Frequency control word switching method for digital controlled oscillator

Abstract

The present invention discloses a method for switching the frequency control word of a numerically controlled oscillator, and belongs to the field of phase-locked loops. Since the frequency control word of a numerically controlled oscillator may be switched in each clock cycle, the nonlinear characteristics will be brought into the output clock during the switching, resulting in an increase in the jitter of the output clock frequency. In order to effectively reduce the nonlinear error, the method for switching the frequency control word of a numerically controlled oscillator of the present invention compares two intermediate variables related to the number of currently opened Class A capacitors and the number of currently opened Class A capacitors, and sets different selection flags accordingly according to the comparison results. Finally, the number of opened Class A capacitors and the number of opened Class T capacitors are updated according to the selection flags. The present invention effectively reduces the nonlinear error of the step length of the numerically controlled oscillator, reduces the jitter of the output clock frequency, and increases the stability of the output clock phase noise spectrum through a set of special methods for switching the frequency control word of a numerically controlled oscillator.

Description

Technical Field

[0001] The invention relates to the field of phase-locked loops, and in particular to a method for switching frequency control words of a digitally controlled oscillator. Background Art

[0002] The All Digital Phase Locked Loop (ADPLL) includes a digitally controlled oscillator, which is used to generate an output clock and is therefore also called a digital oscillator.

[0003] The unit frequency change value of the output clock frequency of the CNC oscillator is the CNC oscillator step length. Due to the influence of process, temperature and voltage changes, the CNC oscillator step length is not a fixed value, but has a certain fluctuation range, which shows the nonlinear characteristics of the CNC oscillator.

[0004] The frequency control word of the digitally controlled oscillator may switch in each clock cycle, which will bring nonlinear characteristics into the output clock, resulting in increased jitter of the output clock frequency.

[0005] Based on this, the present invention discloses a method for switching the frequency control word of a numerically controlled oscillator, which can effectively reduce the nonlinear error. When the frequency control word is switched, the nonlinear error of the step length of the numerically controlled oscillator can be effectively reduced, the jitter of the output clock frequency can be reduced, and the stability of the output clock phase noise spectrum can be increased. Summary of the invention

[0006] In order to alleviate or partially alleviate the above technical problems, the solution of the present invention is as follows:

[0007] A method for switching a frequency control word of a digitally controlled oscillator, before the switching of the frequency control word begins, the number of opened A-level capacitors is the current otw_A, and the number of opened T-level capacitors is the current otw_T, comprising the following steps:

[0008] When the frequency control word starts to switch, determine whether to use the switching function. If so, calculate: otw_A0=f(otw / N); otw_T0=otw-f(otw / N)×N; otw_A1=f(otw / N)-1; otw_T1=otw-(f(otw / N)-1)×N, where N is the ratio of the value of the A-level capacitor to the value of the T-level capacitor, f(·) is a floor function, otw represents the value of the frequency control word after switching, otw_A0 and otw_A1 are two intermediate variables related to otw_A, and otw_T0 and otw_T1 are two intermediate variables related to otw_T;

[0009] Determine whether the current otw_A is equal to otw_A0. If so, set the selection flag to 0. If not, further determine whether the current otw_A is equal to otw_A1. If the current otw_A is equal to otw_A1, set the selection flag to 1. If the current otw_A is not equal to otw_A1, keep the selection flag unchanged.

[0010] After setting or maintaining the selection flag, determine whether the current selection flag is 1. If not, update the current otw_A to otw_A0 and the current otw_T to otw_T0. If yes, update the current otw_A to otw_A1 and the current otw_T to otw_T1.

