Abstract: In a modem device comprising (a) a first surface acoustic wave (SAW) rolloff filter circuit (51) having a first passband delay characteristic for primarily band-limiting into an output signal of a modulator device a multivalued quadrature amplitude modulated signal derived from input digital signal sequences and (b) a second surface acoustic wave rolloff filter circuit (52) having a second passband delay characteristic for rolloff waveform shaping an input signal of a demodulator device into a rolloff waveform shaped signal which is used in producing output digital signal sequences of the demodulator device, the second passband delay characteristic is selected as an inverse characteristic of the first passband delay characteristic. The first and the second passband delay characteristics have first and second ripple amplitudes. Preferably, the first and the second ripple amplitudes are given opposite polarities. More preferably, the second ripple amplitude is substantially equal to the first ripple amplitude.

Abstract: An analog-to-digital converter for use with a stepped square 2.sup.2n -QAM demodulator comprises a first group of (2.sup.n+1 -1) comparators for detecting signal points of a demodulated signal of a stepped square QAM system which are located within a 2.sup.n .times.2.sup.n square of the signal constellation of the stepped square QAM system, and a second group of four comparators for detecting signal points located outside of the square. A first logic circuit is provided to convert the outputs of the comparators of the first group to a first multibit digital signal representative of the signal points located within the square, and a second logic circuit is provided for converting the outputs of the comparators of the second group to a second multibit digital signal representative of the signal points located outside of the square.

Abstract: A phase shifter includes two phase shift circuits connected in parallel with each other between an input terminal for a signal of a voltage e.sub.i and ground and each consisting of a series circuit of a resistor and a capacitor, first and second differential amplifiers for receiving at their first terminals voltages e.sub.1 and e.sub.2 of junctions between the resistors and the capacitors of the phase shift circuits and for receiving input signals at the second input terminals, the collectors of noninverting and inverting transistors of the first differential amplifier being connected to the collectors of inverting and noninverting transistors of the second amplifier, respectively, thereby obtaining a vector sum e.sub.0 of e.sub.1 and e.sub.2, and a third differential amplifier consisting of two transistors having the first and second differential amplifiers as loads at their collector sides, respectively, and capable of arbitrarily changing a phase shift between the input signal e.sub.i and the output e.