Stabilization technique for phase-locked frequency synthesizers

Abstract

A stabilization technique that relaxes the tradeoff between the settling speed and the magnitude of output sidebands in phase-locked frequency synthesizers. The method introduces a zero in the open-loop transfer function through the use of a discrete-time delay element, thereby obviating the need for resistors in the loop filter.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to phase locked loops, and in particular, to a stabilization technique for phase-locked frequency synthesizers.

[0004] 2. Description of the Related Art

[0005] (Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)

[0006] Phase-locked loops (PLLs) typically suffer from a trade-off between the settling time and the ripple on the control voltage, limiting the performance that can be achieved in terms of channel switching speed and output sideband magnitude in radio frequency (RF) synthesizers.

[0007] Accordingly, what is needed in the art is a loop stabilization technique that yields a small ripple while achieving fast settling. The present invention satisfies this need.

BRIEF SUMMARY OF THE INVENTION

[0008] A stabilization technique that relaxes the tradeoff between the settling speed and the magnitude of output sidebands in phase-locked frequency synthesizers. The method introduces a zero in the open-loop transfer function through the use of one or more discrete-time delay elements, thereby obviating the need for resistors in the loop filter.

[0009] In one embodiment, the phase-locked frequency synthesizer of the present invention comprises:

[0010] a phase-frequency detector (PFD) for comparing a phase of a reference signal with a phase of a feedback signal and generating Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal;

[0011] first and second charge pumps (CPs) for generating positive and negative charge pulses in response to the Up and Down signals from the phase-frequency detector;

[0012] a delay element for delaying the positive and negative charge pulses from the second charge pump;

[0013] a loop filter for integrating the positive and negative charge pulses from the first charge pump and the delayed positive and negative charge pulses from the second charge pump, and for generating a control voltage in response thereto;

[0014] a voltage-controlled oscillator (VCO) for accepting the control voltage from the loop filter and for changing a frequency of an output of the VCO in response thereto; and

[0015] a divider for dividing the output of the VCO and generating the feedback signal therefrom.

[0016] The delay element comprises a discrete-time analog delay. The discrete-time analog delay comprises an interleaved sampling network operating at half of the reference frequency. The interleaved sampling network comprises two interleaved master-slave sample-and-hold branches, and provides a delay equal to a reference period, 1/fREF.

[0017] In another embodiment, the phase-locked frequency synthesizer of the present invention includes a delay element for delaying positive and negative charge pulses from a charge pump, wherein the delay element introduces a zero in a open-loop transfer function of the phase-locked frequency synthesizer.

[0018] The phase-locked frequency synthesizer further comprises a phase-frequency detector that compares a phase of a reference signal with a phase of a feedback signal and generates Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal.

[0019] The phase-locked frequency synthesizer further comprises first and second charge pumps (CPs) for generating positive and negative charge pulses in response to the Up and Down signals from the phase-frequency detector, wherein the delay element delays the positive and negative charge pulses from the second charge pump.

[0020] The delay element comprises a discrete-time analog delay. The discrete-time analog delay comprises an interleaved sampling network operating at half of the reference frequency. The interleaved sampling network comprises two interleaved master-slave sample-and-hold branches, and provides a delay equal to a reference period, 1/fREF.

[0021] A loop filter integrates the positive and negative charge pulses from the first charge pump and the delayed positive and negative charge pulses from the second charge pump, and generates a control voltage in response thereto. A voltage-controlled oscillator (VCO) that accepts the control voltage from the loop filter and changes a frequency of an output of the VCO in response thereto. A divider divides the output of the VCO and generates the feedback signal therefrom.

[0022] In yet another embodiment, the phase-locked frequency synthesizer of the present invention comprises:

[0023] N charge pumps (CPs) for generating positive and negative charge pulses in response to Up and Down signals received from a phase-frequency detector; and

[0024] N−1 delay elements for delaying the positive and negative charge pulses from N−1 of the charge pumps;

[0025] wherein the positive and negative charge pulses from a first one of the N charge pumps and the delayed positive and negative charge pulses from the N−1 of the charge pumps are integrated in order to generate a control voltage for a voltage-controlled oscillator (VCO) in response thereto.

