VARIABLE MODULUS INTERPOLATOR, AND A VARIABLE FREQUENCY SYNTHESISER INCORPORATING THE VARIABLE MODULUS INTERPOLATOR

摘要

An indirect variable frequency synthesiser for synthesising selectable frequencies from a reference frequency including a multi-divisor programmable frequency divider located in a feedback loop of the frequency synthesiser for dividing the feedback frequency in the feedback loop, the divider being responsive to a varying control signal applied thereto representative of a rational number of selectable value for selecting the divisor thereof for fractional division of the feedback frequency. A variable modulus interpolator converts a fractional part of the rational number of selectable value to a varying digital code representative of the fractional part of the rational number. The variable modulus interpolator includes an Nth order sigma-delta modulator having N sigma-delta stages cascaded in MASH configuration, where N is an integer greater than one, and is responsive to a selectable value of a numerator and a selectable value of a denominator of the fractional part of the rational number for outputting the varying digital code. A primary summer sums a digital code representative of a selectable value of the integer of the rational number with the varying digital code from the variable modulus interpolator to produce the varying control signal for applying to the programmable frequency divider.