CN116865749A - A direct modulation VCO spread spectrum clock generator based on two-point injection technology - Google Patents

Abstract

The invention discloses a direct modulation VCO spread spectrum clock generator based on a two-point injection technology, which comprises a crystal oscillator F _REF A phase frequency detector PFD, a charge pump CP1, a programmable charge pump CP2, a programmable charge pump CP3, a loop filter LPF, a voltage controlled oscillator VCO, a frequency divider DIV1, a frequency divider DIV2, a frequency divider DIV3. Injecting a current I into the loop filter LPF via the programmable charge pump CP2 _mod And generating a triangular wave modulation signal, controlling the output frequency of the VCO to change within a certain range, and realizing the expansion of the energy of the central frequency in a certain range on the frequency spectrum. Considering that the triangular wave modulation signal is attenuated into sine under the influence of factors such as bandwidth, low pass characteristic of a loop filter LPF and the likeWave, introducing programmable charge pump CP3 to inject current I into loop filter LPF _com And compensating the attenuated sine wave modulation signal, recovering the sine wave modulation signal into triangular wave, and realizing better EMI reduction effect in the frequency spectrum. A fourth order loop filter LPF is used to suppress PLL reference spurs.

Description

A direct modulation VCO spread spectrum clock generator based on two-point injection technology

Technical field

The invention relates to the field of spread spectrum clock generators and is a directly modulated VCO spread spectrum clock generator based on two-point injection technology.

Background technique

With the development of CMOS technology, in order to meet the high-performance application requirements of the system, the operating frequency of electronic devices is also constantly increasing. High-speed clocks are critical in wired communications systems because they provide reliable timing for sending or receiving data. Today, with the rapid increase in data rates in wired systems such as PCIe, USB or SATA, the issue of electromagnetic interference (EMI) caused by high-speed clocks is becoming increasingly important. Excessive electromagnetic interference can cause catastrophic damage to communication systems. As an effective and low-cost method to reduce electromagnetic interference, spread spectrum clock (SSC) has always been a hot topic of research.

In spread spectrum clock generators, modulation signal forms with different contours also have different EMI reduction effects. Common modulation signal contours include triangle wave modulation, sine wave modulation, and Hershey-Kiss modulation. As shown in Figure 2, the output frequency of the sine wave modulated spread spectrum clock changes with the sine wave, and its spectrum side lobes are much higher than the middle frequency, resulting in unsatisfactory EMI reduction effect; the output frequency of the triangle wave modulated spread spectrum clock changes linearly with the triangle wave, and the spectrum side lobes The lobe peak value is lower and the spectrum center is flatter. Therefore, triangular wave modulation has a better EMI reduction effect than sine wave modulation. Compared with triangular wave modulation, Hershey-Kiss modulation can achieve greater attenuation of the peak energy of the output frequency. However, since it is difficult to obtain an ideal Hershey-Kiss modulation signal, we prefer to use a triangular wave modulation signal in circuit implementation. .

Several techniques have been proposed for phase-locked loop (PLL)-based directly modulated VCO spread spectrum clock generator structures. As shown in Figure 3, by injecting the modulation current I _mod between the resistor R ₁ and the resistor R ₂ in the loop filter LPF, the capacitors C ₁ and C ₂ act as an integral to generate a triangular wave modulation signal at the V _point . This structure requires The ideal integration effect can be obtained only by meeting the prerequisite of R ₁ C ₁ =R ₂ C ₂ , and the spikes (Spikes) caused by the spread spectrum operation are suppressed by introducing the resistor R ₃ and the capacitor C ₃ . Based on the same direct modulation VCO principle, as shown in Figure 4, by injecting the modulation current I _mod between the resistor R ₁ and the capacitor C ₁ in the loop filter LPF, an additional current I _P2 and a unity gain buffer U ₁ are introduced, Implement capacitance multiplication technology to reduce the huge capacitance C ₁ caused by the small PLL loop bandwidth of the direct modulation VCO structure, saving area costs. When the condition f _mod <<1/(2πR ₁ C ₂ ) is met, the capacitance C ₁ acts as Integral action generates a triangular wave modulation signal at the V _dump point. Reference spurs are further suppressed by introducing resistor _R3 and capacitor _C3 .