[0026] The delay elements introduce a zero in a open-loop transfer function of the phase-locked frequency synthesizer, thereby obviating a need for resistors in a loop filter of the phase-locked frequency synthesizer.

[0027] The phase-frequency detector compares a phase of a reference signal with a phase of a feedback signal and generates Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal.

[0028] The voltage-controlled oscillator (VCO) accepts the control voltage from the loop filter and changes a frequency of an output of the VCO in response thereto. A divider divides the output of the VCO and generates the feedback signal therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

[0030] FIG. 1 is a block diagram illustrating a conventional phase-locked loop (PLL) architecture;

[0031] FIG. 2 is a block diagram illustrating a proposed phase-locked loop (PLL) architecture with delayed charge pump circuit according to the preferred embodiment of the present invention;

[0032] FIG. 3A is a block diagram illustrating an implementation of a phase-locked loop (PLL) with a delay sampling circuit according to the preferred embodiment of the present invention;

[0033] FIG. 3B is a block diagram illustrating an implementation of a phase-locked loop (PLL) with a delay sampling circuit according to the preferred embodiment of the present invention;

[0034] FIG. 3C is a timing diagram illustrating the operation of a phase-locked loop (PLL) with a delay sampling circuit according to the preferred embodiment of the present invention;

[0035] FIG. 4 is a block diagram illustrating a continuous-time approximation of a delay network according to the preferred embodiment of the present invention;

[0036] FIG. 5 is a block diagram illustrating a proposed phase-locked loop (PLL) architecture using generalized delayed charge pumps according to the preferred embodiment of the present invention;

[0037] FIG. 6 is a block diagram illustrating a synthesizer architecture according to the preferred embodiment of the present invention;

[0038] FIG. 7A is a block diagram illustrating an LC oscillator according to the preferred embodiment of the present invention;

[0039] FIG. 7B is a block diagram illustrating a stacked inductor according to the preferred embodiment of the present invention;

[0040] FIG. 8 is a block diagram illustrating a prescaler according to the preferred embodiment of the present invention;

[0041] FIG. 9A is a block diagram illustrating a divider according to the preferred embodiment of the present invention;

[0042] FIG. 9B is a timing diagram illustrating the operation of the divider according to the preferred embodiment of the present invention;

[0043] FIG. 10 is a die photo of a frequency synthesizer according to the preferred embodiment of the present invention;

[0044] FIG. 11 is a graph illustrating a measured spectrum at the output of the VCO; and

[0045] FIG. 12 is a graph illustrating a control voltage during loop setting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0047] Introduction

[0048] The present invention describes a phased-locked loop (PLL) stabilization technique that yields a small ripple while achieving fast settling. Using a discrete-time analog delay, the present invention creates a zero in an open-loop transfer function of the PLL. Moreover, the present invention uses no resistors in a loop filter for the PLL, thereby lending itself to digital CMOS technologies.

[0049] Stabilization Technique

[0050] FIG. 1 shows a block diagram of a basic phase-locked loop (PLL) used in a frequency synthesizer. The PLL is comprised of a phase-frequency detector (PFD), a charge pump (CP), a loop filter comprised of capacitors C1 and C2 and resister R1, a voltage-controlled oscillator (VCO), and a divider (÷M). The PFD provides means for comparing a phase of a reference signal having a frequency fREF with a phase of a feedback signal (the divided VCO signal output from the divider) and for generating “Up” and “Down” signals to the CP, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal. The CP provides means for generating positive and negative charge pulses from the Up and Down signals from the PFD. The loop filter, which is a low pass filter (LPF), provides means for integrating the positive and negative charge pulses from the CP and for generating a control voltage Vcont in response thereto. The VCO provides means for accepting the control voltage Vcont from the loop filter and for changing a frequency of its output in response thereto. The divider provides means for dividing the output of the VCO and for generating the feedback signal therefrom.

[0051] In the loop filter, resistor R1 provides a stabilizing zero and capacitor C2 suppresses a glitch generated by the CP on every phase comparison instant. The glitch arises from mismatches between the width of Up and Down pulses produced by the PFD, as well as charge injection and clock feedthrough mismatches between PMOS and NMOS devices in the CP.

