FM SIGNAL MEASURING APPARATUS

摘要

PROBLEM TO BE SOLVED: To obtain an FM signal measuring apparatus which can be built easily in a digital modulation system measuring apparatus and exhibiting highly accurate demodulation characteristics without requiring any troublesome adjusting work. SOLUTION: An FM modulated signal S(t) to be measured is converted through an A/D converter 23 into a digital data which is then fed to a quadrature demodulator 24 generating base band components I(t), Q(t) having orthogonal phase through frequency conversion. Subsequently, an FM demodulator 26 calculates the phase ϕ of the signal to be measured by performing arctangent conversion according to a formula ϕ(t)=tan<-1> [Q(t)/I(t)] and demodulates the modulated signal by performing differentiation for that phase ϕ. A second frequency measuring means 32 determines the difference Δf between the carrier frequency of the signal to be measured and the frequency fc of local oscillation signal of the quadrature demodulator from the DC component of demodulate output. The carrier frequency of the signal to be measured is determined by adding the frequency f0 of a signal from the quadrature demodulator 24 and the frequency fc of a local oscillation signal from a local signal generating section 21.