The spread spectrum clock is realized through the above two technologies. Due to the implementation of the PLL negative feedback loop and the spread spectrum clock, in the time domain, the phase frequency detector PFD module will continue to output the UP or DN signal, causing spikes (Spikes) to appear on the control voltage. ), in order to eliminate spikes and further suppress PLL reference spurs, both structures introduce resistor R ₃ and capacitor C ₃ . At the same time, the originally close-to-ideal triangle wave modulation signal will be affected by insufficient selection of the PLL loop bandwidth. Due to the low-pass characteristics of the small and loop filter, its high-frequency component will be filtered out, causing it to attenuate into a modulated signal close to a sine wave, which worsens the EMI reduction effect. If the EMI reduction cannot meet the current communication standards, the communication system will still suffer damage. Therefore, it is of great significance to invent a spread spectrum clock generator with good EMI reduction effect.

Contents of the invention

In view of the shortcomings of the existing technology, the present invention provides a directly modulated VCO spread spectrum clock generator based on two-point injection technology, by injecting modulation current between the resistor R ₁ and the capacitor C ₁ in the loop filter LPF While I _mod , a compensation current I _com is added and injected between the resistor R ₃ and the capacitor C ₃ in the loop filter LPF to solve the problems raised in the above background technology.

In order to achieve the above objectives, the present invention is realized through the following technical solutions: a directly modulated VCO spread spectrum clock generator based on two-point injection technology, including a crystal oscillator F _REF , a phase frequency detector PFD, a charge pump CP1, and a Programmable charge pump CP2, programmable charge pump CP3, loop filter LPF, voltage controlled oscillator VCO, frequency divider DIV1, frequency divider DIV2, frequency divider DIV3. The frequency divider DIV1 is connected to the crystal oscillator F _REF to provide the reference frequency PLL _REF required for PLL locking. The frequency divider DIV2 is connected to the crystal oscillator F _REF to generate two differential square wave modulation signals UP2 and DN2. The frequency and phase identification are The charge pump CP1 is connected to the frequency divider DIV1 to identify the frequency and phase difference between the PLL _REF signal and the PLL _FB signal to generate UP and DN signals. The charge pump CP1 is connected to the phase frequency detector PFD and generates a current I _CP based on the UP and DN signals. Loop filter, the programmable charge pump CP2 and the programmable charge pump CP3 are both connected to the frequency divider DIV2, respectively generating the modulation current I _mod and the compensation current I _com and injecting them into the loop filter LPF to generate a triangular wave modulation signal, the The loop filter LPF is connected to the charge pump CP1, the programmable charge pump CP2 and the programmable charge pump CP3, respectively converting the current I _CP , the modulation current I _mod and the compensation current I _com into the control voltage V _ctrl of the voltage controlled oscillator VCO. , the voltage-controlled oscillator VCO is connected to the loop filter LPF, and an output clock signal is generated according to the loop filter LPF output control voltage V _ctrl to achieve modulation changes in frequency in the time domain. The frequency divider DIV3 is connected to the voltage-controlled The oscillator VCO is connected to generate a feedback signal PLL _FB feedback is connected back to the input terminal of the phase frequency detector PFD to achieve the final locking of the PLL.