[0052] The principal drawback of this architecture is that C1 determines the settling, whereas C2 controls the ripple on the control voltage Vcont (the waveform above the Vcont label in the drawing illustrates this ripple). Since C2 must remain below C1 by roughly a factor of 10 so as to avoid underdamped settling, the loop must be slowed down by a large C1 if C2 is to yield a sufficiently small ripple. It is therefore desirable to seek methods of creating the stabilizing zero without the resistor so that the capacitor that defines the switching speed also directly suppresses the ripple.

[0053] It is important to note that the problem of ripple becomes increasingly more serious as the supply voltage is scaled down and/or the operating frequency goes up. The relative magnitude of the primary sidebands at the output of the VCO is given by AmKVCO=(2ωREF) where Am is the peak amplitude of the first harmonic of the ripple, KVCO is the gain of the VCO, and ωREF is the synthesizer reference frequency. (For a given relative tuning range (e.g. ±10%), the gain of LC VCOs must increase if the supply voltage goes down.) If KVCO=100 MHz/V and fREF=1 MHz, then the fundamental ripple amplitude must be less than 63 μV to guarantee sidebands 60 dB below the carrier.

[0054] In order to arrive at the stabilization technique, consider the PLL architecture shown in FIG. 2. Here, the primary charge pump, CP1, drives a single capacitor C1 while a secondary charge pump, CP2, injects a charge after some delay element ΔT. The total current flowing through C1 is thus equal to: 1$\begin{array}{cc}{I}_p=I_{\mathrm{p1}}+I_{\mathrm{p2}}{e}^{-s\Delta T}& \left(1\right)\\ \approx I_{\mathrm{p1}}+I_{\mathrm{p2}}\left(1-s\Delta T\right)& \left(2\right)\end{array}$

[0055] where Ip is the current flowing through C1, Ip1 is the current value of CP1, Ip2 is the current value of CP2, ΔT is assumed to be much smaller than the loop time constant and s is a complex frequency.

[0056] Consequently, the transfer function of the PFD/CP/LPF combination can be expressed as: 2$\begin{array}{cc}\frac{V_{\mathrm{cont}}}{\Delta \phi }\left(s\right)=\frac{I_{\mathrm{p1}}+I_{\mathrm{p2}}}{2\pi C_{1}s}-\frac{I_{\mathrm{p2}}}{2\pi C_1}\Delta T& \left(3\right)\end{array}$

[0057] where Δφ is the phase difference.

[0058] Assuming Ip2=αIp1, we have: 3$\begin{array}{cc}\frac{V_{\mathrm{cont}}}{\Delta \phi }\left(s\right)=\frac{I_{\mathrm{p1}}}{2\pi }\left(\frac{1-\alpha }{C_{1}s}+\frac{\alpha \Delta T}{C_1}\right)& \left(4\right)\end{array}$

[0059] thereby obtaining a zero frequency at: 4$\begin{array}{cc}{\omega }_z=\frac{1-\alpha }{\alpha }\frac{1}{\Delta T}& \left(5\right)\end{array}$

[0060] In order to achieve a sufficiently low zero frequency, ΔT must be large or a must be close to unity. Since the accuracy in the definition of α is limited by mismatches between the CP1 and CP2, ΔT must still be a large value. For example, if fREF=1 MHz and α=0.9, then a ΔT of approximately 200 ns is required to ensure a well-behaved loop response.

[0061] The architecture of FIG. 2 suffers from a critical drawback: it requires a very long delay element ΔT before CP2, while the Up and Down pulses generated by the PFD can assume a very narrow width during lock. If the bandwidth of each stage in the delay line is reduced so as to produce a large delay, then the narrow Up and Down pulses are heavily attenuated, thus giving rise to a dead zone. Conversely, if the bandwidth of each stage is wide enough to support such pulses, then a very large number of stages is required to obtain the necessary ΔT.

[0062] To resolve the above difficulty, the architecture is modified as shown in FIG. 3A, where the delay element ΔT comprises a discrete-time analog delay placed after CP2 and C2. The discrete-time analog delay comprises a delay network that is realized as depicted in FIG. 3B, comprising two interleaved master-slave sample-and-hold branches operating at half of the reference signal frequency.

