Distorsion of I/Q modulators

The I/Q modulators , when used as continuous phase shifters, generally exhibit a non-linear tranfer function. Fig.1 gives a first-order representation of it . This pattern repeats itself indefinitely modulo 2pi. The (small) deviations from the linear response can be decomposed in Fourier series within the 2pi window.

transfer function

Fig.1 : generic distorsion pattern of a phase shifter

When the I/Q modulator is used as a frequency translator, the different components of the series act as as many PM modulating signals, generating individual lines aside the wanted carrier, as sketched in Fig.2.

output spectrum

Fig.2 : output of an I/Q modulator translating its carrier by ΔF

The carrier-to-sideband ratio is directly related to the phase deviation created by the non-linear transfer. This is an extremely convenient and accurate way of measuring the phase non-linearity.