Preferably, the loop filter LPF is composed of a resistor R ₁ , a resistor R ₃ , a resistor R ₄ , a capacitor C ₁ , a capacitor C ₂ , a capacitor C ₃ and a capacitor C ₄ . One end of the capacitor C ₁ is grounded and the other end is grounded. It is connected to the resistor R _1. One end of the resistor R ₁ is connected to the capacitor C ₁ , and the other end is connected to the capacitor C ₂ and the resistor R _3. One end of the capacitor C ₂ is grounded, and the other end is connected to the resistor R ₁ and the resistor R _3. Connected to one point, one end of the resistor R ₃ is connected to the resistor R ₁ and the capacitor C ₂ , and the other end is connected to the capacitor C ₃ and the resistor R ₄ . One end of the capacitor C ₃ is grounded, and the other end is connected to the resistor R ₃ , resistor R ₄ is connected to one point, one end of the resistor R ₄ is connected to the resistor R ₃ and capacitor C ₃ to one point, and the other end is connected to the capacitor C _{4 . One end of the capacitor C 4 is} grounded, and the other end is connected to the resistor R ₄ . The overall loop filter LPF is composed of fourth order and has four poles, so the overall PLL is a fifth-order type-2 PLL loop.

Preferably, the loop filter LPF is connected to the charge pump CP1, the programmable charge pump CP2, and the programmable charge pump CP3. The output of the charge pump CP1 is connected to a point between the resistor R ₁ and the capacitor C ₂ in the loop filter LPF. The charge pump CP1 generates the PLL main loop current I _CP and injects it into the loop filter LPF between the resistor R ₁ and the capacitor C ₂ . The output of the programmable charge pump CP2 is connected to a point between the resistor R ₁ and the capacitor C ₁ in the loop filter LPF. The programmable charge pump CP2 generates a modulated current I _mod and injects it into the loop filter LPF between the resistor R ₁ and the capacitor C ₁ . The output of the programmable charge pump CP3 and the resistor R ₃ , resistor R ₄ and capacitor C ₃ in the loop filter LPF are connected to one point. The programmable charge pump CP3 generates a compensation current I _com and injects it into the loop filter LPF between the resistor R ₃ and the capacitor C ₃ . Wherein, the current size of the programmable charge pump CP2 and the programmable charge pump CP3 is adjustable.

Preferably, the output of the loop filter LPF is connected to the input of the voltage controlled oscillator VCO, and the resistor R ₄ and capacitor C ₄ in the loop filter LPF are connected to the input of the voltage controlled oscillator VCO at one point. . The loop filter LPF outputs the V _ctrl signal to control the voltage controlled oscillator VCO to produce corresponding frequency changes.

Beneficial effects of the present invention: The present invention is a directly modulated VCO spread spectrum clock generator based on two-point injection technology, and proposes a two-point injection technology to realize the spread spectrum clock.

1. While using the programmable charge pump CP2 to inject the modulation current I _mod into the loop filter LPF, an additional programmable charge pump CP3 is introduced to inject a compensation current between the resistor R ₃ and the capacitor C ₃ in the loop filter LPF. I _com , which compensates for the disadvantage that the triangular wave modulation signal is attenuated into a sine wave modulation signal due to the PLL loop bandwidth and filter low-pass characteristics. It eliminates the spikes (Spikes) on the voltage controlled oscillator VCO control voltage while taking into account the noise. For dissipation suppression, the fourth-order loop filter LPF is introduced to achieve better EMI reduction effect.

2. The present invention realizes linear modulation, can change the current size of programmable charge pump CP2 and programmable charge pump CP3 according to user needs, achieves different spread spectrum depths, has a certain tolerance for PVT changes, and is suitable for application in the field of high-speed communications .

Description of the drawings

Other features, objects and advantages of the present invention will become more apparent by reading the detailed description of the non-limiting examples with reference to the following drawings:

Figure 1 is a schematic structural diagram of a spread spectrum clock generator based on two-point injection technology proposed by the present invention;

Figure 2 shows modulated signals with different contours and the spectrum diagram after modulation;

Figure 3 is a schematic diagram of an existing spread spectrum clock generator based on injection between LPF resistors of a loop filter;

Figure 4 is a schematic diagram of an existing spread spectrum clock generator based on capacitance multiplication technology;

Figure 5 is a circuit schematic diagram of the loop filter LPF and related injection points in the present invention;

Figure 6 is a simulation diagram of the second-order loop filter LPF voltage controlled oscillator VCO control voltage V _ctrl ;

Figure 7 is a simulation diagram of the third-order loop filter LPF voltage controlled oscillator VCO control voltage V _ctrl ;

Figure 8 is a simulation diagram of the control voltage _Vctrl of the voltage controlled oscillator VCO based on the two-point injection technology of the present invention;

Figure 9 is a diagram showing the EMI reduction effect after the spread spectrum is turned on according to the present invention.

