmitter output and achieves excellent spectral containment.
 It uses a strictly bandlimited 4-level frequency modulating symbol pulse to achieve a perfect quadriphase eye pattern at the transmitter output.
 The spectral degradation from hard-limiting of a non-constant carrier envelope offset quadriphase shift keyed (OQPSK) type data signal has also been the subject of some investigation.
That property defines a perfect quadriphase eye pattern.
The effects of filter distortion on intersymbol interference for coherently detected biphase and quadriphase
phase-shif-keyed (BPSK and QPSK) signals have been studied Bit error rate (BER) degradation is minimal for an amplitude ripple of less than 0.1 dB peak-to-peak, increasing to 0.5 dB degradation for 1 dB amplitude ripple.