Method for measuring modulation parameters of digital quadrature-modulated signal

The signal to be measured is converted into digital form and read into a memory 13 using a trigger as the reference, and a signal of a length 1/5 that of the signal to be measured, in the vicinity of the trigger, is orthogonal-transformed (14). A frequency error ω1 of the transformed output I1,Q1 is estimated (15) and is used to correct the output II,QI (16). The corrected output I2,Q2 is subjected to processing by a receiving filter (17), and the filtered output I3,Q3 is synchronized with a PN code to obtain an ideal signal RI,RQ and a phase difference τ1 is calculated. The signal I3,Q3 and the ideal signal RI,RQ are used to obtain a frequency error ω2 and a phase error τ2 of the signal I3,Q3. All signals to be measured, stored in the memory 13, are subjected to orthogonal transformation (23) so that the frequency error ω1+ω2 and the phase error τ1+τ2 are removed, and the transformed output I5,Q5 and the signal RI,RQ are used to calculate modulation parameters as is the case with the prior art.