Detailed ways

In order to make it easy to understand the technical means, creative features, objectives and effects of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, all The described embodiments are only some of the embodiments of the present invention, not all of them (charge pump CP1, programmable charge pump CP2, and programmable charge pump CP3 can be selected in many ways; the selection of the PLL loop bandwidth can be based on the user's requirements. The selection should be made based on comprehensive considerations such as locking time and noise performance. At the same time, the modulation injection method of the unit gain buffer structure shown in Figure 4 can also be used to reduce the capacitor area in the loop filter LPF. These are all within the scope of this patent protection) . Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

Spread Spectrum Clock (SSC): It uses frequency modulation to disperse the energy concentrated in the narrow frequency band of the clock signal spectrum into a set wide frequency range, thereby reducing the fundamental wave of the clock signal. Frequency and the energy (amplitude) of odd harmonic frequencies are effective means of reducing system electromagnetic interference (EMI).

Directly modulate VCO: Spread Spectrum Clock Generator (SSCG) is mostly implemented based on Phase Locked Loop (PLL), by directly modulating the input of Voltage-controlled Oscillator (VCO) The method of controlling voltage to achieve frequency modulation is called direct modulation VCO.

Spread spectrum modulation profile: The spread spectrum modulation profile determines the form of expression on the power spectrum. Common modulation profiles include triangle wave modulation, sine wave modulation, and Hershey-Kiss modulation.

Reference spurs: The phase-locked loop output signal caused by non-ideal factors such as charge pump charge and discharge current mismatch has periodic spurious components, which are reflected in the spectrum as protrusions at the reference source frequency on the left and right sides of the output center frequency.

Please refer to Figure 1. The present invention discloses a directly modulated VCO spread spectrum clock generator based on two-point injection technology, including a crystal oscillator F _REF , a phase frequency detector PFD, a charge pump CP1, a programmable charge pump CP2, a programmable Charge pump CP3, loop filter LPF, voltage controlled oscillator VCO, frequency divider DIV1, frequency divider DIV2, frequency divider DIV3. The frequency divider DIV1 is connected to the crystal oscillator F _REF to provide the reference frequency PLL _REF required for PLL locking. The frequency divider DIV2 is connected to the crystal oscillator F _REF to generate two differential square wave modulation signals UP2 and DN2. The phase frequency detector PFD and frequency divider DIV1 is connected to identify the frequency and phase difference between the PLL _REF signal and the PLL _FB signal to generate UP and DN signals. The charge pump CP1 is connected to the phase frequency detector PFD. The current ICP is generated according to the UP and DN signals. _CP is injected into the loop filter and the charge is programmable. The pump CP2 and the programmable charge pump CP3 are both connected to the frequency divider DIV2, and respectively generate the modulation current I _mod and the compensation current I _{com and} inject them into the loop filter LPF to generate a triangular wave modulation signal. The loop filter LPF is connected with the charge pump CP1 and the programmable charge pump CP2. The charge pump CP2 is connected to the programmable charge pump CP3, which converts the current I _CP , the modulation current I _mod , and the compensation current I _com into the control voltage V _ctrl of the voltage-controlled oscillator VCO, the voltage-controlled oscillator VCO, and the loop filter LPF respectively. Connected, the output clock signal is generated according to the loop filter LPF output control voltage V _ctrl to achieve modulation changes in frequency in the time domain. The frequency divider DIV3 is connected to the voltage controlled oscillator VCO to generate a feedback signal PLL _FB feedback is connected back to the frequency and phase detector. The PFD input terminal of the device realizes the final locking of the PLL.

