This invention relates to an FSK (Frequency Shift Keying) modulator using a surface acoustic wave resonator.
FIG. 11 is a circuit diagram for showing the principles of operation of a prior art FSK modulator using a surface acoustic wave resonator, comprising a transistor Q.sub.31 having a surface acoustic wave resonator of a single port type 31 and a variable-capacity diode 32 connected in series between its collector and base and an input terminal for modulation connected to a junction between the surface acoustic wave resonator 31 and the variable-capacity diode 32. Inductive reactance is provided by the surface acoustic wave resonator 31 and the variable-capacity diode 32. Thus, the circuit shown in FIG. 10 may be said to be of a Colpitts type. When a digital input signal is applied to the variable-capacity diode 32, frequency of oscillation is changed accordingly, and a FSK-modulated signal is outputted.
It may be attempted to produce such a prior art FSK modulator in the form of a single-chip monolithic integrated circuit in order to make it compact, but the transistor Q.sub.31 and the variable-capacity diode 32 cannot be formed on the same chip because they require different semiconductor production processes. In other words, prior art modulators of this type could not be made very compact.