ARRANGEMENT FOR ANGLE MODULATING A PHASE-LOCKED LOOP FREQUENCY SYNTHESIZER

Abstract: Various arrangements for eliminating undesired side effects of modulation deviation within a phase-locked loop frequency synthesizer are presented. Angle modulation is simultaneously applied to two or more points within the synthesizer in order to eliminate the modulation distortion and side band power during modulation otherwise presented in the phase-locked loop. In the preferred embodiment, modulation phase shifts which would otherwise be coupled to one of the signal inputs of the loop phase detector are cancelled before they reach the loop phase detector. Modulation is applied simultaneously to the loop voltage controlled oscillator (30) and a converting oscillator (52). The output signal of converting oscillator (52) is mixed with the output signal of voltage controlled oscillator (30) by a mixer (54) to cancel modulation deviation before the loop signal is coupled to divider (36) and to loop phase detector (24).

Description: ARRANGEMENT FOR ANGLE MODULATING A PHASE- LOCKED LOOP FREQUENCY SYNTHESIZER DESCRIPTION TECHNICAL FIELD This invention relates generally to phase- locked loop frequency synthesizers. Specifically, this invention relates to arrangements for angle modulating phase-locked loop synthesizers. More specifically, this invention relates to improve arrangements for eliminating from within the phase- locked loop the effects of modulation deviation resulting from the angle modulation of its voltage controlled oscillator. BACKGROUND ART Frequency synthesizers provide the capability of generating signals having a frequency that is selectable over an extensive range while maintaining a high degree of frequency stability when operating at the selected frequency. Typical frequency synthesizers are built around a standard phase-locked loop circuit as set forth in Chapter 4 of FREQUENCY SYNTHESIZERS THEORY AND DESIGN by Vadim Manassewitεch and published by John Wiley s Sons, Inc., 1976, the entire book being incorporated herein by reference. A conventional phase-locked loop frequency- synthesizer is shown in block diagram in FIGURE 1. Referring now to FIGURE 1, a reference oscillator 20 provides a signal having a fixed predetermined frequency F RET? . Reference oscillator 20 is usually a highly stable crystal oscillator. The output of reference scillator 20 is coupled to the input of a divider (or multiplier) 22 having a ratio M. The output of divider 22 is coupled to one of the two signal inputs A of a loop phase detector 24. The output of phase detector 24 is coupled to the input o a low pass filter 26. The output of low pass filter 26 is coupled to the control input 28 of a voltage controlled oscillator 30. Voltage controlled oscillator 30 includes a high power output 32 for ' coupling tα an RF amplifier stage and a low power output 34 for supplying the loop feedback signal for coupling to the second input B of loop phase detector 24. The low power output 34 of voltage controlled oscillator 30 is coupled to input B of loop phase detector-24 through a variable ratio divider 36 havin a ratio N. Ratio N is controlled by a signal applied to a divide ratio control input 38 of variable ratio divider 36. This signal is usually a multiple line digital signal responsive to a front panel control of an apparatus such as a transceiver into which the frequency synthesizer has been incorporated. Thus, the frequency of the high and low power output signal of voltage controlled oscillator 30 is controlled by ratio N as set by the user. Loop phase detector 24 generates an output signal proportional to the phase difference between the signals at its signal inputs A and B. This signa controls the frequency of voltage controlled oscillator 30 so as to maintain phase lock. In this manner the frequency F REF of reference oscillator 20 and the values of M and N determine the frequency of the signal appearing at output 32 of voltage controlled oscillator 30. A typical example of a OMPI commercially available integrated circuit phase-locked loop frequency synthesizer is the National Semiconductor MM55124, MM55126. For many phase-locked loop frequency synthesizer applications, such as in communications equipment, it is advantageous to be able to angle modulate high power output 32 of voltage controlled oscillator 30 (also the output of the frequency synthesizer) . Angle modulation is usually accomplished by coupling a modulating signal to a second control input 40 (also referred to as the modulation input) of voltage controlled oscillator 30. -By modulating voltage controlled oscillator 30, modulation