A generic reflectometer is modelled as a device for launching and rece1vmg radiation at a number of microwave frequencies, uniformly distributed over a specified bandwidth. The effect of density fluctuations is modelled as a phase error at each frequency. The phase distortion has a known correlation length in frequency space, allowing a semi-analytical prediction of the resultant error on the time delay. Specific techniques are compared to the generic model: Frequency sweep, differential phase, AM and sine FM modulation. There is no significant difference between them and the model in their response to the same type of phase error. An implication of the model is that there are situations where profile details are unrecoverable by certain types of reflectometer on a short time scale. These arise when the phase fluctuations are strong and the phase correlation length short, conditions found in the plasma edge.