KR101136948B1

KR101136948B1 - Apparatus and Method for Generating Low Jitter Spread Spectrum Clock

Info

Publication number: KR101136948B1
Application number: KR20100010372A
Authority: KR
Inventors: 변상진; 손충환; 심창수
Original assignee: 동국대학교 산학협력단
Priority date: 2010-02-04
Filing date: 2010-02-04
Publication date: 2012-04-19
Also published as: WO2011096627A1; KR20110090533A

Abstract

The spread spectrum clock generator according to an embodiment of the present invention modulates an input clock signal to have a frequency corresponding to a modulation profile to generate an output clock signal. Embodiments of the present invention have a feature of reducing electromagnetic interference (EMI) of the output clock signal and simultaneously reducing jitter of the output clock signal, thereby providing a clock signal having low electromagnetic interference and low jitter.

Description

Apparatus and Method for Generating Low Jitter Spread Spectrum Clock

Embodiments of the present invention relate to clock signal generation techniques, and more particularly, to a spread spectrum clock generator and method for modulating the frequency of an output clock signal according to a modulation profile.

Recently, with the rapid development of electronics and other related science and engineering technology, digital information devices, digital display devices and digital computing devices are all rapidly miniaturized, easy to move, highly integrated, and data processing speed is increased. These high speed digital electrical and electronic devices generally use very precise high speed clock signals, and electromagnetic interference is radiated from these clock signals due to the influence of electromagnetic interference (EMI) caused by the electromagnetic waves. There is a possibility of malfunction of the equipment.

In order to attenuate such electromagnetic interference, a spread spectrum clock generation technique for modulating a frequency of a clock signal according to a modulation profile characterized by a modulation ratio and a modulation frequency has been proposed and used. .

In general, as the modulation rate is increased, the electromagnetic interference of the modulated clock signal is reduced, while the jitter is increased.

Embodiments of the present invention provide the advantage that the electromagnetic interference of the modulated clock signal is reduced by modulating the clock signal using a modulation profile that alternates the frequency above or below the center frequency two or more times within every modulation period. At the same time, there is provided a spread spectrum clock generator that reduces jitter in a modulated clock signal.

The technical problems other than the present invention can be easily understood from the following description.

According to an aspect of the present invention, a spread spectrum clock generator for generating an output clock signal by modulating an input clock signal to have a frequency corresponding to a modulation profile, wherein the modulation profile is switched between a high frequency and a low frequency every modulation period. A spread spectrum clock generator is provided that is at least twice and has a digital waveform.

According to another aspect of the present invention, a method for generating a clock by a spread spectrum clock generator, the method comprising: generating a modulation profile; And modulating an input clock signal to have a frequency corresponding to the modulation profile, wherein the modulation profile has a digital waveform with at least two transitions between a high frequency and a low frequency in every modulation period. A method of generation is provided.

Embodiments of the present invention can reduce the jitter of the modulated clock signal, thereby providing a clock signal having a low jitter characteristic while reducing electromagnetic interference.

1A is a diagram illustrating a spread spectrum clock generator according to an embodiment of the present invention.
Figure 1b is a diagram illustrating the detailed functional portion of the spread spectrum clock generator according to an embodiment of the present invention.
2 is a diagram illustrating a modulation profile used in a spread spectrum clock generator according to a first embodiment of the present invention.
3 is a diagram illustrating a modulation profile of a spread spectrum clock generator according to a second embodiment of the present invention.
4 is a diagram illustrating a modulation profile of a spread spectrum clock generator according to a third embodiment of the present invention.

As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

In the following description of the present invention with reference to the accompanying drawings, the same components are denoted by the same reference numerals regardless of the reference numerals, and redundant explanations thereof will be omitted. In the following description of the present invention, if it is determined that the detailed description of the related known technology may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

1A is a diagram illustrating a spread spectrum clock generator according to an embodiment of the present invention.

Referring to FIG. 1A, a spread spectrum clock generator according to an embodiment of the present invention includes a spread spectrum frequency modulator 110 and a modulation profile generator 120.

The spread spectrum frequency modulator 110 modulates the input clock signal to have a frequency corresponding to the modulation profile received from the modulation profile generator 120 to generate an output clock signal. The modulation profile according to the present invention is a digital modulation profile, and the spread spectrum frequency modulator 110 which frequency modulates to correspond to the digital modulation profile is generally a digital controlled oscillator (DCO) or digital adjustment. Implemented using a digital controlled delay cell.

The modulation profile generator 120 receives an internal clock signal and generates a modulation profile. The modulation profile is a digital modulation profile and is characterized by a modulation ratio and a modulation frequency. In general, as the modulation rate is increased, the electromagnetic interference (EMI) of the modulated output clock signal is reduced, while the jitter is increased. In this case, the internal clock signal may be a signal obtained by dividing an input clock signal or a clock signal output from an external device.

1B is a diagram illustrating a detailed functional unit of a spread spectrum clock generator according to an embodiment of the present invention. FIG. 1B thus includes a spread spectrum frequency modulator 110 and a modulation profile generator 120 as shown in FIG. 1A. The spread spectrum frequency modulator 110 includes a frequency phase detector 111, a charge pump 112, a loop filter 113, an oscillator 114, and clock dividers 115 and 116.

