On Pulse Position Modulation and Its Application to PLLs for Spur Reduction

Randomizing the positions of charge pump current pulses in a PLL breaks their periodicity and redistributes the reference spurs into broadband noise. Closed form expressions for the power spectral density (PSD) of pulse position modulated (PPM) signals are derived and intuitive explanations for the results are given. The redistributed noise has a high-pass shape and does not affect the close in phase noise of the PLL. PPM using a uniformly distributed i.i.d. sequence completely removes the spurs and provides a first-order shaping of redistributed noise. Higher order shaping and reduction of redistributed noise at intermediate offset frequencies are possible using PPM with a high-pass shaped modulating sequence and pulse repetition. Circuit implementations of these techniques are given and their nonidealities are discussed. Simulation results from a 1 GHz PLL operating from a reference frequency of 20 MHz and a bandwidth of 1 MHz confirm the results of the analysis and viability of the proposed techniques. In the presence of nonidealities spurs can be reduced by at least 13 dB without any trimming of the delays in the PPM circuits and by 25 dB after trimming the delays to within 5% of the nominal value.