Pulse frequency modulation (PFM) has been considered by several authors for transmitting wide-band signals. However, the noise performance of PFM has been analyzed only above the SNR threshold. In this correspondence the results on the noise performance of PFM available in the literature are summarized, and a more general expression describing noise performance of PFM, both above and below the SNR threshold, is derived. This expression can be used to evaluate the SNR threshold for channels with both signal-independent noise (e.g., metallic facilities) and signal-dependent noise (e.g., fiber optic facilities). In particular, it is shown that in systems with signal-independent white noise, the SNR threshold is approximately 20 dB.