The frequency phase detector 111 receives a first clock signal obtained by the clock divider 116 dividing an input clock signal and a second clock signal divided by receiving an output clock signal from the oscillator 114. The frequency phase detector 111 detects the frequency and phase difference between the inputted first clock signal and the second clock signal, and the charge pump 112 converts the detected frequency and phase difference into a current signal to loop the filter 113. Will output The loop filter 113 converts the input current signal into an oscillator adjustment voltage so that the center frequency of the output clock signal of the oscillator is exactly N / K times the frequency of the input clock signal of the spread spectrum clock generator. The oscillator 114 shown in FIG. 1B receives not only an oscillator adjustment voltage from the loop filter 113 but also an n-bit modulation profile from the modulation profile generator 120. By setting the loop bandwidth of the spread spectrum frequency modulator 110 to 1/10 or less than the modulation frequency of the modulation profile, the center frequency of the output clock signal is exactly N / K times the frequency of the input clock signal. At the same time, the frequency of the output clock signal can be modulated to correspond to an n-bit modulation profile. The modulation profile generator 120 receives an internal clock signal generated by dividing the input clock signal into the clock divider 130 and generates an n-bit modulation profile.

2 is a diagram illustrating a modulation profile used in a spread spectrum clock generator according to a first embodiment of the present invention. That is, the modulation profile of FIG. 2 represents the frequency over time of the modulated output clock signal.

Priority level is defined as the center frequency f _OUT is called the definition, and the modulation ratio of the output clock signal at 2 f _OUT d as defined, and the modulation frequency f _MOD is called 1 / f _MOD the modulation period, and definition. As shown in FIG. 2, in the spread spectrum clock generating apparatus according to the first embodiment of the present invention, the frequency of the output clock signal is maintained for a specified time interval every modulation period (1 / f _MOD ), Elapsed time immediately uses a digital modulation profile that changes the frequency of the output clock signal to a different frequency. In this case, the modulation profile is a frequency larger than the center frequency f _OUT (that is, a frequency within an interval from f _OUT to f _OUT + f _OUT d: hereinafter referred to as high frequency) and a frequency smaller than the center frequency f _OUT (that is, Frequency in the interval from f _OUT to f _OUT -f _OUT d: (hereinafter referred to as low frequency) is a modulation profile that is alternately located in order. The specified time interval means a time for which the frequency of the output clock signal is maintained at the same frequency within one modulation period (1 / f _MOD ) of the modulation profile.

Hereinafter, the n th frequency refers to a frequency maintained in an n th section within one modulation period 1 / f _MOD .

In the modulation profile according to the first embodiment of the present invention, when the n th frequency is a high frequency, the n + 1 frequency is a low frequency. On the contrary, when the n th frequency is a low frequency, the n + 1 frequency is a high frequency. Is a modulation profile that is a natural number. In addition, the modulation profile is such that the absolute value of the difference between the n th frequency and the center frequency f _OUT is equal to the absolute value of the difference between the n + 1 frequency and the center frequency f _OUT , or the n th frequency and the center frequency ( f _OUT) can be a difference between the absolute value of the n-1 frequency and the center frequency (the same modulation profile and the absolute value of a difference between f _OUT) of the liver.

For example, when the n th frequency according to the first embodiment of the present invention is f _OUT + A, the n + 1 th frequency may be f _OUT −A. Where A is a real number greater than or equal to 0 and less than or equal to the modulation rate f _OUT d.

The purpose of the spread spectrum clock generator according to the present invention to modulate the frequency of the output clock signal by alternating the high frequency and the low frequency two or more times within one modulation period is to reduce jitter of the output clock signal. Jitter of the output clock signal may be represented by Equation 1 below.

[Equation 1]

At this time, t is the time at a specific time, f (t) is the frequency of the output clock signal at time t, and Jitter (t) is the jitter of the output clock signal at time t. This jitter may be negative or positive depending on time t. To reduce jitter of the output clock signal, as shown in FIG. 2, the frequency of the output clock signal may be modulated by alternating high and low frequencies within one modulation period.

According to Equation 1, the jitter of the output clock signal modulated according to the modulation profile illustrated in FIG. 2 increases during the first period, decreases during the second period, and increases again during the third period. That is, Jitter (t), which is an integral value of the frequency difference, is integrated alternately between negative and positive frequencies f (t) -f _OUT for each interval, so that the maximum jitter of the modulation profile according to the first embodiment of the present invention is It is jitter at the end of the seventh section and corresponds to the painted area of FIG.

That is, since the modulation profile of the spread spectrum clock generator according to the first embodiment of the present invention repeats the increase and decrease of jitter for one modulation period, the jitter of the output clock signal is a conventional triangular modulation profile. ) Or a Hershey-Kiss modulation profile.

3 is a diagram illustrating a modulation profile of a spread spectrum clock generator according to a second embodiment of the present invention.

