Phase-locked loop circuit

[0002] The present invention relates to an improvement of a phase-locked loop circuit, and more particularly to an improved design of a phase-locked loop circuit for improving the operating frequency range and the stability of middle/low frequency thereof, and for adjusting accurately the frequency continuously.

[0003] A conventional phase-locked loop circuit is shown in FIG. 1, in which a signal f_inof some frequency is inputted into a divider 11 for being frequency divided by M to become a signal f_in/M, and then inputted into a phase frequency detector 12.

[0004] The phase frequency detector 12 will be inputted with another feedback signal f_rhaving the same frequency with f_in/M. The signal f_ris a modified reference signal for adjusting the phase of f_in/M. If the phase of f_in/Mis ahead of f_r, then the phase frequency detector 12 will generate an UP signal. If the phase of f_in/Mfalls behind f_r, then the phase frequency detector 12 will generate a DN signal.

[0005] The UP signal or the DN signal will be inputted into a charge pump circuit 13 for generating a corresponding voltage V_ctrlto be inputted into a low pass filter 14.

[0006] The output V_oof the low pass filter 14 will be inputted into a voltage controlled oscillator 15 for generating an oscillating signal to be inputted into a prescaler circuit 16. After being processed by the prescaler circuit 16, an oscillating signal f_owill be inputted into a divider 17 for generating a required accurate frequency.

[0007] The signal f_ois also inputted into another divider 18 for being frequency divided by N to generate the signal f_r, and then inputted into the phase frequency detector 12. The frequency of f_ris the same as that of f_in/M.

[0008] Referring to FIG. 2, the detailed circuit of the voltage-controlled oscillator 15 will be described. A voltage signal V_Mis inputted into the control terminals of the inverters 21, 22, 23, 24, 25 simultaneously. The inverters 21, 22, 23, 24, 25 are connected serially to form a ring, as shown in the figure. Capacitors C₁, C₂, C₃, C₄, C₅are each parallelly connected at the output terminals a, b, c, d, e of the inverters. Therefore, oscillating signals having same frequency but different phases are each generated at the output terminals a, b, c, d, e of the inverters.

[0009] Referring to FIG. 3, which shows an improvement for the circuit in FIG. 2. The improvement is that capacitors C11, C12, C13, capacitors C21, C22, C23, capacitors C31, C32, C33, capacitors C41, C42, C43, and capacitors C51, C52, C53 are connected parallelly at the output terminals a, b, c, d, e of the inverters respectively. Signals CC1, CC2 and CC2 are used to enable switches S11, S12, S13, switches S21, S22, S23, switches S31, S32, S33, switches S41, S42, S43 and switches S51, S52, S53 that are connected serially with related capacitors respectively. By selecting signals CC1, CC2 or CC3 to turn related switches on and to enable related capacitors to become conductive, the frequency of the signals at the output terminals a, b, c, d, e can be adjusted.

[0010] Although the frequency of the signals at the output terminals a, b, c, d and e in FIG. 3 can be adjusted, the adjustment cannot be continuous. Another disadvantage is that the sensitivity of the frequency to voltage is too high in middle/low frequency range, so it is hard to do the frequency adjustment accurately.

[0011] It is therefore an object of the present invention to provide an improved design of a voltage-controlled oscillator of a phase-locked loop circuit for improving the operating frequency range and the stability of middle/low frequency therefore, and for adjusting accurately the frequency continuously.

[0012] FIG. 1 shows the block diagram of a conventional phase-locked loop circuit.

[0013] FIG. 2 shows the circuit diagram of the voltage-controlled oscillator in a conventional phase-locked loop circuit.

[0014] FIG. 3 shows a prior improved circuit design of the voltage-controlled oscillator in a conventional phase-locked loop circuit.

[0015] FIG. 4 shows an improved circuit design of the voltage-controlled oscillator in a phase-locked loop circuit according to the present invention.

[0016] Referring to FIG. 4, which shows an improved circuit design according to the present invention. The ouput voltage V_oof the low pass filter 14 in the phase-locked loop circuit is inputted into the gate of an NMOS MN1. One terminal of the MN1 is connected with a resistor R1 to the ground, another terminal of the MN1 is connected with a PMOS MP1 to the Vcc. The gate of MP1 is connected with the connection between MP1 and MN1, as shown in the figure, thus a signal V_Mis generated at the gate of MP1.

[0017] V_Mis a voltage signal and will be inputted into the gate of each PMOS MP2, MP3, MP4, MP5 and MP6 in the voltage-controlled oscillator. One terminal of each PMOS MP2, MP3, MP4, MP5 and MP6 is connected with Vcc, while another terminal is connected with the control terminal of inverters 41, 42, 43, 44, 45 respectively. Inverters 41, 42, 43, 44, 45 are connected serially to form a ring, as shown in the figure. The output of MP6 is inputted into an amplifier 46, and the output of the amplifier 46 is inputted into a multiplexer 47. Another input terminal of the multiplexer 47 can be inputted with a voltage transferred from digital data DATA by a digital-to-analog converter 48. The multiplexer 47 has two switches S1 and S2 for selecting the output of either the amplifier 46 or the digital-to-analog converter 48 as the CC1 signal to be inputted simultaneously into the gate of each PMOS MP7, MP8, MP9, MP10 and MP11. One terminal of each PMOS MP7, MP8, MP9, MP10 and MP11 is connected respectively with capacious C6, C7, C8, C9 and C10 to the Vcc, while another terminal is connected respectively with the output points f, g, h, i, j of inverters 41, 42, 43, 44, 45. Thus, signals having a same frequency but different phases are generated respectively at the output points f, g, h, i, j of inverters 41, 42, 43, 44, 45.

[0018] The voltage of the CC1 signal will control the width of the channel between the source and the drain of each PMOS MP7, MP8, MP9, MP10 and MP11. Therefore, current flowing through each of the channels is controlled and the frequency of the signals at points f, g, h, i, j can be adjusted. Since the voltage of the CC1 signal is varied continuously, the frequency of the signals at points f, g, h, i, j, which serve as output terminals for the voltage controlled oscillator and correspond to output terminals a, b, c, d, and e illustrated in FIG. 3 and described above.

[0019] The CC1 signal can be selected from outputs of the amplifier 46 or the digital-to-analog converter 48. If the output of the amplifier 46 is selected, then the voltage V_Mdominates. If the output of the digital-to-analog converter 48 is selected, then an external accurate digital signal DATA will dominate to adjust the frequency of the signals at points f, g, h, i, j.

[0020] Each of the inverters in the voltage-controlled oscillator according to the present invention can be replaced by a differential amplifier. Therefore, MOS devices and capacitors have to be connected at two sides of the differential amplifiers respectively to achieve the same effect as the above-mentioned embodiment.

[0021] The scope of the present invention depends only upon the following Claims, and is not limited by the above embodiment.