The loop filter LPF is composed of resistor R ₁ , resistor R ₃ , resistor R ₄ , capacitor C ₁ , capacitor C ₂ , capacitor C ₃ , and capacitor C ₄ . One end of capacitor C ₁ is grounded, and the other end is connected to resistor R ₁ . One end of R ₁ is connected to capacitor C ₁ , the other end is connected to capacitor C ₂ and resistor R ₃ at one end. One end of capacitor C ₂ is connected to ground. The other end is connected to resistor R ₁ and resistor R ₃ at one point. One end of resistor R ₃ is connected to resistor R _1. Capacitor C ₂ is connected to one point, and the other end is connected to capacitor C ₃ and resistor R _4. One end of capacitor C ₃ is connected to ground, and the other end is connected to resistor R ₃ and resistor R _4. One end of resistor R ₄ is connected to resistor R. _3. Capacitor C ₃ is connected to one point, and the other end is connected to capacitor C _4. One end of capacitor C ₄ is connected to ground, and the other end is connected to resistor R ₄ . The overall loop filter LPF is composed of fourth order and has four poles, so the overall PLL is a fifth-order type 2 PLL loop.

The loop filter LPF is connected to the charge pump CP1, the programmable charge pump CP2, and the programmable charge pump CP3. The output of the charge pump CP1 is connected to a point between the resistor R ₁ and the capacitor C ₂ in the loop filter LPF. The charge pump CP1 generates the PLL main loop current I _CP and injects it into the loop filter LPF between the resistor R ₁ and the capacitor C ₂ . The output of the programmable charge pump CP2 is connected to a point between the resistor R ₁ and the capacitor C ₁ in the loop filter LPF. The programmable charge pump CP2 generates a modulated current I _mod and injects it into the loop filter LPF between the resistor R ₁ and the capacitor C ₁ . The output of the programmable charge pump CP3 is connected to one point between the resistor R ₃ , the resistor R ₄ and the capacitor C ₃ in the loop filter LPF. The programmable charge pump CP3 generates a compensation current I _com and injects it into the loop filter LPF between the resistor R ₃ and the capacitor C ₃ . Among them, the current size of programmable charge pump CP2 and programmable charge pump CP3 is adjustable.

The output of the loop filter LPF is connected to the input of the voltage controlled oscillator VCO. The resistor R ₄ and capacitor C ₄ in the loop filter LPF are connected to the input of the voltage controlled oscillator VCO at one point. The loop filter LPF outputs the V _ctrl signal to control the voltage controlled oscillator VCO to produce corresponding frequency changes.

Working principle of the invention:

The invention discloses a direct modulation VCO spread spectrum clock generator based on two-point injection technology, as shown in Figure 1, frequency divider DIV1, phase frequency detector PFD, charge pump CP1, loop filter LPF, voltage controlled oscillation The VCO and the frequency divider DIV3 form the main structure of a conventional PLL, enabling the PLL to be locked to the target frequency.

The frequency divider DIV2 divides the crystal oscillator F _REF to generate two differential periodic modulation signals UP2(f _modp ) and DN2(f _modn ). Normally, the frequency range of f _modp and f _modn is between 30kHz and 70kHz. The programmable charge pump CP2 generates a modulated current I _mod based on the periodic signals UP2(f _modp ) and DN2(f _modn ) and injects it between the resistor R ₁ and the capacitor C ₁ in the loop filter LPF. From this, we can write from The transfer function from I _mod to V _ctrl is as follows:

In the formula, R ₁ , C ₁ and C ₂ are the resistor R ₁ , capacitor C ₁ and capacitor C ₂ in the loop filter LPF respectively, maintaining the modulation frequency f _mod <<(C ₁ +C ₂ )/(2πR ₁ C ₁ C ₂ ), an ideal integral effect can be obtained, so the above formula can be simplified to

Then the triangular wave modulation signal can be obtained by integrating the modulation current I _mod , and a periodic triangular signal can be generated at the V _ctrl node, and the frequency of the periodic signals UP2(f _modp ) and DN2(f _modn ) and the modulation current I _mod and the generated triangular wave The three modulated signals have the same frequency, so this injection method can be used to implement spread spectrum clocking.