[0063] The delay network emulates ΔT as follows. Consider the timing diagram of FIG. 3C, which shows fREF, CKeven and CKodd. When CKeven is high, Cs1 shares a charge packet corresponding to the previous phase comparison with C1 while Cs2 samples a level proportional to the present phase difference. In the next period, Cs1 and Cs2 exchange roles: when CKodd is high, Cs2 shares a charge packet corresponding to the previous phase comparison with C1 while Cs1 samples a level proportional to the present phase difference. (Note that Cs1=Cs2=Cs, as indicated in FIG. 3B.) The interleaved sampling network therefore provides a delay equal to the reference signal period, 1/fREF or TREF.

[0064] The discrete-time analog delay of FIG. 3A allows a precise definition of the zero frequency without the use of resistors. To quantify the behavior of a PLL incorporating this method, we assume the loop settling time is much greater than I/fREF, so that the delay network can be represented by the continuous-time model shown in FIG. 4. Here, Req=(fREFCs)−1 approximates the interleaved branches. Equation (4) can then be rewritten as: 5$\begin{array}{cc}\frac{V_{\mathrm{cont}}}{\Delta \phi }\left(s\right)=\frac{I_{\mathrm{p1}}}{2\pi }\left(\frac{1}{f_{\mathrm{REF}}{C}_s}+\frac{I_{\mathrm{p1}}+I_{\mathrm{p2}}}{I_{\mathrm{p1}}}\frac{1}{s{C}_2}\right)& \left(6\right)\end{array}$

[0065] where it is assumed C2>>C1 and the current through C1 is neglected. This equation exhibits two interesting properties.

[0066] First, if Ip2=−αIp1, then (Ip1+Ip2)/Ip1=1−α and the value of C2 is “amplified” by (1−α)−1. For example, if α=0.9, then C2 is multiplied by a factor of 10, saving substantial area.

[0067] Second, the zero frequency is equal to: 6$\begin{array}{cc}{\omega }_z=\left(1+\frac{I_{\mathrm{p2}}}{I_{\mathrm{p1}}}\right)\frac{C_s}{C_2}{f}_{\mathrm{REF}}& \left(7\right)\end{array}$

[0068] which is a value independent of process and temperature.

[0069] Assuming C2=Cs and Ip1=−αIp2=Ip, we obtain the damping factor and the settling time constant of the loop as: 7$\begin{array}{cc}=\frac{1}{2f_{\mathrm{REF}}}\sqrt{\frac{I_p}{2\pi \left(1-\alpha \right)C_2}\frac{K_{\mathrm{VCO}}}{M}}& \left(8\right)\\ {\left({\omega }_n\right)}^{-1}=\frac{4\pi f_{\mathrm{REF}}{C}_2}{I_p}\frac{M}{K_{\mathrm{VCO}}}& \left(9\right)\end{array}$

[0070] The key observation here is that the factor (1−α)−1 appears in ζ but not in (ζωn)−1. Thus, the two parameters can be optimized relatively independently.

[0071] Furthermore, the damping factor exhibits much less process and temperature dependence than in the conventional loop of FIG. 1. Note that for Ip2=0, the proposed circuit resembles the topology of FIG. 1, but with the resistor replaced with a switched-capacitor network.

[0072] For a frequency synthesizer, the delay network of FIG. 2 must be designed carefully so as to minimize ripple on the control voltage. Since in the locked condition, the voltages at nodes A and B are nearly equal, the charge sharing between Cs1 or Cs2 and C1 creates only a small ripple. Furthermore, the switches in the delay network are realized as small, complementary devices to introduce negligible charge injection and clock feedthrough.

[0073] Generalization of the Concept

[0074] The concept of adding a charge pump and a delay element to a conventional PLL is a special case of a more general and powerful concept. As illustrated in FIG. 5, which is a block diagram illustrating a proposed PPL architecture using generalized delayed charge pumps, the general concept employs n charge pumps (CP1, CP2, CP3, . . . , CPn) having current values (Ip1, Ip2, Ip1, . . . , Ipn) n−1 capacitors (C2, C3, . . . , Cn), and n−1 delay elements (ΔT2, ΔT3, . . . , ΔTn), with the current values and delay values chosen to optimize the tradeoff between the settling speed and the sideband magnitudes.

