Patent ID: 7127219

Claim:
A transmitter outputting a microwave signal, comprising: a circuit for making constant a level of an input intermediate frequency signal; and a frequency converting circuit frequency-converting the intermediate frequency signal of which level is made constant by said circuit for making constant the level, to said microwave signal, wherein the detection is made at a point between the circuit and the frequency converting circuit, wherein said circuit for making constant the level of the input intermediate frequency signal includes a π-type variable attenuation circuit receiving said intermediate frequency signal, of which amount of level attenuation is variable, a detecting circuit detecting an output signal of said variable attenuation circuit and providing a detection signal in accordance with an input level, a reference voltage generating circuit generating a reference voltage, and a comparing circuit comparing the reference voltage from said reference voltage generating circuit with the detection signal from said detecting circuit, and varying the amount of attenuation of said variable attenuation circuit based on a comparison output signal, wherein said variable attenuation circuit includes a PIN diode having each amount of attenuation changed as terminal to terminal resistance changes in accordance with the comparison output signal from said comparing circuit, wherein said detecting circuit includes a detecting diode rectifying an output signal of said variable attenuation circuit, a capacitor smoothing the rectified output signal to provide a DC signal, and a resistance outputting the rectified DC signal, and wherein said reference voltage generating circuit includes a plurality of resistance circuits.