[0011] Furthermore, if the switching function is not used, the following are calculated: otw_A0=f(otw / N); otw_T0=otw-f(otw / N)×N; otw_A1=f(otw / N)-1; otw_T1=otw-(f(otw / N)-1)×N, where N is the ratio of the value of the A-level capacitor to the value of the T-level capacitor, f(·) is a floor function, otw represents the value of the frequency control word after switching, and otw_A0 and otw_A1 are two intermediate variables related to otw_A, and otw_T0 and otw_T1 are two intermediate variables related to otw_T; then set the selection flag to 0; after setting or maintaining the selection flag, determine whether the current selection flag is 1, if not, update the current otw_A to otw_A0, and update the current otw_T to otw_T0, if so, update the current otw_A to otw_A1, and update the current otw_T to otw_T1.

[0012] Furthermore, the digitally controlled oscillator frequency control word switching method is applied to ADPLL.

[0013] The technical solution of the present invention has one or more of the following beneficial technical effects:

[0014] (1) Correction can be achieved only in the background without affecting the normal operation of the system;

[0015] (2) It can effectively reduce the nonlinear characteristics caused by the frequency control word;

[0016] (3) Ensure the stability of the phase noise spectrum of the output clock;

[0017] (4) Low implementation cost and low silicon cost.

[0018] In addition, other beneficial effects of the present invention will be mentioned in the specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is an architecture diagram of ADPLL in the prior art;

[0020] Figure 2 The present invention is a flow chart of a method for switching a frequency control word of a digitally controlled oscillator. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] In order to clearly describe the technical solutions of the embodiments of the present invention, in the embodiments of the present invention, words such as "first" and "second" are used to distinguish the same or similar items with substantially the same functions and effects. Those skilled in the art can understand that words such as "first" and "second" do not limit the quantity and execution order.

[0023] Figure 1 The figure is an architecture diagram of the ADPLL in the prior art. The ADPLL includes an integrator, a digital-to-time converter, a time-to-digital converter, a counter, a digitally controlled oscillator, and a loop filter. The loop filter generates a frequency control word to control the capacitor switch of the digitally controlled oscillator and tune the frequency.

[0024] In order to achieve a smaller numerically controlled oscillator step size and higher adjustment accuracy, in the field, the capacitors in the numerically controlled oscillator are generally divided into two different levels of capacitors: Class A capacitors and Class T capacitors. A stands for capture and T stands for temperature. Usually, the value of the Class A capacitor is N times the value of the Class T capacitor, and N is a positive integer not less than 4.

[0025] When N T-level capacitors need to be turned on, the replacement is achieved by turning on one more A-level capacitor. These are all conventional means in the art.

[0026] Figure 2 The present invention is a flow chart of a method for switching a frequency control word of a digitally controlled oscillator.

[0027] As an example, the method for switching the frequency control word of a digitally controlled oscillator of the present invention is applied to ADPLL.

[0028] In the present invention, otw represents the value of the frequency control word, otw_A represents the number of A-level capacitors turned on when the value of the frequency control word is otw, otw_T represents the number of T-level capacitors turned on when the value of the frequency control word is otw, and otw_A0 and otw_A1 are two intermediate variables related to otw_A, and otw_T0 and otw_T1 are two intermediate variables related to otw_T.

[0029] Before the switching frequency control word starts, the number of turned-on A-level capacitors is the current otw_A, and the number of turned-on T-level capacitors is the current otw_T.

[0030] When the frequency control word starts to switch, determine whether to use the switching function. If not, calculate: otw_A0=f(otw / N); otw_T0=otw-f(otw / N)×N; otw_A1=f(otw / N)-1; otw_T1=otw-(f(otw / N)-1)×N, where N is the ratio of the value of the A-level capacitor to the value of the T-level capacitor, f(·) is the floor rounding function, and otw represents the value of the frequency control word after switching.

[0031] Further, when the switching function is not used, then the selection flag sel is set to 0.

[0032] If so, that is, when the switching function is selected, the following calculations are performed in the same manner: otw_A0=f(otw / N); otw_T0=otw-f(otw / N)×N; otw_A1=f(otw / N)-1; otw_T1=otw-(f(otw / N)-1)×N, where N is the ratio of the value of the A-level capacitor to the value of the T-level capacitor, and f(·) is a floor function.