In the traditional PLL-based direct modulation VCO spread spectrum clock generator, since the triangular wave modulation signal is applied to the voltage controlled oscillator VCO control voltage, the entire modulation path exhibits high-pass characteristics, so it is necessary to select an extremely low PLL loop bandwidth to ensure modulation The signal can pass through the PLL loop completely. In addition, the extremely low PLL loop bandwidth is also helpful in filtering out the modulation component of the phase frequency detector PFD output due to the spread spectrum operation. As mentioned earlier, the frequency range of the modulation signals f _modp and f _modn is between 30kHz and 70kHz, so in order to implement the spread spectrum clock, the loop bandwidth of the PLL needs to be set between 3kHz and 7kHz. Unfortunately, the extremely small loop bandwidth requires a huge loop filter LPF capacitor C ₁ , which increases the area cost and, on the other hand, lengthens the locking time of the PLL. Therefore, it is of great significance to invent a directly modulated VCO spread spectrum clock generator that has good spread spectrum performance without increasing area cost and locking time.

As analyzed previously, the transfer function from I _mod to V _ctrl point shows that the V _ctrl point can generate a periodic triangular wave modulation signal. If the second-order loop filter LPF is used, due to the injection of the modulation current I _mod and the implementation of the spread spectrum clock, in the time domain, the phase frequency detector (PFD) will identify the phase difference between PLL _FB and PLL _REF , outputting UP and DN signals, causing the charge pump CP1 to periodically charge and discharge the loop filter, thereby seeing spikes (Spikes) at the V _a point, which is undesirable. Fortunately, it can be filtered by introducing resistor R ₃ and capacitor C ₃ in the loop filter LPF, but while filtering out the spikes, the high-frequency component of the triangle wave modulation signal at point _Va will also be filtered out. , which will attenuate the peak value of the triangular modulated signal. Moreover, if the PLL loop bandwidth is selected too large, at point V _a it is already a periodic signal that has been low-pass filtered by the resistor R ₁ and capacitor C ₂ in the loop filter LPF, and then filtered by the resistor R ₃ and capacitor C ₃ , in severe cases, it will attenuate to a modulation signal close to a sine wave, worsening the EMI reduction effect.

In view of the above problems, the present invention proposes to introduce an additional programmable charge pump CP3, which has the same operation as the programmable charge pump CP2. The difference is their current size. The compensation current I _com of the programmable charge pump CP3 is smaller than the programmable charge pump CP3. The modulation current I _mod of the pump CP2 is similarly required to keep the sum of the equivalent impedances of the resistor R ₁ , capacitor C ₁ , capacitor C ₂ and capacitor R ₃ in the loop filter LPF not exceeding the equivalent impedance of the capacitor C ₃ , the transfer function from the compensation current I _com injection point to V _ctrl is:

In the formula, R ₄ , C ₃ and C ₄ are the resistor R ₄ , capacitor C ₃ and capacitor C ₄ in the loop filter LPF respectively. If the modulation frequency f _mod <<(C ₃ +C ₄ )/(2πR ₄ C ₃ C ₄ ), we can get an ideal integral effect, so we can simplify the above formula to:

According to the above formula, it can be known that the injection of the compensation current I _com into the voltage controlled oscillator VCO control voltage V _ctrl still has an integral effect, that is, the compensation effect on the attenuated sine wave modulation signal can be achieved. It only needs to reasonably adjust the current size of the programmable charge pump CP3 to achieve compensation and restore it to triangular wave modulation. In addition, the introduction of the compensation current I _com can relax the limitation of the traditional direct modulation VCO spread spectrum clock generator structure on the PLL loop bandwidth, even if the originally close-to-ideal triangle wave modulation signal is filtered into a sine wave modulation by the loop filter LPF signal, it can also be compensated back by adjusting the size of the compensation current I _com , effectively reducing the locking time of the PLL.

