Feed-Forward Compensation Circuit for Digital Random-Walk Filters
Original Publication Date: 1988-Dec-01
Included in the Prior Art Database: 2005-Feb-15
A technique is described whereby a feed-forward compensation circuit is implemented in a class of digital filters, known as random-walk filters (RWFs) or sequential filters [*]. The concept is especially useful in second-order phase-locked loop (PLL) applications, so as to stabilize the overall loop response. In a typical application of a RWF to a first-order PLL, as shown in Fig. 1, the output of the RWF is used to control the phase of the oscillator. This is done by controlling an adjustable divider, which adds or subtracts '1' or '0' pulses from the oscillator (OSC). The resulting pulse stream is fed to a divide-by-N counter so as to provide a phase change of 360/N degrees for each output of the filter. To achieve a reasonable resolution in the oscillator phase, N must be larger than ten and preferably greater than thirty.