Frequency division/multiplication with jitter minimization
View Patent ↗A method and system described for producing frequency multiplication/division by any non-integer output signal frequency relative to a reference signal frequency of a Phase Lock-Loop (PLL), while simultaneously maintaining low jitter. In one embodiment, the invention increases the number of the available clock phases to M and then shifts the output clock phase by one, every K/M cycle. In one aspect of the present invention, this is accomplished by adding a multiplexer (MUX) to the output of the PLL to implement the phase shifting every K/M cycles. In another aspect, the MUX is placed in the feedback loop of the PLL. In one embodiment, a quantizer is used to drive the MUX.
1. A method comprising:
(a) using a phase-lock loop (PLL) to produce M clock phases from a reference signal;
(b) producing a count signal from the M clock phases;
(c) producing a shifted signal from the M clock phases and the count signal; and
(d) dividing the shifted signal to produce a feedback signal that is fed back to the PLL.
2. The method of claim 1 , further comprising:
quantizing the count signal to produce a quantized signal, wherein the quantized signal in used to perform step (c).
3. The method of claim 2 , wherein the quantizing step comprises using a truncator that outputs the most significant j bits of the count signal.
4. The method of claim 2 , wherein the quantizing step comprises using a random number generator that adds one to or subtracts one from the count signal at a random time interval.
5. The method of claim 2 , wherein the quantizing step comprises using a Sigma-Delta noise shaping method.
6. The method of claim 5 , wherein the Sigma-Delta noise shaping method comprises using a noise shaping transfer function of Z −1 /(1−Z −1 ).
7. The method of claim 1 , wherein step (c) comprises using a multiplexer (MUX) to perform the producing of the shifted signal.
8. The method of claim 1 , wherein step (c) further comprises:
shifting the reference signal one phase every K/M cycles, wherein the reference signal is at least one of decreased by (K+1)/K and increased by K/(K−1).
9. The method of claim 1 , wherein step (d) further comprises:
setting a frequency of the feedback signal using a divide-by-N element.
10. The method of claim 1 , wherein step (b) further comprises:
extending a period T of the reference signal to at least one of T+T/M to perform frequency division of the reference signal and to T−T/M to perform frequency multiplication of the reference signal.
11. A method comprising:
(a) receiving a reference signal and a feedback signal at a clock generator;
(b) using the clock generator to generate M clock phases from at least one of the reference signal and the feedback signal;
(c) generating a count signal from the M clock phases;
(d) generating a shifted signal from the M clock phases and the count signal; and
(e) dividing the shifted signal to produce the feedback signal.
12. The method of claim 11 , further comprising quantizing the count signal before it is used in step (d).
13. The method of claim 11 , wherein step (d) comprises shifting the reference signal by one phase every K/M cycles.
14. The method of claim 11 , wherein step (b) comprises using a phase-lock loop (PLL) as at least part of the clock generator.
15. The method of claim 11 , wherein step (d) comprises using a multiplexer (MUX) that receives the M clock phases at one terminal and the count signal at another terminal to generate the shifted signal.
16. The method of claim 11 , wherein step (c) further comprises extending a period of the reference signal at least to T+T/M to perform frequency division of the reference signal and to T+T/M to perform frequency multiplication of the reference signal.