Phase lock loop and the control method thereof

[0001] The invention relates to a phase lock loop, and more specifically to a phase lock loop capable of adjusting states automatically and control method thereof.

[0002] FIG. 1A is a block diagram of a conventional phase lock loop (PLL), having a phase/frequency detector (PFD) 90, a charge pump 92, a voltage-controlled oscillator (VCO) 94, and a frequency divider 96. The PFD 90 detects the transitional edges of the feedback clock Clk_fb and the reference clock Clk_ref to generate comparative signals UP and DN which, in turn, charge and discharge the charge pump 92 to generate a control voltage V_c. The VCO 94 generates an output clock Clk_vco of variable frequency in response to the control voltage V_c. The frequency divider 96 divides the frequency of the clock Clk_vco to generate the feedback clock Clk_fb.

[0003] VCO is generally designed to operate in a single state, having a single voltage frequency transfer curve. There are two types of VCOs in terms of the voltage-frequency transfer curve slope, high gain and low gain VCOs. FIG. 1B shows two voltage-frequency transfer curves A and B, corresponding to high gain and low gain VCOs respectively. As depicted, high gain VCO provides the advantage of broader tuning frequency range. The performance of the output clock stability in high gain VCO, however, is inferior since frequency of the output clock is susceptible to the change in control voltage V_c. Conversely, low gain VCO provides a more stable frequency of the output clock but a narrower tuning frequency range. As a result, selecting a suitable voltage-frequency transfer curve for a VCO has been important for circuit designers.

[0004] The present invention is generally directed to a phase lock loop capable of adjusting operating states automatically. According to one aspect of the invention, the phase lock loop generates a feedback clock for tracing a reference clock, the phase lock loop comprising a phase/frequency detector, a charge pump, a voltage-controlled oscillator and a state select circuit. The phase/frequency detector (PFD) compares phase difference between the feedback clock and the reference clock to generate a comparative signal. According to the comparative signal, the charge pump generates a control voltage to the voltage-controlled oscillator (VCO). The VCO then operates in one of the select states to generate an oscillating clock according to the control voltage and a select voltage wherein the ratio of the frequency of the oscillating clock to the frequency of the feedback clock is an integer. Moreover, the state select circuit adjusts the select voltage in accordance with the reference clock to select one of the select sates.

[0005] According to another aspect of the invention, a method for controlling a phase lock loop is disclosed. The phase lock loop capable of operating in a plurality of select states generates a feedback clock for tracing a reference clock and the method comprises generating first and second clocks corresponding to the highest and lowest frequency oscillating clocks respectively, generated by the phase lock loop when operating in one of the select states, comparing the frequencies of the first and second clocks to the frequency of the reference clock respectively, thereby holding the select state of the phase lock loop when the first, second, and reference clocks are in a first predetermined condition; or changing the select state of the phase lock loop when in a second predetermined condition.

[0018] FIG. 2 shows the voltage-frequency transfer curves of a VCO according to an embodiment of the invention. The VCO is capable of operating in a plurality of select states, S, and each select state S corresponds to a voltage-frequency transfer curve. In FIG. 2, V_min and V_max in x-axis, denote the maximum and minimum values of the input control voltage V_C in the VCO, respectively and the denotation, f_L,S in y-axis, denotes the frequency of the feedback clock generated by the frequency divider with the VCO when the VCO operates in a select state S and the control voltage V_C is V_min. In other words, frequency f_L,S is the lowest frequency of the feedback clock generated by the frequency divider and the VCO in select state S. Conversely, frequency f_H,S is the highest frequency of the feedback clock generated by the frequency divider and the VCO in select state S. The tuning range of the feedback clock in each select sate preferably overlaps a small part of tuning range in the neighboring select state, thus ensuring that every frequency is covered by the tuning range of the feedback clock in at least one select state.

[0019] When changing or selecting select state S of the VCO properly, this VCO is capable of tracing a reference clock Clk_ref with frequency f_ref. In FIG. 3, a flow diagram illustrating an embodiment of the invention, corresponding frequencies f_L,S and f_H,S are provided according to the select state S of the VCO (step 12). It is determined whether the condition is fulfilled, f_L,S<f_ref<f_H,S (step 14). If so, the combination of the VCO and the frequency divider in the current select state S is capable of generating an output clock with frequency f_ref. Consequently, the select state S of the VCO is held and tracing of the reference clock Clk_ref is begun with the feedback clock generated by the VCO and frequency divider (step 18). If not, the select state S of the VCO (step 16) is changed. The rule of changing select states is, for example, changing select state S to another select state capable of generating a feedback clock with higher frequency if f_ref>f_H,S or a lower frequency if f_ref<f_L,S.

[0020] FIG. 4 shows a phase lock loop 40 according to an embodiment of the invention, comprising a phase/frequency detector (PFD) 30, a charge pump 32, a voltage-controlled oscillator (VCO) 34a, a frequency divider 36 and a state select circuit 38.

