Advanced Frequency Synthesis by Phase Lock

CHAPTER 3

OTHER SPURIOUS REDUCTION TECHNIQUES

We have considered two methods for reducing the discrete spurs that tend to occur when n_fract uses only more significant bits, causing a short repetition period in the accumulators. One was offsetting n_fract by a small number and the other was presetting a small odd number into the accumulator every time n_fract is changed¹⁵ (a capability that some ICs may not provide). Here, we will consider two additional methods, randomly changing the least significant bit (LSB), at a rate of f_ref, and modifying the MASH structure to maximize the sequence length.

3.1   LSB DITHER

A popular method for eliminating discrete spurs is to dither (randomly change) the LSB at the input to the modulator. Figure 3.1 shows the results of dithering the LSB of a 10 bit modulator, using a pseudorandom sequence of 0s and 1s. The synthesizer output spectrum with such dithering is displayed above the spectrum obtained with 2⁻¹⁶ initially in the first accumulator. We see immediately that the center of the spectrum is shifted by the average value of the dither, half an LSB (2⁻¹¹ × 10 MHz = 4.88kHz), and that the PSD near spectral center is increased significantly.

The value of with this dither is plotted as q = 0 curve in Fig. 3.2. Also shown is the quantization noise of a MASH-111 ΣΔ modulator (p = 3). These are the levels that appear in Fig 3.1,¹⁶ but less the loop ...