Abstract:
A device includes: a plurality of sampling phase detectors, each receiving a sampling signal and a VCO output signal and in response thereto outputting a beat signal representing a frequency and phase difference between the VCO output signal and the sampling signal; a frequency/phase detector receiving a reference signal and a combined beat signal produced by combining the beat signals, and in response thereto producing an error signal representing a phase difference between the reference signal and the combined beat signal; a loop integrator receiving the error signal and in response thereto producing the VCO control signal; a power detector detecting a power level of the combined beat signal; and at least one offset voltage generator adjusting a value of a bias voltage in response to the detected power level of the combined beat signal, and applying the adjusted bias voltage to one of the sampling phase detectors.

Description:
BACKGROUND 
       [0001]    A voltage controlled oscillator (VCO) is an important component in many communication systems and radar systems. In general, a VCO receives an input control voltage (aka “tuning voltage”) and generates an output signal whose frequency changes in response to the control voltage. Any given VCO will have a frequency or tuning range defined by the minimum and maximum frequencies that are generated in response to the range of the control voltage which it is designed to receive. VCOs have been designed to operate in a variety of different frequency bands, including particularly VHF, UHF, and/or microwave frequency bands. 
         [0002]    In many applications, the frequency accuracy and stability of a VCO operating in an “open loop” mode is unsatisfactory, because of component tolerances, production tolerances, variations in voltage levels, temperature drift, aging, etc. Accordingly, in many applications a VCO&#39;s output signal is frequency and/or phase locked to a more stable frequency source, such as a crystal oscillator, an acoustic resonator device, or the like. 
         [0003]    A VCO can be phase locked to a reference oscillator by digital or analog phase lock techniques. 
         [0004]      FIG. 1  illustrates one embodiment of a phase-locked loop (PLL). PLL  100  includes a reference oscillator  110  (e.g., a crystal oscillator), a reference frequency divider  120 , a digital phase detector  130 , a loop filter  140 , a VCO  150 , and feedback frequency divider  160 . In operation, the VCO output frequency F VCO  is divided by N to produce the feedback signal of frequency F VCO /N, while the reference frequency F REF  is divided by M to produce a comparison signal of frequency F REF /M. The frequencies F VCO /N and F REF /M are compared by the digital phase detector  130  to produce a control signal which is filtered by loop filter  140  to produce the control voltage V CONTROL  for tuning the frequency of VCO  150 . Loop filter  140  may take a variety of forms, but it typically a single pole “RC” low pass filter, or a lag-lead filter. 
         [0005]    When PLL  100  is locked, then F VCO /N=F REF /M, yielding: 
         [0000]        F   VCO =( M/N )* F   REF    (1) 
         [0006]    So given a reference frequency F REF , by properly selecting divider values M and N one can select a desired output frequency F VCO . When the divider values M and N are programmable, then digital PLL  100  may operate as a PLL frequency synthesizer. 
         [0007]    One important performance characteristic for a frequency source is the phase noise response of the VCO output signal, x(t). In the case of PLL  100 , reference oscillator  110  and VCO  150  both contribute to the phase noise of the output signal. More specifically, the phase noise of the output signal at lower offset or modulation frequencies (e.g., frequencies within the bandwidth of loop filter  140 ) is controlled primarily by the phase noise of reference oscillator  110 , and the phase noise of the output signal at higher offset or modulation frequencies (e.g., frequencies outside the bandwidth of loop filter  140 ) is controlled primarily by the phase noise of VCO  150 . 
         [0008]    PLL  100  is self-acquiring and can operate with VCO frequencies over a very wide range. The upper frequency limit of a digital PLL is set by the loop divider. Current high-speed dividers easily allow VCO frequencies up to 20 GHz. However, PLL  100  also has some disadvantages. First, the cost is sometimes uneconomical. Second, the combined noise floor of the feedback frequency divider  160 , the reference divider  120 , and the digital phase detector  130  can set a floor on the phase noise of the VCO output signal x(t). 
         [0009]      FIG. 2  illustrates another embodiment of a phase-locked loop (PLL). PLL  200  includes a reference oscillator  210  (e.g., a crystal oscillator), a sampling phase detector (SPD)  230 , a loop amplifier  240 , and a VCO  250 . 