[0021] The PFD 30 compares the transitional edges of the feedback clock Clk_fb and the reference clock Clk_ref to generate comparative signals UP and DN. For example, when the PFD 30 detects the rising edge of the feedback clock Clk_fb later than that of reference clock Clk_ref, the PFD 30 then generates a comparative signal UP wherein the signal UP takes the form of a pulse having a width or duration corresponding to the phase difference between rising edges of the reference and feedback clocks, Clk_ref and Clk_fb. In a similar fashion, when the PFD 30 detects the rising edge of the feedback clock Clk_fb earlier than that of the reference clock Clk_ref, the PFD 30 generates a comparative signal DN.

[0022] When receiving the respective comparative signals, UP and DN, the charge pump 32 charges and discharges the VCO 34a accordingly. For example, the magnitude of control voltage V_c generated by the charge pump 32 is increased with the comparative signal UP and decreased with the comparative signal DN.

[0023] The VCO 34a operates in one of the select states, S, with the voltage-frequency transfer curve of the combination of the VCO 34a and the frequency divider 36 shown in FIG. 2. The phase lock loop 40 operates in one of the select states, S. Moreover, the select state S of the VCO 34a is controlled by a select voltage V_s and the VCO 34a generates an oscillating clock Clk_vco in response to the control voltage V_c generated by the charge pump 32.

[0024] The frequency divider 36 divides the frequency of the oscillating clock Clk_vco and generates the feedback clock Clk_fb for an oscillating clock Clk_vco with a frequency N times the frequency of the reference clock Clk_ref and N is an integer.

[0025] When the select state S of the VCO 34a is fixed, that is, the select voltage V_s is fixed, the operation of the phase/frequency detector (PFD) 30, the charge pump 32, the voltage-controlled oscillator (VCO) 34a, and the frequency divider 36 is the same as the operation of conventional PLLs and is not further described in detail here.

[0026] The state select circuit 38 receives the reference clock Clk_ref, examining the reference clock to generate a select voltage V_s for adjusting select state S of the VCO 34a.

[0027] FIG. 5 is a block diagram of the state select circuit 38 in FIG. 4. The state select circuit 38 checks if the frequency of the reference clock Clk_ref is within the tuning frequency range of the VCO 34a and frequency divider 36 in the current select state S. VCO 34b and VCO 34c are similar to the VCO 34a in terms of internal circuit structure and all three VCOs receive the same select voltage V_s. The inputs V_in of the VCO 34a, VCO 34b and VCO 34c, however, receive the control voltage V_c, a fixed voltage V_max and another fixed voltage V_min respectively. The fixed voltages V_max and V_min denote the maximum and minimum values of the control voltage V_c respectively. Thus, the frequency of the oscillating clock Clk_max,s generated by the VCO 34b corresponds to the highest frequency, f_max,s, by which VCO 34a is able to generate in the select state S. Similarly, the frequency of the oscillating clock Clk_min,s generated by the VCO 34c corresponding to the lowest frequency, f_min,s, by which VCO 34a is able to generate in the select state S. As well the clocks Clk_H,S and Clk_L,S are the frequency divided versions of the clocks Clk_max,s and Clk_min,s and generated by frequency divider 36. The frequency comparator 42 then compares the frequency of the reference clock Clk_ref, f_ref, to determine whether the frequency f_ref is between the frequencies f_max,s and f_min,s, thereby generating digital signals b0 and b1 to the weight refreshing circuit 44. The weight refreshing circuit 44 then changes or holds the select state S in accordance with digital signals b0 and b1 and generate digital signals D0˜D2 which record the select state S of the VCO 34a to the select voltage generating circuit 46. The select voltage generating circuit 46, such as a digital to analog converter converts the digital signals D0˜D2 to the select voltage Vs which determines the select state S of the VCO 34a.

[0028] Further, there is provided a simple method to compare the frequencies of two clocks, calculating the number of transition edges of one clock during one period of another clock. Since there are only two transitional edges (rising and falling edge) in one period of a clock theoretically, the period of clock Clk_b must be smaller than that of clock Clk_a if there are three transitional edges of clock Clk_b during one period of clock Clk_a. Thus, the frequency of clock Clk_b, f_b, is higher than the frequency of clock Clk_a, f_a. Conversely, if there are two or less transitional edges of clock Clk_b during one period of clock Clk_a, the frequency of clock Clk_b, f_b, is equal to or lower than the frequency of clock Clk_a, f_a.

