PURPOSE:To improve the spectrum analyzing sensitivity of an FFT spectrum analyzer by performing addition or subtraction on signals obtained by multiplying one parts of input signals divided into two parts by a window function and signals obtained by multiplying the other part of the input signals by a Hilbert-transformed window function after the other part of the input signals are Hilbert-transformed. CONSTITUTION:A halving device 2 divides inputs signals into two parts and another halving device 11 divides window function signals from a window function generator 9 into two parts. A multiplier 4 multiplies one part of the input signals divided into two parts by means of the device 2 by the window function signals. A Hilbert transformer 7 performs Hilbert transformation on the other part of the input signals. Another Hilbert transformer 14 performs Hilbert transformation on the window function signals. A multiplier 16 multiplies signals from the transformer 7 by the Hilbert-transformed window function signals. An adder 18 adds signals from the multipliers 4 and 16 to each other. Even when the signals from the multipliers 4 and 16 are subject to subtraction, substantially the same acting effect can be obtained.
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