         [0010]    In operation SPD  230  receives the VCO output signal from VCO  250  at frequency F VCO , and the reference signal from reference oscillator  210  at frequency F REF . SPD  230  acts as a non-continuous phase detector that periodically samples the phase of the VCO output signal from VCO  250 . The period of the sample pulse is 1/F REF , and the sample pulse directly samples the VCO output signal. Thus SPD  230  compares the phase of the two signals which are different in frequency, and outputs a beat signal or error signal Vbeat=Em sin Ø representing the differential phase error between the output signals of VCO  250  and reference oscillator  210 . Loop amplifier  240  amplifies the error signal and provides the amplified error signal to VCO  250  to correct the VCO output signal to be in phase with the reference signal from reference oscillator  210 . At the same time, loop amplifier  240  acts as a low pass filter and filters out the high frequency mixing products. As a result, the VCO output signal is frequency and phase locked to a harmonic of the reference frequency. 
         [0011]      FIG. 3  is a schematic diagram of one embodiment of a sampling phase detector (SPD)  300 . SPD  300  includes balancing transformer  310 , module  320 , and coupling circuit  330 . Module  320  includes a step-recovery-diode (SRD)  322  which is connected in parallel with a pair of series-connected diodes (e.g., Schottky diodes)  324  by means of two equal-dimensioned coupling capacitors  326 . In one embodiment, module  310  includes a ceramic substrate on which circuit elements  322 ,  324  and  326  are mounted in film circuit technology (thick or thin film technology). Example embodiments of module  320  include models SPD1101-111, SPD1102-111, and SPD1103-111 by SKYWORKS SOLUTIONS, INC., and the MSPD series SPDs from AEROFLEX/METELICS. 
         [0012]    In operation, SRD  322  is controlled by a sampling signal (which in  FIG. 2  corresponds to reference signal of frequency F REF ). The sampling signal arrives at SRD  322  via balancing transformer  3   10 . The terminals of SRD  322  are each connected electrically with an R/C network, and each terminal includes a parasitic inductance L not shown in  FIG. 3 . 
         [0013]    SRD  322  operates by alternately producing and consuming a charge, based on the frequency of the sampling signal. Operation of SRD  322  will be explained with respect to  FIG. 4  which illustrates the current through SRD  322  as a function of time in response to a sinusoidal sampling signal. 
         [0014]    During a first portion of each sampling cycle of the sampling signal, SRD  322  is forward biased and conducts current as a “normal” diode while it builds up an internal charge. During a subsequent second portion of each sampling cycle of the sampling signal, SRD  322  is then reverse-biased. During the reverse-bias portion of the cycle, initially SRD  322  appears to act as a low impedance and maintains conduction by consuming the internal stored charge that was accumulated during the forward-bias portion of the sampling cycle. However when the stored charge has been fully consumed, the impedance of SRD  322  very quickly returns to its normal reverse impedance, which is very high. As a result, as shown in  FIG. 4 , SRD begins to “snap-off” at snap time T SNAP , and very quickly reverts to zero conduction during a transition time T T . 
         [0015]    In SPD  300 , SRD  322  depends upon on extremely fast transition time T T  to generate pulses rich in harmonics. By storing charge during the positive-going half cycle of an input sinusoidal sampling signal and then “extracting” that charge during the negative-going half cycle, a voltage pulse with a rise time equivalent to transition time T T  is generated. Accordingly, each time SRD  322  receives a charge during the “forward-bias” portion of the sampling cycle, a comparatively rapid discharge pulse occurs during the “reverse-bias” portion of the sampling cycle which is conducted by means of the coupling capacitors  326  to the diode pair  324 . 
         [0016]    The discharge pulse from SRD  322  connects the diode pair  324  (a diode arrangement comprising more than two diodes can be used instead of diode pair  324 ). The phase state of the VCO input signal applied to diode pair  324  from a VCO input  332  is detected by the switching operation of the diode pair  324 . Coupling circuit  330  connected to diode pair  324  charges or discharges according to the phase shift between the VCO input signal and the sampling signal. A beat signal representing the above-described phase shift is provided at an IF output  334  of coupling circuit  330 . Coupling circuit  330  decouples the beat signal supplied at IF output  334 , from the VCO input signal received at VCO input  332 . 