[0029] FIG. 6 shows the frequency comparator 42 in FIG. 5. The D type flip-flop 50 is a rising edge triggered flip flop used as a divider with dividing ratio, 2, to divide the frequency of clock Clk_ref by 2, thereby generating clock 2Clk_ref and D type flip-flops 52a˜52c and 54a˜54c are double edge triggered flip-flops wherein the resets thereof are connected to clock 2Clk_ref. D type flip-flops 52a˜52c are coupled in series wherein the positive output Q of each D type flip-flops is connected to the input D of the next D-type flip-flop and the clock inputs of D-type flip-flops 52a˜52c receive clock Clk_L,S. The input D of D-type flip-flop 52a is tied to a high voltage VDD (i.e. logic value “1”). D-type flip-flop 52c generates digital signal b0 at positive output Q therein. Thus, the D-type flip-flops operate as a comparison circuit to determine the number of transitional edges of clock Clk_L,S during one period of clock Clk_ref. Since all the resets of D-type flip-flops 52a˜52c receive clock 2Clk_ref, that is, the D-type flip-flops 52a˜52c operate normally during one period of clock Clk_ref but are reset in the following period of clock Clk_ref. After reset, all the outputs Q of D-type flip-flops 52a˜52c are logic “0”. Hence, when D-type flip-flops 52a˜52c operate normally, the output Q of a double triggered D-type flip-flop is logic “1” with input D having a logic value “1” in response to the transitional edge of clock Clk_L,S. The digital signal b0, therefore, will only be logic “1” when there are three or more transitional edges of clock Clk_L,S during one period of clock Clk_ref, otherwise signal b0 is “0”. The inter-connection and operation of D-type flip-flops 54a˜54c are similar to D-type flip-flops 52a˜52c. Therefore, only there are three or more transitional edges of clock Clk_H,S during one period of clock Clk_ref that digital signal b1 will be logic “1”, otherwise logic “0”.

[0030] Whether the frequency of clock Clk_ref, f_ref, falls within the tuning frequency range of VCO 34a in current select state S can be obtained by signals b0 and b1. Table 1 shows the possible logic combinations of signals b0 and b1, and the corresponding illustrations and subsequent action that are taken.

[0031]

	TABLE 1
	b0	b1	illustrations	subsequent action
	0	0	fref < fL, S	Change select state S to
				generate a lower frequency
				oscillating clock
	0	1	Not allowed	No action
	1	0	fL, S =< fref < fH, S	Hold current select state S
	1	1	fref >= fH, S	Change select state S to
				generate a higher frequency
				oscillating clock

[0032] FIG. 7 is a schematic diagram of the weight refreshing circuit in FIG. 5 implementing subsequent actions as shown in Table 1. D-type flip-flops 56a and 56b are rising edge triggered flip-flops. D-type flip-flops 56a and 56b record the values of signals b0 and b1 respectively before the D-type flip-flops 52a˜c and 54a˜c are reset. D-type flip-flop 58 is a falling edge triggered flip-flop which generates a clock 2Clk_refP having 90° phase difference with clock 2Clk_ref. Signals D0˜D2 indicate the current select state of VCOs 34a˜34c. The logic processor 60 determines the values of signals D0˜D2 according to digital signals b0, b1 and D0˜D2 after a predetermined time and outputs them at outputs, B0, B1 and B2 to D-type flip-flops 62a˜62c respectively. When the rising edge of clock 2Clk_refP, the digital signals D0˜D2 are refreshed by D-type flip-flops 62a˜62c. Thus, the select state of VCO 34a˜34c is changed or held accordingly.

[0033] FIG. 8 is a schematic diagram of the select voltage generating circuit 46 in FIG. 5. PMOS transistors P0˜P2 act as three current-mirrors to provide currents with a ratio of 1:2:4 and NMOS transistors N0˜N2 are controlled by digital signals D0˜D2. Thus, the select voltage generating circuit 46 converts the digital signals D0˜D2 to an analog current I_s which is then converted into an analog voltage, that is, the select voltage V_s by transistor ND.

[0034] FIG. 9 is an embodiment of VCO 34 which can also be used as VCOs 34a˜34c. VCO 34 is a ring oscillator formed by cascading a plurality of delay devices 80. The control voltage V_c and select voltage V_s determine the delay time of each delay device 80, thereby the frequency of output clock Clk_out in the ring oscillator.

[0035] FIG. 10 is a schematic diagram of the delay device 80 in FIG. 9, which can be seen as an improved latch circuit. As shown in FIG. 10, the left side is a first inverter and the right side is another inverter. The control voltage V_c and select voltage V_s control the bias current of each inverter, thereby determining the timing for latch signal IN to output signal OUT. Fine tuning of bias current in inverters is controlled by voltage V_c as the coarse tuning is controlled by select voltage V_s.

[0036] In one embodiment of the invention, a phase lock loop traces a reference clock Clk_ref according to a flatter voltage-frequency transfer curve. The output clock, therefore, maintains high stability. Moreover, the tuning frequency range of the phase lock loop is extended with automatic select state S selection. As a result, the phase lock loop maintains the stability of output clock and provides wide tuning frequency range at the same time, which conventional technologies do not provide.

[0037] While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.