Considering spurious suppression, in order to maintain good reference spurious suppression performance, a smaller pole position needs to be selected. Considering that the position of the main pole of the loop filter LPF is:

It can be seen from the above formula that in order to obtain better reference spurious suppression effect, the value of C _eq needs to be increased, where C _eq =C ₁ ·C ₂ /(C ₁ + C ₂ ), C ₁ and C ₂ are respectively Capacitor C ₁ and capacitor C ₂ in the loop filter LPF. Combined with the previous prerequisite of modulation frequency f _mod , the increase of C _eq will increase the nonlinearity of the capacitor in the loop filter LPF integrating the modulation current I _mod , so this structure has a trade-off between modulation linearity and spurious suppression. However, the present invention gives priority to modulation linearity and reduces the value of C _eq so that the triangular wave modulated signal can achieve a more ideal integration effect and be more linear by adding an additional resistor R ₄ and capacitor in the loop filter LPF. C ₄ achieves better reference spur suppression.

The structure of the loop filter LPF used in the present invention is shown in Figure 5. Based on the fourth-order loop filter LPF, better triangular wave linear modulation is achieved.

Specific simulation results are given to assist in verifying the theoretical correctness of the present invention. As shown in Figure 6, the second-order loop filter LPF is used, and the control voltage V _ctrl entering the voltage-controlled oscillator VCO will carry spikes (Spikes). Figure 7 shows the introduction of the third-order loop filter LPF to filter out the spikes. But the modulated signal will be further attenuated. Finally, the effect of introducing the compensation current I _com , resistor R ₄ and capacitor C ₄ proposed by the present invention is shown in Figure 9. The control voltage V _ctrl curve entering the voltage controlled oscillator VCO can be restored to an approximate triangular modulated wave signal. Figure 9 is a spectrum comparison chart before and after the spread spectrum is turned on according to the present invention. It can be seen from the figure that the EMI of the spread spectrum clock generator proposed by the present invention can be reduced by up to 14.48dB.

The above content describes the working principle, main features and advantages of the present invention compared with traditional structures. It is obvious to those skilled in the art that the present invention is not limited to the detailed description of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the theory or basic characteristics of the present invention. Therefore, this embodiment should be exemplary and non-restrictive from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that the meaning and scope of the claims will fall All changes within are included in the novelty of the present invention, and any reference signs in the claims should not be construed as limiting the involved claims.

Furthermore, it should be understood that such description in the specification is merely for clarity and ease of understanding. Those skilled in the art should take the description as a whole and may modify the technical solutions adopted in the foregoing embodiments, or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims (4)

