Phase-locked loops (PLL's) may be used to implement signal combiners which coherently sum multiple signals from an array of sensors. In each combiner channel, the sensor signal is simultaneously downconverted to an intermediate frequency (IF) signal and phase-locked to an appropriately generated reference signal by a "long-loop" PLL. This loop maintains a nominal 90° phase difference between the IF signal and the reference signal irrespective of phase of the channel input (sensor output) signal. The channel IF signals are summed to generate the combiner output signal. The reference signal may be a locally generated sine wave or a delayed version of the combiner output signal. Imperfect phase control and, thus, imperfect signal combining results when noise voltages are associated with the channel signals. In this paper, a lincarized model of a PLL coherent combiner is developed. This model applies when the desired channel signals are equal amplitude and angle modulated; the channel noise voltages are equal level, Gaussian distributed, and independent; and the combiner phase errors are appropriately small. This model is then used to derive equations for the variance of differential phase errors associated with combiner operation and to show the effect of these phase errors on the average power in the combiner output signal. Relevant experimental results from a four-channel combiner are compared with the performance predicted by the linear model.