         [0017]    Turning back to  FIG. 2 , noise contributions from the reference signal from reference oscillator  210 , and loop components such as VCO  250  itself, SPD  230 , and loop amplifier  240  can all add to the phase noise of VCO output signal. SPD  230  is typically the dominant noise contributor and thus improving its phase noise directly results in a corresponding improvement to the output noise of the VCO. 
         [0018]    What is needed, therefore, is an arrangement for phase-locking a VCO to a reference signal where the VCO output signal&#39;s phase noise response is improved. 
       SUMMARY 
       [0019]    In an example embodiment, a phase-locked loop (PLL) comprises: a voltage controlled oscillator (VCO) adapted to receive a control signal and in response thereto to output a VCO output signal having a VCO frequency; a plurality, N, of sampling phase detectors, each of the sampling detectors adapted to receive a sampling signal and the VCO output signal and in response thereto to output a beat signal representing a frequency and phase difference between the VCO output signal and the sampling signal; a phase &amp; frequency detector adapted to receive a reference signal from a reference oscillator, and adapted to receive a combined beat signal produced by combining the beat signals output by the plurality of sampling phase detectors, and in response thereto to produce an error signal representing a phase difference between the reference signal and the combined beat signal; a loop integrator adapted to receive the error signal and in response thereto to produce the control signal for the VCO; a power detector adapted to detect a power level of the combined beat signal; and at least N−1 offset voltage generators, each offset voltage generator being adapted to adjust a value of a corresponding bias voltage in response to the detected power level of the combined beat signal, and to apply the adjusted corresponding bias voltage to a corresponding one of the sampling phase detectors. 
         [0020]    In another example embodiment, a method comprises: providing a sampling signal and the VCO output signal to each of a plurality, N, of sampling phase detectors; at each of the sampling phase detectors, outputting a beat signal representing a frequency and phase difference between the VCO output signal and the sampling signal; producing a combined beat signal from the beat signals output by the plurality of sampling phase detectors; providing the combined beat signal and a reference signal from a reference oscillator to a phase &amp; frequency detector; producing at the phase &amp; frequency detector an error signal representing a phase difference between the reference signal and the combined beat signal; integrating the error signal to produce a control signal for tuning a frequency of the VCO output signal; detecting a power level of the combined beat signal; adjusting values of at least N−1 bias voltages in response to the detected power level of the combined beat signal; and applying each of the bias voltages to a corresponding one of the sampling phase detectors. 
         [0021]    In yet another embodiment, a device comprises: a plurality of sampling phase detectors, each receiving a sampling signal and a VCO output signal and in response thereto outputting a beat signal representing a frequency and phase difference between the VCO output signal and the sampling signal, each of the sampling phase detectors including a step recovery diode; a phase &amp; frequency detector receiving a reference signal and a combined beat signal produced by combining the beat signals output by the plurality of sampling phase detectors, and in response thereto producing an error signal representing a phase difference between the reference signal and the combined beat signal; a loop integrator receiving the error signal and in response thereto producing the VCO control signal; a power detector detecting a power level of the combined beat signal; and at least one offset voltage generator adjusting a value of a bias voltage in response to the detected power level of the combined beat signal, and applying the adjusted bias voltage to one of the sampling phase detectors so as to adjust a snap time of the step recovery diode in the sampling phase detector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
           [0023]      FIG. 1  illustrates one embodiment of a phase-locked loop (PLL). 
           [0024]      FIG. 2  illustrates another embodiment of a PLL. 
           [0025]      FIG. 3  is a schematic diagram of one embodiment of a sampling phase detector (SPD). 
           [0026]      FIG. 4  illustrates the current through a step recovery diode as a function of time in response to a sinusoidal sampling signal. 
           [0027]      FIG. 5  illustrates an offset PLL. 
           [0028]      FIG. 6  illustrates on embodiment of a PLL that includes two or more sampling phase detectors (SPD). 
           [0029]      FIG. 7  shows a phase noise characteristics of the output signals of two different PLLs. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparati and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparati are clearly within the scope of the present teachings. 
         [0031]    Unless otherwise noted, when a first device is said to be connected to a second device, this encompasses cases where one or more intermediate devices may be employed to connect the two devices to each other. However, when a first device is said to be directly connected to a second device, this encompasses only cases where the two devices are connected to each other without any intermediate or intervening devices. Similarly, when a signal is said to be coupled to a device, this encompasses cases where one or more intermediate devices may be employed to couple the signal to the device. However, when a signal is said to be directly coupled to a device, this encompasses only cases where the signal is directly coupled to the device without any intermediate or intervening devices. 