[0033] Next, it is determined whether the current otw_A is equal to otw_A0. If so, the selection flag sel is set to 0. If not, it is further determined whether the current otw_A is equal to otw_A1. If the current otw_A is equal to otw_A1, the selection flag sel is set to 1. If the current otw_A is not equal to otw_A1, the selection flag sel remains unchanged.

[0034] Regardless of whether the switching function is used, after setting or maintaining the selection flag sel, determine whether the current selection flag sel is 1. If not, update the current otw_A to otw_A0, and update the current otw_T to otw_T0. If so, update the current otw_A to otw_A1, and update the current otw_T to otw_T1.

[0035] For example, Table 1 shows a comparison table of errors that would be generated if the switching function is not enabled. In the second clock cycle, the fifth clock cycle, and the sixth clock cycle, a frequency error that can be generated by a T-class capacitor will be generated. In this example, N=7.

[0036] Table 1: Error comparison table if the switching function is not enabled

[0037]

[0038] By way of example and comparison, Table 2 shows a comparison table of errors generated when the switching function is turned on. Here, not only the characteristic of 0 frequency error in other clock cycles is maintained, but also the frequency error is effectively reduced in the second, fifth, and sixth clock cycles, thereby achieving the purpose of the invention.

[0039] Table 2: Error comparison table caused by turning on the switching function

[0040]

[0041] In order to better illustrate the present invention, numerous specific details are provided in the above specific embodiments. It should be understood by those skilled in the art that the present invention can also be implemented without certain specific details. In some examples, methods, means, components and circuits well known to those skilled in the art are not described in detail in order to highlight the subject matter of the present invention.

[0042] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for switching the frequency control word of a digitally controlled oscillator, before the switching of the frequency control word begins, the number of A-level capacitors turned on is the current otw_A, and the number of T-level capacitors turned on is the current otw_T, characterized in that: The steps include: When the frequency control word starts to switch, determine whether to use the switching function. If so, calculate: otw_A0=f(otw / N); otw_T0=otw-f(otw / N)×N; otw_A1=f(otw / N)-1; otw_T1=otw-(f(otw / N)-1)×N, where N is the ratio of the value of the A-level capacitor to the value of the T-level capacitor, f(·) is a floor function, otw represents the value of the frequency control word after switching, otw_A0 and otw_A1 are two intermediate variables related to otw_A, and otw_T0 and otw_T1 are two intermediate variables related to otw_T; Determine whether the current otw_A is equal to otw_A0. If so, set the selection flag to 0. If not, further determine whether the current otw_A is equal to otw_A1. If the current otw_A is equal to otw_A1, set the selection flag to 1. If the current otw_A is not equal to otw_A1, keep the selection flag unchanged. After setting or maintaining the selection flag, determine whether the current selection flag is 1. If not, update the current otw_A to otw_A0 and the current otw_T to otw_T0. If yes, update the current otw_A to otw_A1 and the current otw_T to otw_T1.

2. The method for switching the frequency control word of a numerically controlled oscillator according to claim 1, characterized in that: If the switching function is not used, the following are calculated: otw_A0=f(otw / N); otw_T0=otw-f(otw / N)×N; otw_A1=f(otw / N)-1; otw_T1=otw-(f(otw / N)-1)×N, where N is the ratio of the value of the A-level capacitor to the value of the T-level capacitor, f(·) is a floor function, otw represents the value of the frequency control word after switching, and otw_A0 and otw_A1 are two intermediate variables related to otw_A, and otw_T0 and otw_T1 are two intermediate variables related to otw_T; Then set the selection flag to 0; After setting or maintaining the selection flag, determine whether the current selection flag is 1. If not, update the current otw_A to otw_A0 and the current otw_T to otw_T0. If yes, update the current otw_A to otw_A1 and the current otw_T to otw_T1.

3. The method for switching the frequency control word of a numerically controlled oscillator according to claim 2, characterized in that: The digital controlled oscillator frequency control word switching method is applied to ADPLL.

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