[0075] Another important generalization is that the n−1 delay elements (ΔT2, ΔT3, ΔTn) need not be equal to a multiple of the reference signal period TREF; these delay values can be a fraction of TREF if staggered edges of fREF are used.

[0076] Frequency Synthesizer Design

[0077] A 2.4-GHz CMOS frequency synthesizer targeting Bluetooth™ applications has been designed using the stabilization technique described above. This section presents the architecture and building blocks of the frequency synthesizer.

[0078] Shown in FIG. 6, the frequency synthesizer uses an architecture similar to FIG. 3A, but having an integer-N architecture with a feedback divider comprised of a Prescaler (÷4/5), Level Converter (LE), Program Counter (÷600) and Swallow Counter, wherein the Swallow Counter includes a Channel Selection (0-127) and Flip-flop (FF1). The feedback divider comprised of these elements has a modulus given by M=NP+S+1, where N 4 (Prescaler), P=600 (Program Counter), and S=0-127 (Channel Selection).

[0079] With fREF=1 MHz, the output frequency covers the 2.4-GHz ISM (Industrial Scientific and Medical) band. The output of the Swallow Counter is pipelined by the Flip-flop FF1 to allow a relaxed design for the Level Converter and the Swallow Counter [1]. The Buffer (BF) following the VCO suppresses the kickback noise of the Prescaler when the modulus changes. It also avoids limiting the tuning range of the VCO by the input capacitance of the Prescaler.

[0080] The VCO topology is shown in FIG. 7A. The VCO is driven by the control voltage Vcont. To provide both negative and positive voltages across the MOS varactors C1 and C2, the sources of M1 and M2 are grounded and the circuit is biased on top by IDD, which results from VDD. The inductors Lp are realized as shown in FIG. 7B, with the bottom spiral moved down to metal layer 2 so as to reduce the parasitic capacitance [2].

[0081] The Prescaler must divide the 2.4-GHz signal while consuming a small power dissipation. Depicted in FIG. 8, the circuit employs three current-steering flip-flops (NOR-FF, FF and NOR-FF) with diode-connected loads, wherein the flip-flops have inputs D, {overscore (D)}, outputs Q, {overscore (Q)}, are clocked by the CK signal and are controlled by the MC signal. The output of the Prescaler is indicated as Vout, which is subsequently input to the Level Converter. The use of NOR gates with two of the flip-flops (NOR-FF) obviates the need for power-hungry and headroom-hungry level shift circuits required in NAND implementations.

[0082] Simulation

[0083] A 2.4-GHz frequency synthesizer with a reference frequency of 1 MHz requires a transient simulation step of approximately 20 ps for a total settling time on the order of 100 μs. The simulation therefore requires an extremely long time owing to both the vastly different time scales and the large number of devices (especially in the divider).

[0084] In order to study the loop dynamics with realistic transistor level PFD, CP and VCO design, two speed-up techniques have been employed. First, the reference frequency is scaled up by a factor of 100 and the loop filter capacitor and the divide ratio are scaled down by the same factor. Since the PFD operates reliably at 100 MHz with no dead zone, this method directly reduces the simulation time by a factor of 100. From Eqs. (8) and (9), we note that scaling C2 and M by 100 maintains a constant damping factor while scaling the settling time by 100.

[0085] Second, the divider is realized as a simple behavioral model in HSPICE that uses a handful of ideal devices and its complexity is independent of the divide ratio. FIG. 9 is a block diagram of the divider, which includes an input Vb, two sets of switches CK and {overscore (CK)}, capacitors C, an op-amp (OA) and two comparators (CO) driven by the output of the OA, and VREF and VTH, respectively. The first comparator compares the voltage VA at node A with a reference voltage VREF, thereby generating a signal, having a voltage VB at node B, which controls a switched capacitor network for the op-amp OA. The second comparator compares the voltage VA at node A with a threshold voltage VTH, thereby generating the Vout signal. The timing diagram of FIG. 8B shows the relationships between VTH, VA, VB, and Vout.

