The performance of limiter discriminator detection ofM-ary FSK signals is analysed at arbitrary modulation indices. It is shown that the error rate performance of limiter discriminator detection can be significantly improved by increasing the modulation index above 1/M. The optimum modulation index that minimises the overall error probability is determined for the casesM= 2, 4 and 8. The analysis is carried out for wideband and bandlimited channels with Gaussian and second-order Butterworth filters. It is shown that the optimum modulation index depends on the signal/noise ratio (SNR), in a wideband channel, and on both SNR and timebandwidth product in a bandlimited channel. Finally, it is shown that the optimum sampling instance in presence of a nonzero phase IF filter can be approximately determined by using only the worst case symbol pattern.