Parallel fractional-N phase locked loop circuit
View Patent ↗Novel phase locked loop architectures that can overcome the limitation of the maximum operating frequency of the fractional-N phase-locked loop (PLL) for fast-chirp frequency modulated continuous wave (FMCW) radars are suggested. Several phase frequency detector and charge pumps (PFD&CPs) are put in parallel and are operated with reference signals that are generated by using a delay-locked loop (DLL) instead of further increasing the operating frequency of the PFD&CP. The proposed DLL supported parallel PLL architectures enable further speeding up the FMCW chirp as well as improving its linearity and the performance of Range Doppler Radars based on fast-chirp FMCW radar. Methods for operating the parallel fractional N phase locked loop are proposed.
1. A phase locked loop circuit comprising a reference signal source and a voltage controlled oscillator for generating a phase and/or frequency controlled output signal, wherein a multi-modulus divider divides the output signal of the voltage controlled oscillator to generate a divider signal that is compared with a reference signal in a phase and/or frequency detector outputting an error signal that controls the voltage controlled oscillator, wherein the phase locked loop circuit comprises:
a delay locked loop as the reference signal source that provides n reference signals that have the same frequency and are shifted among each other by a fixed phase difference, wherein n is a natural number,
generator means for generating one associated divider signal for each reference signal,
n phase frequency detectors and charge pumps configured such that each one of the reference signals and its associated divider signal are connected as a pair of input signals to one phase frequency detector and charge pump outputting an error signal if a frequency and/or phase difference is detected between the input signals;
a node where all error signals generated by the phase frequency detectors and charge pumps are summed up to generate a control voltage signal for controlling the voltage-controlled oscillator, wherein
the multi-modulus divider is operatively connected with an associated delta sigma modulator to form a fractional-N divider, and wherein
the phase locked loop circuit comprises a plurality of multi-modulus dividers and associated delta sigma modulators to form parallel fractional-N dividers, wherein a multi-phase splitter receives the output signal of the voltage controlled oscillator or of the pre-scaler and outputs a plurality of phase shifted output signals to the plurality of fractional-N dividers such that each one of the plurality of fractional-N dividers receives one respective signal of the plurality of phase-shifted output signals.
2. The phase locked loop circuit according to claim 1 , wherein the generator means includes a plurality of multi-phase splitters each receives the output signal of one of the multi-modulus dividers for generating n divider signals, that are shifted by an equal phase difference relative to one another, and wherein their frequency is n-times smaller than the frequency of the output signal of the plurality of multi-modulus dividers.
3. The phase locked loop circuit according to claim 1 , wherein each output of the plurality of multi-modulus dividers is connected to one of the associated delta sigma modulators as a clock signal.
4. The phase locked loop circuit according to claim 1 , wherein the phase locked loop circuit comprises a pre-scaler receiving the output of the voltage-controlled oscillator and outputting a lower frequency signal to the plurality of multi-modulus dividers.
5. The phase locked loop circuit according to claim 1 , wherein the output signal of each multi-modulus divider is provided as a clock signal to its associated delta sigma modulator forming one of the fractional-N dividers.
6. The phase locked loop circuit according to claim 5 , wherein each reference signal is provided as clock signal to one of the delta sigma modulators forming one of the fractional-N dividers.
7. The phase locked loop according to claim 1 , wherein a frequency ramper controls the delta sigma modulators.
8. The phase locked loop according to claim 1 , wherein the multi-phase splitter receives an input signal from the pre-scaler.
9. The phase locked loop according to claim 1 , wherein the output of each one of the multi-modulus dividers is connected with a multi-phase splitter outputting a plurality of divider signals.
10. The phase locked loop circuit according to claim 1 , wherein the delay locked loop includes a multiplier multiplying the reference signal by a factor n to generate a clock signal to be provided to each one of the delta sigma modulators of the parallel fractional-N dividers.
11. The phase locked loop circuit according to claim 1 , further comprising a combination of parallel phase frequency detectors and charge pump, parallel fractional-N dividers, one or multiple multipliers for multiplying a reference signal, one or several multi-phase splitters connected with an output of a fractional-N divider and/or one or several multi-phase splitters providing an input to the one or several fractional-N dividers.
12. A radar sensor comprising a phase locked loop circuit according to claim 1 .
13. A method for operating a phase locked loop, the method comprising
providing n reference signals that have the same frequency and are shifted among each other by a fixed phase difference, wherein n is a natural number,
receiving in a multiphase splitter an output signal of a voltage controlled oscillator or a pre-scaler;
generating in the multiphase splitter a plurality of phase shifted output signals;
outputting the plurality of phase shifted output signals to a plurality of fractional-N dividers, such that each one of the plurality of fractional-N dividers receives one respective signal of the plurality of phase shifted output signals from the multiphase splitter;
generating in the plurality of fractional-N dividers one associated divider signal for each reference signal;
connecting each one of the reference signals and the associated divider signal as a pair of input signals to one phase frequency detector and charge pump;
outputting an error signal if a frequency and/or phase difference is detected between the input signals; and
summing up all error signals to control a voltage-controlled oscillator such that a potentially existing frequency and/or phase difference between the pairs of input signals is reduced.