Hybrid phase-locked loop

[0002] 1. Field of the Invention

[0003] The present invention generally relates to a phase-locked loop (PLL) and, more particularly, to a hybrid phase-locked loop with digital processing and analog processing mixed therein.

[0004] 2. Description of the Prior Art

[0005] FIG. 1 shows a conventional analog phase-locked loop. The analog phase-locked loop 10 includes frequency dividers 11 and 12, a phase-frequency detector (PFD) 13, a charge pump 14, a loop filter 15 and a voltage-controlled oscillator (VCO) 16. The phase-locked loop 10 divides an input reference signal F_refand an oscillation signal F_VCOby the same/different multiple(s) such as M or/and N by using the frequency dividers 11 and 12. Then, the phase-locked loop 10 evaluates the differences of phase and frequency between the output signals from the frequency dividers 11 and 12 by using the phase-frequency detector (PFD) 13, so as to generate difference signals. The charge pump 14 and the loop filter 15 generate a control voltage V_Caccording to the difference signals. The voltage-controlled oscillator 16 outputs an oscillation signal F_VCOcorresponding to the control voltage V_C. The phase-locked loop 10 may change the frequency of the oscillation signal F_VCOby varying the frequency-dividing multiple(s) of the frequency dividers 11 and 12.

[0006] However, there exists a dilemma situation considering the circuit design. On the one hand, concerning the jitter of the reference signal F_ref, the loop bandwidth F_LBWof the PLL should be designed to be sufficiently narrow in order to filter out the F_refjitter; on the other hand, considering the jitter of the voltage-controlled oscillator 16, the loop bandwidth of the PLL should be wide enough to eliminate the F_VCOjitter. In addition to the dilemma situation in circuit design, there is also a problem related to the stabilization of the PLL. Generally, the PLL is reliable only when the ratio of F_LBWof the PLL to the frequency of F_refsatisfies the following inequality (1): $\begin{array}{cc}\frac{F_{\mathrm{ref}}}{F_{\mathrm{LBW}}}\ge K& \left(1\right)\end{array}$

[0007] The inequality (1) comes into existence when the frequency of F_refis relatively high. However, when the frequency of F_refis low and the output frequency of the voltage-controlled oscillator is high, the jitter of the voltage-controlled oscillator cannot be effectively eliminated since the loop bandwidth F_LBWof the PLL is not sufficiently large for the sake of stabilization. For example, in a liquid crystal display (LCD) controller chip, the frequency of the reference signal (i.e., the horizontal synchronous signal, HSYNC) F_refis in the range of 30 KHz˜100 KHz and the frequency of the voltage-controlled oscillator is in the range of 25 MHz˜200 MHz. The multiple is in the range of 800˜2000. Therefore, the jitter of the voltage-controlled oscillator cannot be suppressed by using a conventional PLL.

[0008] In view of the aforementioned problems, it is the primary object of the present invention to provide a hybrid phase-locked loop with digital processing and analog processing mixed therein.

[0009] It is another object of the present invention to provide a hybrid phase-locked loop, in which the long-term jitter of a voltage-controlled oscillator is effectively reduced when the frequency of a reference signal is low and the jitter of the reference signal is large.

[0010] In order to achieve the foregoing objects, the present invention provides a hybrid phase-locked loop, comprising: a phase-frequency detector, for detecting the phase difference between a reference signal and an oscillation feedback signal and generating a digital phase-difference signal according to a mean-frequency signal; a digit pump, for receiving the phase-difference signal and generating a proportional gain signal and an accumulative gain signal according to a proportional gain value and an accumulative gain value; a digital filter, for receiving the proportional gain signal and the accumulative gain signal so as to generate a digital control signal; a digital-controlled oscillator (DCO), for receiving the control signal and the mean-frequency signal so as to generate a phase-swap signal; a phase selector, for receiving a plurality of multi-phase signals and the phase-swap signal so as to select one among neighboring phases to be the mean-frequency signal according to the phase-swap signal; an analog phase-locked loop, for receiving the mean-frequency signal and filtering out the cycle-to-cycle jitter thereof so as to generate an output signal; and a frequency divider, for receiving the output signal and dividing the frequency thereof so as to generate an oscillation feedback signal.