[0086] As illustrated in the block diagram of FIG. 9A and the timing diagram of FIG. 9B, the principle of the behavioral divider is to pump a well-defined charge packet into an integrator in every period and reset the integrator when its output exceeds a certain level, VREF. Using an ideal op amp (OA), comparator (CO), and switches (CK and {overscore (CK)}) with proper choice of Vb and VREF, the circuit can achieve arbitrarily long high divide ratios. This techniques yields another factor of 20 improvement in the simulation speed, allowing the synthesizer to be simulated in less than 3 minutes on an Ultra 10 Sun workstation.

[0087] Experimental Results

[0088] The frequency synthesizer has been fabricated in a digital 0.25-μm CMOS technology. Shown in FIG. 10 is a photograph of the die, whose active area measures 0.65 mm×0.45 mm. The circuit has been tested in a chip-on-board assembly while running from a 2.5-V power supply.

[0089] FIG. 11 shows the output spectrum in the locked condition. The phase noise is equal to −112 dBc/Hz at 1 MHz offset, well exceeding the Bluetooth™ requirement. The primary reference sidebands are at approximately −58.7 dBc. This level is lower than that achieved in [3] with differential VCO control and an 86.4-MHz reference frequency. FIG. 12 plots the measured settling behavior of the frequency synthesizer when its channel number is switched by 64. The settling time is about 60 μs, i.e., 60 input cycles.

[0090] Table 1 below provides a performance summary of the frequency synthesizer.

	Center Frequency	2.4	GHz
	Channel Spacing	1	MHz
	No. of channels	128
	Phase Noise at 1 MHz Offset	−112	dBc/Hz
	Reference Sidebands	−58.7	dBc
	Settling Time	60	μs
	Power Dissipation
	VCO	10	mW
	VCO Buffer	3	mW
	Divider	6	mW
	Charge Pump	0.5	mW
	Others	0.5	mW
	Total	20	mW
	Supply Voltage	2.5	V
	Die Area	0.65 mm × 0.45 mm
	Technology	0.25-μm CMOS

REFERENCES

[0091] The following references are incorporated by reference herein:

[0092] [1] C. Lam and B. Razavi, “A 2.6-GHz/5.2-GHz Frequency Synthesizer in 0.4-μm CMOS Technology,” IEEE J. Solid-State Circuits, vol. 35, pp. 788-794, May 2000.

[0093] [2] A. Zolfaghari, A. Chan, and B. Razavi, “Stacked Inductors and 1-to-2 Transformers in CMOS Technology,” Proc. of CICC, pp. 345-348, May 2000.

[0094] [3] L. Lin, L. Tee, and P. R. Gray, “A 1.4-GHz Differential Low-Noise CMOS Frequency Synthesizer using a Wideband PLL Architecture,” ISSCC Dig. Tech. Paper, pp. 204-205, February 2000.

CONCLUSION

[0095] This concludes the description of the preferred embodiment of the invention. The following describes some alternative embodiments for accomplishing the present invention.

[0096] For example, the present invention could be used with many types of circuits, and not just those described herein. Moreover, any number of different components or different configurations of components could be used without departing from the scope of the present invention. Finally, any number of n charge pumps and n−1 delay elements could be used in the present invention.

[0097] The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

1. A phase-locked frequency synthesizer, comprising: a phase-frequency detector (PFD) for comparing a phase of a reference signal with a phase of a feedback signal and generating Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal; first and second charge pumps (CPs) for generating positive and negative charge pulses in response to the Up and Down signals from the phase-frequency detector; a delay element for delaying the positive and negative charge pulses from the second charge pump; a loop filter for integrating the positive and negative charge pulses from the first charge pump and the delayed positive and negative charge pulses from the second charge pump, and for generating a control voltage in response thereto; a voltage-controlled oscillator (VCO) for accepting the control voltage from the loop filter and for changing a frequency of an output of the VCO in response thereto; and a divider for dividing the output of the VCO and generating the feedback signal therefrom.

2. The phase-locked frequency synthesizer of claim 1, wherein the delay element comprises a discrete-time analog delay.

3. The phase-locked frequency synthesizer of claim 2, wherein the discrete-time analog delay comprises an interleaved sampling network operating at half of the reference frequency.

4. The phase-locked frequency synthesizer of claim 3, wherein the interleaved sampling network comprises two interleaved master-slave sample-and-hold branches.

5. The phase-locked frequency synthesizer of claim 4, wherein the interleaved sampling network provides a delay equal to a reference period, 1/fREF.