1. A direct modulation VCO spread spectrum clock generator based on two-point injection technology, including crystal oscillator F _REF , phase frequency detector PFD, charge pump CP1, programmable charge pump CP2, programmable charge pump CP3, and loop filter LPF, voltage controlled oscillator VCO, frequency divider DIV1, frequency divider DIV2, frequency divider DIV3. The frequency divider DIV1 is connected to the crystal oscillator F _REF to provide the reference frequency PLL _REF required for PLL locking. The frequency divider DIV2 is connected to the crystal oscillator F _REF to generate two differential square wave modulation signals UP2 and DN2. The frequency and phase identification are The charge pump CP1 is connected to the frequency divider DIV1 to identify the frequency and phase difference between the PLL _REF signal and the PLL _FB signal to generate UP and DN signals. The charge pump CP1 is connected to the phase frequency detector PFD and generates a current I _CP based on the UP and DN signals. Loop filter, the programmable charge pump CP2 and the programmable charge pump CP3 are both connected to the frequency divider DIV2, respectively generating the modulation current I _mod and the compensation current I _com and injecting them into the loop filter LPF to generate a triangular wave modulation signal, the The loop filter LPF is connected to the charge pump CP1, the programmable charge pump CP2 and the programmable charge pump CP3, respectively converting the current I _CP , the modulation current I _mod and the compensation current I _com into the control voltage V _ctrl of the voltage controlled oscillator VCO. , the voltage-controlled oscillator VCO is connected to the loop filter LPF, and an output clock signal is generated according to the loop filter LPF output control voltage V _ctrl to achieve modulation changes in frequency in the time domain. The frequency divider DIV3 is connected to the voltage-controlled The oscillator VCO is connected to generate a feedback signal PLL _FB feedback is connected back to the input terminal of the phase frequency detector PFD to achieve the final locking of the PLL. 2. The direct modulation VCO spread spectrum clock generator based on two-point injection technology according to claim 1, characterized in that: the loop filter LPF consists of a resistor R ₁ , a resistor R ₃ , a resistor R ₄ , and a capacitor C _1. It consists of capacitor C ₂ , capacitor C ₃ and capacitor C ₄ . One end of the capacitor C ₁ is connected to the ground and the other end is connected to the resistor R ₁ . One end of the resistor R ₁ is connected to the capacitor C ₁ and the other end is connected to the capacitor C ₂ . The resistor R ₃ is connected to one point. One end of the capacitor C ₂ is connected to the ground. The other end is connected to the resistor R ₁ and the resistor R _{3 . One end of the resistor R 3 is} connected to the resistor R ₁ and the capacitor C ₂ . The other end is connected to the ground. Capacitor C ₃ and resistor R ₄ are connected to one point. One end of capacitor C ₃ is connected to ground, and the other end is connected to resistor R ₃ and resistor R ₄ . One end of resistor R ₄ is connected to one end of resistor R ₃ and capacitor C ₃ . , the other end is connected to the capacitor C ₄ , one end of the capacitor C ₄ is connected to ground, and the other end is connected to the resistor R ₄ . The overall loop filter LPF is composed of fourth order and has four poles, so the overall PLL is a fifth-order type-2 PLL loop. 3. The direct modulation VCO spread spectrum clock generator based on two-point injection technology according to claim 1, characterized in that: the loop filter LPF and the charge pump CP1, the programmable charge pump CP2, The programmable charge pump CP3 is connected. The output of the charge pump CP1 is connected to a point between the resistor R ₁ and the capacitor C ₂ in the loop filter LPF. The charge pump CP1 generates the PLL main loop current I _CP and injects it into the loop filter LPF between the resistor R ₁ and the capacitor C ₂ . The output of the programmable charge pump CP2 is connected to a point between the resistor R ₁ and the capacitor C ₁ in the loop filter LPF. The programmable charge pump CP2 generates a modulated current I _mod and injects it into the loop filter LPF between the resistor R ₁ and the capacitor C ₁ . The output of the programmable charge pump CP3 and the resistor R ₃ , resistor R ₄ and capacitor C ₃ in the loop filter LPF are connected to one point. The programmable charge pump CP3 generates a compensation current I _com and injects it into the loop filter LPF between the resistor R ₃ and the capacitor C ₃ . Wherein, the current size of the programmable charge pump CP2 and the programmable charge pump CP3 is adjustable. 4. The direct modulation VCO spread spectrum clock generator based on two-point injection technology according to claim 1, characterized in that: the loop filter LPF output is connected to the voltage controlled oscillator VCO input, and the The resistor R ₄ and the capacitor C ₄ in the loop filter LPF are connected to the input of the voltage controlled oscillator VCO at one point. The loop filter LPF outputs the V _ctrl signal to control the voltage controlled oscillator VCO to produce corresponding frequency changes.