         [0032]      FIG. 5  illustrates an embodiment of a phase-locked loop (PLL) that utilizes a sampling phase detector. PLL  500  includes a reference oscillator  510  (e.g., a crystal oscillator), a sampling signal generator  520 , a sampling phase detector (SPD)  530 , a loop integrator  540 , a VCO  550 , and a phase &amp; frequency detector (PFD)  560 . PLL  500  is an offset PLL, and as such in general provides increased flexibility in frequency tuning compared to PLL  200  shown in  FIG. 2 . 
         [0033]    In operation of PLL  500 , a VCO output signal is supplied to a VCO input of SPD  530 , and a sampling signal is supplied to a sampling signal input of SPD  530 , and in response thereto SPD  530  provides a beat signal to phase &amp; frequency detector  560  via an IF output of SPD  530 . Phase &amp; frequency detector  560  compares the phase and frequency of the beat signal to the phase and frequency of the reference signal from reference oscillator  510 , and in response thereto generates an error signal, which is integrated by loop integrator  540  to provide the control signal for tuning the frequency of VCO  550 . When PLL  500  is locked, the frequency of VCO  550  is: 
         [0000]      Fvco= H·F   S   ±F   REF ,   (2) 
         [0000]    where F S  is the sampling frequency of the sampling signal from sampling signal generator  520 , H is the harmonic of Fs, and F REF  is the reference frequency of the reference signal from reference oscillator  510 . 
         [0034]    From equation (2) it can be seen that the output frequency of VCO  550 , F VCO , thus tracks 1:1 with changes in the frequency F REF  of reference oscillator  510 , and tracks H:1 with respect to changes in the frequency of sampling signal generator  520 . Likewise, phase noise from reference oscillator  510  is transferred with a gain of 0 dB to the output signal of VCO  550 , and phase noise from sampling signal generator  520  is transferred with a gain of 20*log(H) dB to the output signal of VCO  550 . In addition noise contributions from the input signals, loop components such as VCO  550  itself, SPD  530 , phase &amp; frequency detector  560 , and loop amplifier  540  can all add to the phase noise of the VCO output signal. SPD  530  is typically the dominant noise contributor, and thus improving its phase noise directly results in a corresponding improvement to the output noise the VCO output signal. 
         [0035]    Now consider a PLL where N identical SPD devices are each connected “in parallel” such that each SPD device receives the same sampling signal and the same VCO output signal. If the “beat signals” output by N identical SPD devices are summed together to produce a combined beat signal, and if all of the beat signals are coherent, then the signal power in the combined beat signal is increased by 20*log(N) dB with respect to the signal power of any one of the beat signals. Beneficially, however, in general the noise output by each of the N SPD devices is uncorrelated to the noise of each of the other N−1 SPD devices. Accordingly, the noise power in the combined eat signal is increased only by 10*log 10 (N) dB with respect to the noise power of any one of the beat signals. The net result is an improvement in the signal-to-noise ratio (SNR) of 10*log 10 (N) dB of the combined beat signal with respect to the SNR of any one of the individual beat signals. 
         [0036]    However when multiple SPDs are used to reduce the overall phase noise, the main difficulty is to maintain the same phase relationship among all of beat signals to be summed so that the beat signals are all coherent and the signal power in the combined beat signal is increased by 20*log 10 (N) dB as described above. There is often considerable variation in the characteristics of each SPD, and particularly the “snap time” of the SRDs in the SPDs, requiring careful matching of devices to achieve a good result. Trying to achieve good matching of SPD devices across a broad operating frequency range is even more difficult to achieve, so that a device-matching technique is generally not suitable for production designs. 
         [0037]      FIG. 6  illustrates one embodiment of a phase-locked loop (PLL)  600  that includes two or more sampling phase detectors (SPD). PLL  600  includes a reference oscillator  610  (e.g., a crystal oscillator), a sampling signal generator  620 , N sampling phase detectors (SPD)  630 - 1  . . .  630 -N, a loop integrator  640 , a VCO  650 , and a phase &amp; frequency detector (PFD)  660 . PLL  600  also includes a memory device  635 , a sampling signal level controller  645 , N beat signal amplifiers  655 - 1 ˜ 655 -N, N beat signal sampling circuits  665 - 1 ˜ 665 -N, N−1 offset voltage generators  675 - 1 ˜ 675 -(N−1), a power detector  685 , and an analog-digital-converter (ADC)  695 . 
