System and method for suppressing noise in a phase-locked loop circuit

Abstract

Systems and methods for increasing the signal-to-noise ratio of a frequency generator suppress phase noise and noise generated by mismatches in internal generator circuits. This is accomplished using a modulation scheme that moves the spurious noise signal out of the generator's loop bandwidth. When shifted in this manner, the noise signal can be removed entirely or to any desired degree using, for example, filters located along the signal path of the generator. In one embodiment, the sigma-delta modulator controls the value of a swallow divider placed along the feedback path of the phase-locked loop in order to achieve the desired degree of noise rejection. In another embodiment, the reference signal input to the phase locked loop is modulated to achieve noise suppression. In another embodiment, the above forms of modulation are combined in order to achieve the desired frequency offset. With these modulation techniques, the signal-to-noise ratio of the frequency generator can be substantially improved while achieving faster lock times.

Description

technical field [0001] The invention relates to the generation of frequency signals in signal processing systems. Background technique [0002] Phase-locked loop (PLI) circuits have been used in many wired and wireless applications to generate carrier frequency and timing reference signals. [0003] Figure 1(a) shows a PLL typically used in a communication transceiver. The circuit consists of a phase and frequency detector 1, a loop filter 2 and outputs a signal f of the desired frequency out The VCO3. The feedback loop connecting the oscillator to the input of the phase and frequency detector comprises a frequency divider 4 which divides the output of the oscillator by the value (N+1) / N. The analog-to-digital control circuit outputs a signal to the frequency divider for controlling the value N. [0004] In PLLs of the type described above, it is known that there is a trade-off between loop bandwidth and channel spacing. Also know that the channel spacing is the same as...

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Problems solved by technology

In fact, the spurious signals are so close to the output frequency (Δf is very small) that they lie within the loop bandwidth of the circuit and ...

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