An actively mode-locked laser (AMLL) is subjected to RF modulation frequency detuning away from its fundamental resonant frequency. The standard theoretical model of mode-locked lasers is extended with perturbative analysis and injection locking theory. The proposed model shows that with increasing detuning, the number of modes with phase-lock decreases. For a mode n, the injection range Rn is found to decrease as n−1, thus making higher modes more susceptible to unlocking by RF detuning. Experimental verification of these theoretical predictions is obtained by using a custom built Yb 3+ doped fiber based ring AMLL. The AMLL operates at 1064nm, with a repetition rate of f0=26.69MHz. The unlocking of modes is studied in the electrical domain by varying the RF detuning over a range of ±13kHz with a step-size of 130Hz. The locking limits of a mode decrease as n−1 as predicted by the theory. The fundamental mode (n=1) has an injection range of 143.4kHz, with the higher modes, n>1, having smaller injection ranges as predicted by the theory. © 2018 Elsevier B.V.