6. A phase-locked frequency synthesizer including a delay element for delaying positive and negative charge pulses from a charge pump, wherein the delay element introduces a zero in a open-loop transfer function of the phase-locked frequency synthesizer.

7. The phase-locked frequency synthesizer of claim 6, further comprising a phase-frequency detector that compares a phase of a reference signal with a phase of a feedback signal and generates Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal.

8. The phase-locked frequency synthesizer of claim 7, further comprising first and second charge pumps (CPs) for generating positive and negative charge pulses in response to the Up and Down signals from the phase-frequency detector, wherein the delay element delays the positive and negative charge pulses from the second charge pump.

9. The phase-locked frequency synthesizer of claim 8, wherein a loop filter integrates the positive and negative charge pulses from the first charge pump and the delayed positive and negative charge pulses from the second charge pump, and generates a control voltage in response thereto.

9. The phase-locked frequency synthesizer of claim 8, further comprising a voltage-controlled oscillator (VCO) that accepts the control voltage from the loop filter and changes a frequency of an output of the VCO in response thereto.

10. The phase-locked frequency synthesizer of claim 9, further comprising a divider that divides the output of the VCO and generates the feedback signal therefrom.

11. The phase-locked frequency synthesizer of claim 6, wherein the delay element comprises a discrete-time analog delay.

12. The phase-locked frequency synthesizer of claim 11, wherein the discrete-time analog delay comprises an interleaved sampling network operating at half of the reference frequency.

13. The phase-locked frequency synthesizer of claim 12, wherein the interleaved sampling network comprises two interleaved master-slave sample-and-hold branches.

14. The phase-locked frequency synthesizer of claim 13, wherein the interleaved sampling network provides a delay equal to a reference period, 1/fREF.

15. A phase-locked frequency synthesizer, comprising: N charge pumps (CPs) for generating positive and negative charge pulses in response to Up and Down signals received from a phase-frequency detector; and N−1 delay elements for delaying the positive and negative charge pulses from N−1 of the charge pumps; wherein the positive and negative charge pulses from a first one of the N charge pumps and the delayed positive and negative charge pulses from the N−1 of the charge pumps are integrated in order to generate a control voltage for a voltage-controlled oscillator (VCO) in response thereto.

16. The phase-locked frequency synthesizer of claim 15, wherein the delay elements introduce a zero in a open-loop transfer function of the phase-locked frequency synthesizer, thereby obviating a need for resistors in a loop filter of the phase-locked frequency synthesizer.

17. The phase-locked frequency synthesizer of claim 16, wherein the phase-frequency detector compares a phase of a reference signal with a phase of a feedback signal and generates Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal.

18. The phase-locked frequency synthesizer of claim 17, wherein the voltage-controlled oscillator (VCO) accepts the control voltage from the loop filter and changes a frequency of an output of the VCO in response thereto.

19. The phase-locked frequency synthesizer of claim 18, wherein a divider divides the output of the VCO and generates the feedback signal therefrom.

20. A method for operating a phase-locked frequency synthesizer, comprising: generating positive and negative charge pulses in N charge pumps (CPs) in response to Up and Down signals received from a phase-frequency detector; and delaying the positive and negative charge pulses from N−1 of the charge pumps in N−1 delay elements; wherein the positive and negative charge pulses from a first one of the N charge pumps and the delayed positive and negative charge pulses from the N−1 of the charge pumps are integrated in order to generate a control voltage for a voltage-controlled oscillator (VCO) in response thereto.

21. The method of claim 20, wherein the delay elements introduce a zero in a open-loop transfer function of the phase-locked frequency synthesizer, thereby obviating a need for resistors in a loop filter of the phase-locked frequency synthesizer.

22. The method of claim 21, wherein the phase-frequency detector compares a phase of a reference signal with a phase of a feedback signal and generates Up and Down signals, depending on whether the phase of the reference signal leads or lags the phase of the feedback signal.

23. The method of claim 22, wherein the voltage-controlled oscillator (VCO) accepts the control voltage from the loop filter and changes a frequency of an output of the VCO in response thereto.

24. The method of claim 23, wherein a divider divides the output of the VCO and generates the feedback signal therefrom.