Multi-point modulation and VCO compensation
View Patent ↗The present invention, generally speaking, provides a VCO linearization technique applicable to advanced loop architectures. In particular, the linearization technique is applicable to a mostly-digital frequency locked loop (FLL), phase locked loop (PLL) or the like using multi-point modulation. In an exemplary embodiment, a correction table is used to form a corrected control variable that affects one modulation point only (e.g., a fast modulation path) of the multi-point modulation circuit. The other modulation point (e.g., a slow modulation path) of the multi-point modulation circuit is controlled in accordance with an error-forming circuit including a loop filter. The use of correction within the fast path enables the VCO to achieve more rapid phase changes than would otherwise be possible, an advantage in high-data-rate communications applications, for example.
1. A circuit including a feedback loop for tracking an input signal having modulation applied thereto, comprising:
a controlled oscillator producing an output signal;
a slow modulation circuit, including a loop filter, coupled to the input signal and to the controlled oscillator; and
a fast modulation circuit coupled to the input signal and to the controlled oscillator, the fast modulation circuit including a compensation circuit configured to output an altered input signal, the compensation circuit referring to predetermined information to alter the input signal in accordance with an output frequency characteristic of the controlled oscillator, thereby improving linearity of the output signal.
2. The apparatus of claim 1 , wherein the compensation circuit is configured to alter the input signal by referring to a lookup table.
3. A method of controlling a variable oscillator to produce an output signal, using circuitry including a slow modulation circuit having a loop filter and a fast modulation circuit having a compensation circuit, both the slow and the fast modulation circuit being coupled to an input signal and to the variable oscillator, the method comprising:
determining an output frequency characteristic of the variable oscillator; and
within the compensation circuit configured to output an altered input signal, the compensation circuit referring to predetermined information to alter the input signal in accordance with the output frequency characteristic of the controlled oscillator, thereby improving linearity of the output signal.
4. The method of claim 3 , wherein the variable oscillator and said circuitry are incorporated within a device, and the output frequency characteristic of the variable oscillator is determined automatically during use of the device by a user.
5. The method of claim 4 , wherein the compensation circuit includes a lookup table, the method further comprising updating contents of the lookup table during use of the device.
6. The apparatus of claim 1 , wherein the slow modulation circuit is configured to receive a feedback signal based on the output signal, and
the compensation circuit is configured to output the altered input signal without referring to the feedback signal.
7. The apparatus of claim 1 wherein the slow modulation circuit further includes a difference engine receiving the input signal and the output signal and generating a phase error signal which is transmitted to the loop filter.
8. The apparatus of claim 7 wherein the difference engine generates the phase error signal and a frequency error signal which are transmitted to the loop filter.
9. The apparatus of claim 8 wherein the slow modulation circuit further includes a charge pump connected to the loop filter and the controlled oscillator.
10. The apparatus of claim 2 wherein the lookup table is periodically updated to account for changes in characteristics of the controlled oscillator.
11. The method of claim 3 , wherein the slow modulation circuit is configured to receive a feedback signal based on the output signal, and
the compensation circuit is configured to output the altered input signal without referring to the feedback signal.
12. In a phase lock loop including a controlled oscillator producing an output signal for high-data-rate transmission, the improvement comprising:
a slow modulation circuit, including a loop filter, coupled to an input signal and to the controlled oscillator, the slow modulation circuit configured to receive a feedback signal based on the output signal; and
a fast modulation circuit coupled to the input signal and to the controlled oscillator, the fast modulation circuit including a compensation circuit configured to output an altered input signal without referring to the feedback signal, the compensation circuit referring to a lookup table, which is periodically updated to account for changes in characteristics of the controlled oscillator, to alter the input signal in accordance with an output frequency characteristic of the controlled oscillator, thereby improving linearity of the output signal.