System and method for reducing spectral pollution in a signal
View Patent ↗A system and a method for modulating an input signal are provided. The system includes a fractional-N phase locked loop (PLL) for frequency multiplying the input signal by a multiplication factor to generate an output signal. The fractional-N PLL includes an input signal path and a feedback signal path. The system includes a controllable delay line for inserting a linearizing tone into the input signal path or the feedback signal path of the fractional-N PLL.
1. A system comprising:
a fractional-N phase locked loop (PLL) for frequency multiplying an input signal by a multiplication factor to generate an output signal, the fractional-N PLL comprising an input signal path and a feedback signal path; and
a dynamically adjustable delay line controllable by a linearizing phase modulation signal for inserting a linearizing tone into the input signal path or the feedback signal path of the fractional-N PLL to suppress spurious spectral content generated by a non-linearity in a phase detector and charge pump of the fractional-N PLL.
2. The system of claim 1 , wherein a frequency of the linearizing tone is unrelated to a quantization noise repetition frequency of the fractional-N PLL.
3. The system of claim 1 , wherein the fractional-N PLL comprises a phase detector and a charge pump, and wherein the linearizing tone is selected such that a phase detector and charge pump transfer function of the fractional-N PLL is linearized over a time scale near or below an inherent low pass time constant of the fractional-N PLL.
4. The system of claim 1 , wherein the linearizing tone comprises a signal having a frequency at least ten times a bandwidth of the fractional-N PLL.
5. The system of claim 1 , wherein the linearizing tone comprises a plurality of tonal phase modulation signals and wherein each of the plurality of tonal phase modulation signals has a unique frequency.
6. The system of claim 5 , wherein the plurality of tonal phase modulation signals avoid intermodulation below a low pass bandwidth of the fractional-N PLL.
7. The system of claim 3 , wherein a frequency and amplitude of the linearizing tone is optimized for a fraction value of the fractional-N PLL.
8. The system of claim 1 , wherein the linearizing phase modulation signal is generated by a multi-modulus divider in the feedback signal path of the fractional-N PLL.
9. The system of claim 8 , wherein the multi-modulus divider produces the linearizing tone, and wherein the linearizing tone is superimposed on an underlying fractional-N phase modulation signal of the fractional-N PLL.
10. The system of claim 1 , wherein the linearizing phase modulation signal is a modulated analog delay signal or a modulated digital delay signal.
11. The system of claim 1 , wherein the linearizing tone comprises a simple tone with a frequency equal to half of a frequency of the input signal.
12. The system of claim 1 , wherein the linearizing tone comprises equal magnitude positive and negative phase offsets for alternating input signal cycles.
13. A method comprising:
frequency multiplying, using a fractional-N phase locked loop (PLL), an input signal by a multiplication factor to generate an output signal, the fractional-N PLL comprising an input signal path and a feedback signal path; and
inserting a linearizing tone into the input signal path or the feedback signal path of the fractional-N PLL using a dynamically adjustable delay line controllable by a linearizing phase modulation signal to suppress spurious spectral content generated by a non-linearity in a phase detector and charge pump of the fractional-N PLL.
14. The method of claim 13 , wherein a frequency of the linearizing tone is unrelated to a quantization noise repetition frequency of the fractional-N PLL.
15. The method of claim 13 , wherein the fractional-N PLL comprises a phase detector and a charge pump, and wherein the method further comprises selecting the linearizing tone such that a phase detector and charge pump transfer function of the fractional-N PLL is linearized over a time scale near or below an inherent low pass time constant of the fractional-N PLL.
16. The method of claim 13 , wherein the linearizing tone comprises a signal having a frequency at least ten times a bandwidth of the fractional-N PLL.
17. The method of claim 13 , wherein the linearizing tone comprises a plurality of tonal phase modulation signals and wherein each of the plurality of tonal phase modulation signals has a unique frequency.
18. The method of claim 17 , wherein the plurality of tonal phase modulation signals avoid intermodulation below a low pass bandwidth of the fractional-N PLL.
19. The method of claim 15 further comprising:
optimizing a frequency and amplitude of the linearizing tone for a fraction value of the fractional-N PLL.
20. The method of claim 13 further comprising:
generating the linearizing phase modulation signal using a multi-modulus divider in the feedback signal path of the fractional-N PLL.
21. The method of claim 20 further comprising:
superimposing the linearizing tone on an underlying fractional-N phase modulation signal of the fractional-N PLL.
22. The method of claim 13 wherein:
the linearizing phase modulation signal is a modulated analog delay signal or a modulated digital delay signal.
23. The method of claim 13 , wherein the linearizing tone comprises a simple tone with a frequency equal to half of a frequency of the input signal.
24. The method of claim 13 , wherein the linearizing tone comprises equal magnitude positive and negative phase offsets for alternating input signal cycles.