Digital QP detecting apparatus, spectrum analyzer having the same, and digital QP detecting method

A QP detection unit includes digital circuit elements. There is provided a digital QP detector which detects an electric power signal Vi, which is an input signal, and outputs a detection signal Vo. The digital QP detector includes a register which records input digital data, a first multiplier which multiplies the digital data recorded in the register by a first coefficient, a second multiplier which multiplies the digital data recorded in the register by a second coefficient, an adder which adds the electric power signal Vi and the output from the first multiplier to each other, a comparator which compares the level of the electric power signal Vi and the level of the detection signal Vo, and a first switch which switches the digital data to be fed to the register between the output from the adder (Vi>Vo) and the output from the second multiplier (Vi