In many commonly used frequency‐shift modulators, a phase error occurs at the time of switching. If a demodulator is used which utilizes only the zero‐crossing information, then this phase error will cause lime jitter in the received data transitions. The magnitude of the peak time jitter for various modulators is derived, assuming an ideal zero‐crossing detector. The modulators considered include the reactance tube and variable reactance modulators, the basic switched reactance modulators, and the multivibrator modulator. It is found that the switched reactance modulators cause the most jitter, and that the multivibrator modulator may be designed to cause as small a jitter as desired. The theory agrees well with some experimental measurements made on existing data sets, which show that this jitter accounts for most of the back‐to‐back data distortion in many wideband data systems. Finally, a set of sufficient conditions is derived for jitter‐free frequency‐shift modulation, and an implementation of a modulator satisfying these conditions is described.