Effect of aliasing on spurs and PM noise in frequency dividers

We show that the noise and spurious signals over the entire input frequency response of a binary digital divider contribute with equal weight to the output signal. Similar results are also obtained using non-binary digital dividers. We present a simple model that allows one to understand the mechanisms that produce these aliasing effects. Only filters at the input or intermediate stages of division can reduce these effects in digital dividers. We also show that direct-digital-synthesis where the output of a counter is used to drive a shift register and a digital-to-analog converter to produce a sinewave output is largely free from these effects.