FRACTIONAL-N FREQUENCY SYNTHESIZER WITH A DELTA-SIGMA FREQUENCY DISCRIMINATOR

摘要

A fractional - N frequency synthesizer comprises a voltage-controlled oscillator (107) for generating an output signal (Fo) in response to a contr ol voltage derived by a digital-to-analog converter (105) from a digital error signal (e). The error signal is derived by a differencing device (103) which subtracts a digital signal (Do) representing the actuated frequency of the output signal from an input signal (Fd) having the desired frequency for the output signal. The digital signal representing the output signal frequency i s derived by a frequency discrimination device (101) which determines the instant frequency of the analog output signal and provides a corresponding digital representation with zero static frequency error. In preferred embodiments, the frequency discrimination device is a delta-sigma frequency synthesizer in combination with a decimator (102). This frequency synthesize r configuration avoids deficiencies due to non-linearity and noise sensitivity of analog phase detectors. The invention also provides a frequency discriminator (101') comprising a feedforward path including a multimodulus divider (201) for dividing the frequency of a digital signal (Fo) in dependence upon a division ratio control signal, comparison (202) for comparing the divided frequency signal with a reference frequency signal (Fref) to provide an error signal, and a feedback path comprising a circuit having a transfer function of 2-Z-1 and responsive to the error signal and t he reference frequency signal to provide the division ratio control signal.