         [0038]    In the embodiment of  FIG. 6 : sampling signal level controller  645  includes an amplifier with a level control input; beat signal sampling circuits  665 - 1 ˜ 655 -N each comprise an analog-to-digital converter (ADC); and offset bias voltage generators  675 - 1 ˜ 675 -N−1 each comprise a digital-to-analog converter (DAC). 
         [0039]    In operation, in PLL  600  SPDs  630 - 1  to  630 -N are all driven by the same sampling signal and all sample the same VCO output signal. Beneficially, sampling signal level controller  645  includes a level-controlled amplifier, and the sampling signal is amplified by the amplifier to produce enough signal drive to all of the SPDs  630 - 1 ˜ 630 -N. The sampling diodes of each SPD, in addition to producing the beat signal, produce a DC voltage proportional to the sampler drive level and this DC voltage is output at the IF output of the SPD. By monitoring the DC output voltage of one of the SPDs using beat signal sampling circuit  665 - 1 , the sampling signal drive level can be monitored and adjusted through the level control of sampling signal level controller  645 . Additional beat signal sampling circuits  665 - 2 ˜ 665 -N can be employed to monitor the DC output of all of the SPDs  630 - 1 ˜ 630 -N to ascertain the operation of the SPDs. 
         [0040]    Beat signal amplifiers  655 - 1 ˜ 655 -N are buffer amplifiers that match the high output impedance of the corresponding SPD  630 - 2 ˜ 630 -N. The beat signals output by beat signal amplifiers  655 - 1 ˜ 655 -N are summed together to produce a combined beat signal. Power detector  685  detects the power level of the combined beat signal, and the detected power level of the combined beat signal is digitized by ADC  695 . 
         [0041]    Meanwhile, the combined beat signal is applied to phase &amp; frequency detector (PFD)  660  to provide feedback for PLL  600 . The other input to PFD  660  is the reference signal from reference oscillator  610 . The output of PFD  660  is an error signal, and the error signal is applied to loop integrator  640  to produce the control voltage to tune VCO  650  to keep it locked to the sampling signal from sampling signal generator  620  and the reference signal from reference oscillator  610 . 
         [0042]    Beneficially, offset voltage generators  675 - 1 ˜ 675 -(N−1) are used to adjust the snap times of SPDs  630 - 2 ˜ 630 -N. Beneficially, offset voltage generators  675 - 1 ˜ 675 -(N−1) align the snap times of all of the step recovery diodes in all of the SPDs  630 - 2 ˜ 630 -N to insure that the beat signals from all of the SPDs add coherently to produce the combined beat signal, so that the combined beat signal exhibits the SNR improvement described above, and the phase noise of the VCO output of PLL  600  is optimized. Since one of the SPDs can be fixed while the others are all adjusted to align to it, only N−1 offset voltage generators  675 - 1 ˜ 675 -(N−1) are needed to support the N SPDs  630 - 2 ˜ 630 -N. 
         [0043]    In one embodiment, each of the offset voltage generators  675 - 1 ˜ 675 -(N−1) comprises a DAC that adjusts a bias voltage that is supplied to a corresponding one of the SPDs  630 - 2 ˜ 630 -N. In one embodiment, the output of the DAC is connected to one of the terminals  632  or  634  of the SPD  300  as shown in  FIG. 3  to adjust the bias voltage applied to SRD  322  and thereby adjust the snap time of SRD  322 . 
         [0044]    To provide a concrete example that illustrates exemplary values, one embodiment of PLL  600  of  FIG. 6  is now described in greater detail in conjunction with SPD  300  of  FIG. 3 . In this exemplary embodiment: the sampling signal may have a frequency of 100 MHz and have a power level of 18-19 dBm; the reference oscillator may have a frequency of 25 MHz; the VCO input signal may have a frequency range of 2000 MHz-4000 MHz such that the phase locked VCO can produce output frequencies from 2025 MHz to 3975 MHz in 50 MHz steps; balancing transformer  310  also provides an impedance transformation to present a 50Ω input impedance to the sampling signal; the peak-to-peak sampling signal voltage applied across SRD  322  is in a range of 3-4 volts; the bias voltage applied to terminal  632  by the DAC of an offset voltage generator  675 - i  is in a range of +0.05 Volts to −0.05 volts; and the transition time TT of SRD  322  is in a range of 30-50 picoseconds. Of course an infinite number of other embodiments may be provided having different values. These values are only provided to illustrate more fully a concrete example, and are not in any way limiting of the scope of embodiments of PLL  600 . 
