High frequency transformer and mixer using the same

Abstract

The present invention provides a high frequency transformer capable of stable operation from DC to high frequency. Additionally the present invention provides a mixer with low insertion loss and low distortion, in which a common mode choke is removable from an input portion thereof. To form the stable high frequency transformer according to the present invention, a short circuiting pattern couples two strip lines together to form a U-shaped pattern on a printed circuit board and a lead wire is insulatedly coupled to each strip line and short-circuited at the tip of the U-shape. According to the present invention, a local oscillator is connected to a first port of the high frequency transformer, a diode switch is connected to a second port of the high frequency transformer, thereby serving as an IF output terminal, and a third port is formed at the short-circuiting pattern, thereby serving as the RF input terminal. Each of the strip lines is designed so as to have a characteristic impedance twice as large as the characteristic impedance of the RF input system providing the RF input. Matching the characteristic impedance of the strip lines and the characteristic impedance of the RF input system is achieved even when a common mode choke is coupled to the local oscillator. In this way, the inductance of the common mode choke viewed from the RF input terminal can be neglected, so that a mixer with low insertion loss and low distortion is operable over a broad range of frequencies.