Patent ID: 6329862
Filing Date: 2001-12-11
Classification: H03K

Abstract:
A reference frequency signal switching circuit comprising:an internal oscillator to generate an internal reference frequency signal;an external reference frequency signal input terminal to which an external reference frequency signal is supplied;a signal switching circuit;an output level adjusting circuit; anda reference frequency signal output terminal from which the internal reference frequency signal and the external reference frequency signal are outputted,wherein when the external reference frequency is not supplied, the signal switching circuit outputs the internal reference frequency signal from the internal reference frequency oscillator to the reference frequency signal output terminal via the output level adjusting circuit, when the external reference frequency is supplied, the signal switching circuit outputs the external reference frequency signal to the reference frequency signal output terminal via the output level adjusting circuit,wherein the output level adjusting circuit adjusts an output level of the external reference frequency signal outputted to the reference frequency signal output terminal to be constant irrespective of the level of an input,wherein the output level can be changed, andwherein the output level adjusting circuit comprises a PIN diode and a differential amplifier,the PIN diode is connected in series between the signal switching circuit and the reference frequency signal output terminal,a reference voltage is supplied to one of input terminals of the differential amplifier, a detection voltage proportional to the level of the external reference frequency signal is supplied to another input terminal of the differential amplifier, an output terminal of the differential amplifier is connected to one end of a PIN diode, the magnitude of a current passing through the PIN diode is changed according to an output voltage of the differential amplifier, and the output level is changed according to the reference voltage.