Referring to FIG. 3, the modulation profile of the spread spectrum clock generator according to the second embodiment of the present invention has a specified number of sections having a high frequency or a low frequency, and then a specified number of sections has a frequency corresponding to a previously specified number of sections. Modulation profile that maintains a frequency symmetrical about the center frequency. For example, in the modulation profile shown in FIG. 3, the first frequency and the second frequency may be high frequencies, and the third and fourth frequencies may be low frequencies. At this time, the absolute value of the difference between the third frequency and the center frequency (f _OUT) may be the same as any one of the absolute value of the difference between the first frequency and the second frequency and the central frequency (f _OUT). Also, the absolute value of the difference between the fourth frequency and the center frequency f _OUT is the same as the other one except the absolute value corresponding to the third frequency among the absolute values of the difference between the first frequency and the second frequency and the center frequency f _OUT . can do.

That is, in the modulation profile according to the second embodiment of the present invention, when the n th frequency to the n + a frequency is a high frequency, the n + a + 1 frequency to the n + a + 1 + b frequency are low frequencies, and vice versa. When the nth frequency to the n + a frequency is a low frequency, the n + a + 1 frequency to the n + a + 1 + b frequency are high frequencies, where n is a natural number and a and b are integers of 0 or more. . The modulation profile illustrated in FIG. 3 is simply a case where a = b = 1.

In addition, although the modulation profile of the spread spectrum clock generator described above with reference to FIGS. 2 and 3 is illustrated as having the same time in each section in which the frequency of the output clock signal is maintained in the drawing, the time in each section may be set to be different. It is self-evident.

The modulation profile of the spread spectrum clock generating apparatus described above with reference to FIGS. 2 and 3 has been described as the frequency of the corresponding section is set to the low frequency or the high frequency as the frequency of the nth previous section is the high frequency or the low frequency. However, the embodiment of the present invention is not limited thereto, and it is obvious that the frequency of the corresponding section may be set to a high frequency or a low frequency regardless of whether the frequency of the previous section is a high frequency. Hereinafter, a modulation profile in which a frequency of a corresponding section is set regardless of whether a frequency of a previous section is a high frequency will be described with reference to FIG. 4.

4 is a diagram illustrating a modulation profile of a spread spectrum clock generator according to a third embodiment of the present invention.

Referring to FIG. 4, the modulation profile of the spread spectrum clock generator according to the third embodiment of the present invention is a modulation profile in which the frequency of each section is arbitrarily selected as high frequency or low frequency. In this case, the maximum jitter of the output clock signal corresponds to the shaded area, and due to the randomness of this frequency selection, the electromagnetic interference (EMI) of the output clock signal is lower than using a triangular modulation profile. You lose.

That is, the general triangular modulation profile (triangular modulation profile) is a switch between a high frequency and a low frequency occurs once per modulation period, the modulation profile according to the embodiments of the present invention is shown in Figures 2 to 4 Likewise, it is a modulation profile that is switched between two or more high and low frequencies per modulation period.

So far I looked at the center of the embodiment for the present invention. Many embodiments other than the above-described embodiments are within the claims of the present invention. Those skilled in the art will appreciate that the present invention can be implemented in a modified form without departing from the essential features of the present invention. The disclosed embodiments should, therefore, be considered in an illustrative rather than a restrictive sense. The scope of the present invention is shown in the claims rather than the foregoing description, and all differences within the scope will be construed as being included in the present invention.

Claims

A spread spectrum clock generator which modulates an input clock signal to have a frequency corresponding to a modulation profile to generate an output clock signal.
The modulation profile is a transition between high frequency and low frequency two or more times in each modulation period and has a digital waveform,
The modulation profile is the n + a + 1th frequency to the n + a + 1 + b frequency is a low frequency when the nth frequency to n + a frequency is a high frequency,
When the n th frequency to the n + a frequency is a low frequency, the n + a + 1 frequency to the n + a + 1 + b frequency are high frequencies,
And n is a natural number, and a and b are integers of 0 or more.

delete

The method according to claim 1,
The modulation profile is a spread spectrum clock generator, characterized in that the time intervals in which the frequency of the output clock signal is maintained are the same time interval.

The method according to claim 1,
Wherein each frequency of the modulation profile has a constant difference from a neighboring frequency.

A method for generating a clock by a spread spectrum clock generator,
Generating a modulation profile; And
Modulating an input clock signal to have a frequency corresponding to the modulation profile,
The modulation profile is a transition between high frequency and low frequency two or more times in each modulation period and has a digital waveform,
The modulation profile is the n + a + 1th frequency to the n + a + 1 + b frequency is a low frequency when the nth frequency to n + a frequency is a high frequency,
When the n th frequency to the n + a frequency is a low frequency, the n + a + 1 frequency to the n + a + 1 + b frequency are high frequencies,
N is a natural number, and a and b are integers greater than or equal to zero.

delete

The method of claim 7, wherein
The modulation profile is spread spectrum clock generation method, characterized in that the time intervals in which the frequency of the output clock signal is maintained are the same time interval.

The method of claim 7, wherein
Wherein each frequency of the modulation profile has a constant difference from a neighboring frequency.