[0011] Other and further features, advantages and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings are incorporated in and constitute a part of this application and, together with the description, serve to explain the principles of the invention in general terms.

[0012] The objects, spirits and advantages of the preferred embodiment of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein:

[0013] FIG. 1 is a schematic block diagram showing a conventional analog phase-locked loop;

[0014] FIG. 2 is a schematic block diagram showing a hybrid phase-locked loop in accordance with the present invention;

[0015] FIG. 3 is a timing diagram showing waveforms of a reference signal, an oscillation feedback signal and a phase-difference signal; and

[0016] FIG. 4 is a circuit diagram comprising a digit pump and a digital filter in FIG. 2.

[0017] The present invention providing a hybrid phase-locked loop can be exemplified by the preferred embodiment and the accompanied drawings as described hereinafter. In the prior-art PLL design, there is a dilemma situation when it comes to suppression of the jitter of the reference signal as well as the jitter of the oscillation signal from the voltage-controlled oscillator. However, in the present invention, the hybrid phase-locked loop suppresses the jitter of the reference signal by using digital processing and suppresses the jitter of the voltage-controlled oscillator by using analog processing.

[0018] FIG. 2 is a schematic diagram showing a hybrid phase-locked loop in accordance with the present invention. As shown in FIG. 2, the hybrid phase-locked loop 20 comprises: a phase-frequency detector 21; a digit pump 22; a digital filter 23; a digit-controlled oscillator 24; a phase selector 25; a multi-phase frequency generator 26; an analog phase-locked loop 27; and a frequency divider 28. More particularly, the phase-frequency detector 21, the digit pump 22, the digital filter 23, the digit-controlled oscillator 24, the phase selector 25 and the frequency divider 28 are included in digital processing, while the multi-phase frequency generator 26 and the analog phase-locked loop 27 are included in analog processing.

[0019] The phase-frequency detector 21 detects the phase difference between the reference signal F_refand the oscillation feedback signal F_band outputs a phase-difference signal PE. The phase-difference signal PE is a digit signal, indicating the number of pulses generated by the mean-frequency signal Fav during the phase-difference period between the reference signal F_refand the oscillation feedback signal F_b. The reference signal F_refis the horizontal synchronous signal (HSYNC) in a LCD controller chip. FIG. 3 is a timing diagram showing waveforms of a reference signal, an oscillation feedback signal and a phase-difference signal. As shown in FIG. 3, the larger the phase difference between the reference signal F_refand the oscillation feedback signal F_bis, the longer the phase-difference period would be, resulting in a larger phase-difference signal PE.

[0020] FIG. 4 is a circuit diagram comprising a digit pump 22 and a digital filter 23. The digit pump 22 receives the phase-difference signal PE and outputs a proportional output signal P and an accumulative output signal I. The digital filter 23 receives the proportional output signal P and the accumulative output signal I so as to generate a control signal PCW. The digit pump 22 comprises two multipliers 221 and 222. The multiplier 221 multiplies the phase-difference signal PE by a proportional gain value (P-gain) so as to generate a proportional output signal P. On the other hand, the multiplier 222 multiplies the phase-difference signal PE by an accumulative gain value (I-gain) so as to generate an accumulative output signal I. The accumulative output signal I is input into an adder 232 and an integrator 233 for integration and is then added to with the proportional output signal P by an adder 231 so as to generate the control signal PCW. As shown in FIG. 4, the adder 231 is equivalent to the resistor R_Sin a conventional analog loop filter, while the adder 232 together with the integrator 233 is equivalent to the capacitor C_Sin a conventional analog loop filter.

[0021] The digit-controlled oscillator 24 receives the control signal PCW and generates a phase-select signal PS. The digit-controlled oscillator 24 can be implemented by using an accumulator and the mean-frequency signal F_avas a triggering clock pulse. The control signal PCW is accumulated so as to output a carry signal functioning as a phase-select signal PS. Therefore, the larger the value of the control signal PCW is, the higher the probability that the phase-select signal PS appears would be. The relation between the frequency 𝑓_avof the mean-frequency signal F_av, the control signal PCW and the frequency 𝑓_psof the phase-select signal PS is described in Equation (2): $\begin{array}{cc}{f}_{\mathrm{ps}}=f_{\mathrm{av}}·\frac{\mathrm{PCW}}{2^n}& \left(2\right)\end{array}$

[0022] where 2ⁿindicates the maximum value of the digit-controlled oscillator 24, equivalent to an n-bit accumulator.

