SIGMA DELTA MODULATOR WITH A RANDOM OUTPUT BIT STREAM OF SPECIFIED I/O AVERAGE RATIO
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2002-Feb-05
A 2nd order digital Sigma-Delta (SD) modulator with a digital input representing a rational fraction (e.g. 9/16) produces at its output a bit stream at the clock rate with the "1" ratio equal the input fraction. In our case the problem is that with fractions like 9/16, the output bit stream is periodic with period of 16 or 32, so its frequency spectrum has many sideband tones. In our application-a functional-N synthesizer with divider modulus control as shown in Figure 2-we wish to con- trol the divider modulus with a random output bit stream with the correct "l"/"O" ratio so that its frequency spec- trum is spread and is not periodic.