Abstract : Instantaneous frequency measurement (IFM) system is the initial stage in the electronics intelligence (ELINT) application, prior to further analysis to estimate the RF frequency instantaneously. Recently optical instantaneous frequency measurement (OIFM) system attracted the attention due to its various features such as immunity to electromagnetic and high operating range. However, most OIFM systems conducted in this field assumed using ideal components except one work done by Fandino et al. (2015), that considered the impact of non-ideality on the amplitude comparison function (ACF) used in the OIFM system. In this work, two OIFM systems are investigated under non-idealities of Mach Zehnder Modulator (MZM). The two systems are both based on Single Mode Fiber as a source for dispersion. The firs is based on a single ACF and the second is an improvement on the first that is based on a dual ACF to extend the operating frequency. The non-idealities of MZM are the extension ratio (ER), drift in MZM bias and RF input power. First, the impact of each imperfect parameter individually was studied then the impact of combining them all on the OIFM system performance was studied. The results show that ACF is affected by all the individual parameters under study. However, the high RF input has the highest impact in changing the shape of the ACF and amplitude of the ACF, which limits the dynamic range of both OIFM systems. Although using the dual ACFs system to estimate RF frequency it operates over wider range under ideal conditions, this is not the case under non-ideal conditions. Thus, more attention should be paid to the imperfect parameters and their impact on the performance of OIFM system.