[0023] The multi-phase frequency generator 26 provides a plurality of multi-phase signals PH1˜PHn. The frequency of these signals PH1˜PHn is close to the frequency 𝑓_avof the mean-frequency signal F_av. The multi-phase frequency generator 26 has been disclosed in the prior art, and thus the description thereof is omitted. The phase selector 25 receives the multi-phase signals PH1˜PHn and selects among different phase signals according to the phase-select signal PS. In other words, the phase selector 25 selects a neighboring (preceding or successive) phase according to the frequency of the multi-phase signals PH1˜PHn and the frequency 𝑓_avof the mean-frequency signal F_avonce the phase-select signal PS is triggered. Therefore, when the system is at a steady state, the control signal PCW is fixed at a constant value such that the frequency of the phase-select signal PS allows the frequency of the mean-frequency signal F_avoutput from the phase selector 25 in an acceptable range.

[0024] Certainly, the frequency range of the mean-frequency signal F_avis limited by the frequency as well as the number n of the multi-phase signals PH1˜PHn generated by the multi-phase frequency generator 26. For example, if the frequency of the multi-phase signals PH1˜PHn is 35 MHz and the number n of the multi-phase signals is 16, the frequency range for the mean-frequency signal F_avis F_max˜F_min, where $\begin{array}{c}{F}_{\max }=35\mathrm{MHz}·\frac{17}{16}=37.1875\mathrm{MHz}\\ F_{\min }=35\mathrm{MHz}·\frac{15}{16}=32.8125\mathrm{MHz}\end{array}$

[0025] The analog phase-locked loop 27 is a general PLL for filtering out the cycle-to-cycle jitter of the mean-frequency signal F_av. Since the frequency of the mean-frequency signal F_avis much higher than that of the reference signal F_ref, the bandwidth of the analog phase-locked loop 27 can be designed to be wider so as to suppress the long-term jitter of the voltage-controlled oscillator in the analog phase-locked loop 27, reducing the jitter of the output signal F_VCO. Furthermore, the proportional gain value (P-gain) and the accumulative gain value (I-gain) can be adjusted smaller so as to filter out the jitter of the reference signal F_refby making the bandwidth of the loop narrower. The frequency divider 28 divides the output signal F_VCOby a multiple and then outputs an oscillation feedback signal F_b. Accordingly, the PLL controls the frequency of the output signal F_VCOby controlling the multiple.

[0026] Moreover, the present invention further provides a method for controlling a hybrid phase-locked loop. The method comprises steps of.

[0027] Step 1: detecting a phase frequency, wherein the phase difference between a reference signal and an oscillation feedback signal is detected so as to generate a digital phase-difference signal according to a mean-frequency signal.

[0028] Step 2: calculating gain signals, wherein the phase-difference signal is received and digital processing is used so as to calculate a proportional gain signal and an accumulative gain signal according to a proportional gain value and an accumulative gain value.

[0029] Step 3: calculating a control signal, wherein digital processing is used so as to calculate a digital control signal according to the proportional gain signal and the accumulative gain signal.

[0030] Step 4: digital oscillation, using a digit-controlled approach so as to generate a phase-swap signal according to the control signal and the mean-frequency signal, wherein the digit-controlled approach accumulates the control signal so as to output a carry signal to be the phase-swap signal.

[0031] Step 5: phase selecting, wherein a plurality of multi-phase signals and the phase-swap signal are received so as to select one among neighboring phases to be the mean-frequency signal according to the phase-swap signal.

[0032] Step 6: filtering out the jitter, wherein an analog phase-locked loop is used for filtering out the cycle-to-cycle jitter of the mean-frequency signal so as to generate an output signal.

[0033] Step 7: frequency dividing, wherein the output signal is received and the frequency thereof is divided so as to generate the oscillation feedback signal.

[0034] According to the above discussion, the present invention discloses a hybrid phase-locked loop with digital processing and analog processing mixed therein. Therefore, the present invention has been examined to be novel, unobvious and useful.

[0035] Although this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art. This invention is, therefore, to be limited only as indicated by the scope of the appended claims.