         [0045]    In one embodiment, the SNR of the VCO output signal of PLL  600  can be optimized by monitoring the phase noise of the VCO output signal while adjusting offset voltage generators  675 - 1 ˜ 675 -(N−1). However, this method is not suitable for general usage due to the fact that phase noise measurements are time consuming and often requires specialized equipment not commonly available. 
         [0046]    In a beneficial arrangement, the combined beat signal power level and the corresponding phase noise response of the VCO output signal of VCO  650  are characterized at different values of the combined beat signal power level and, beneficially, also at different frequencies spanning the range of operating frequencies of PLL  600 . The phase noise profile of the VCO output signal is then correlated to the combined beat signal power level so that offset voltage generators  675 - 1 ˜ 675 -(N−1) can be controlled to adjust the bias voltages applied to SPDs  630 - 2 ˜ 630 -N for optimum phase noise using an alignment algorithm. Beneficially, in one embodiment SNR of the VCO output signal is optimized by offset voltage generators  675 - 1 ˜ 675 -(N−1) adjusting the bias voltages applied to SPDs  630 - 2 ˜ 630 -N so as to maximize the combined beat signal power level. 
         [0047]    In one embodiment, a digital word output by ADC  695  representing the combined beat signal power level is stored in memory  635 . The digitized combined beat signal power level may be read from memory  635  and employed by a processor (not shown) to execute a feedback algorithm to determine how each of the offset voltage generators  675 - 1 ˜ 675 -(N−1) should adjust the bias voltage it applies to its corresponding SPD  630 - 2 ˜ 630 -N so as to maximize the combined beat signal power level, and thereby optimize the SNR of the VCO output signal. 
         [0048]    In one embodiment, a table stored in memory  635  stores digitized bias voltage values to be applied to the N−1 sampling phase detectors by offset voltage generators  675 - 1 ˜ 675 -(N−1) corresponding to different values of the power level of the combined beat signal. 
         [0049]    In one embodiment, memory  635  may be addressed by the digital word output by ADC  695  directly to read and output stored digitized bias voltage values to be utilized by offset voltage generators  675 - 1 ˜ 675 -(N−1) to adjust the bias voltages applied to the N−1 sampling phase detectors SPD  630 - 2 ˜ 630 -N. 
         [0050]    A variety of other arrangements are possible to generate digitized bias voltage values to be utilized by offset voltage generators  675 - 1 ˜ 675 -(N−1) to adjust the bias voltages applied to the N−1 sampling phase detectors SPD  630 - 2 ˜ 630 -N in response to the detected combined beat signal power level. 
         [0051]      FIG. 7  shows exemplary phase noise characteristics of the output signals of two different embodiments of phase-locked loops (PLLs). A top curve  710  illustrates an exemplary phase noise of the beat signal from a single SPD, while bottom curve  720  illustrates an exemplary phase noise of the combined beat signal from two SPDs (N=2). As can be seen in  FIG. 7 , when N=2 SPDs are employed, the phase noise of the sampler output signal can be generally reduced by 10*log 10 (2)=3 dB, yielding a corresponding improvement in the SNR of the VCO output signal when the sampler is the dominant noise contributor. 
         [0052]    While example embodiments are disclosed herein, one of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. For example, in  FIG. 6  the output signal of VCO  650  is illustrated as being applied directly to the VCO inputs of the SPDs  630 - 1 ˜ 630 -N. However, it should be understood that the output signal from VCO  650  could have its frequency multiplied, divided, or changed by mixing with another oscillator signal, before it is applied to the VCO inputs of the SPDs  630 - 1 ˜ 630 -N. Also, the combined beat signal can be applied directly to the loop integrator  640  without needing the reference signal  610  and the phase frequency detector  660  if the VCO output is restricted to only harmonics of the sampling signal. Such variations are in accordance with the present teachings and remain within the scope of the appended claims. The invention therefore is not to be restricted except within the scope of the appended claims.