deviation appears on the output signal at high power output 32. However, modulation deviation also appears at low power output 34 of the voltage controlled oscillator and is fed back to input B of loop phase detector 24 through variable ratio divider 36. Loop phase detector 24 sees this deviation as a phase difference between the signals coupled to its A and B signal inputs and generates a compensating signal at its output. This compensating signal is a demodulated replica of the modulating signal applied to input 40 of voltage controlled oscillator 30. The inherent characteristics of phase detectors such as analog, digital, double-balanced mixers, three-state, etc., cause their output compensating signal to include signal components having the frequency of reference oscillator 20 (as divided or multipled by divider 22) as well as the modulating signal. The amplitude of the reference oscillator frequency content present at the output of loop phase detector 24 is proportional to the modulation deviation of the signal at output 34 of voltage controlled oscillator 30. These frequency _ OMPI components appearing at the output of loop phase detector 24 are undesirable.. In order to eliminate the undesirable frequency content within the loop, the cut off frequency of low pass filter 26 is selected to be below the lowest modulation frequency. Thus modulation deviation products are suppressed within the loop before they are coupled to control input 28 of voltage control oscillator 30. The disadvantage associated with relying upon low pass filter 26 for suppression of undesired modulation products within the loop is that the cut off frequency of the filter must be selected to be so low that the dynamic response of the phase-locked loop is much more sluggish than is desirable. The bandwidth of the loop must be made smaller than it would have to be made in the absence of modulation. Thus, it is desirable to find alternative arrangements for preventing modulation products from being coupled through the loop to control input 28 of voltage controlled oscillator 30 without the necessity of lowering the cut off frequency of low pass filter 26. Various arrangements have been utilized for angle modulating phase-locked loops in such a manner that the output modulated signal is rendered less dependent upon loop transfer characteristics. One such arrangement is set forth in U.S. Patent 3,662,913 assigned by Don Glen Shipley to the RCA Corporation and. granted November 23, 1971. In Shipley's arrangement, modulation is applied to the VCO and is also applied to a VCXO coupled to one input of the loop phase detector. In essence, Shipley provides- a "crossover" network. At low modulatinc frecuencies O " l the signal applied to the reference input of the phase comparator is a frequency modulated signal that is modulated at the same rate as the output signal. But, for higher modulating frequencies, the signal applied to the reference input of the phase detector is a constant frequency reference source. His arrangement thereby allows the use of modulation at low as well as high frequencies, and is not limited by the cut-off point of the loop filter.. In another arrangement set forth in U.S. Patent 4,074,209, assigned by Morris Lysobey to the RCA Corporation and granted February 14, 1978, a special modulator is used to bypass the phase-locked loop to inject modulation voltage directly into the voltage controlled oscillator of the loop to broaden the bandwidth of modulation. However, it is desirable to find less complex and less expensive arrangements for angle modulating phase-locked loop synthesizers over a wide frequency range while preventing extraneous and undesirable modulation products from being coupled through the loop to control input 28 of voltage controlled oscillator 30 shown in FIGURE 1. DISCLOSURE OF THE INVENTION Therefore, the present invention is directed to arrangements for suppressing angle modulation deviation products within a phase-locked loop frequency synthesizer without relying upon a low pass loop filter for attenuating the undesired frequency components. Utilizing such alternate arrangements permits the designer of a phase-locked loop synthesizer to use a higher cut-off frequency for the loop low pass filter thereby enhancing loop bandwidth and dynamic response. Two different circuit arrangements are presented wherein modulation is applied simultaneously to two points within a phase-locked loop synthesizer so as to cancel or compensate for phase differences between the signals at inputs A and B of loop phase detector 24 resulting from the angle modulation of its voltage controlled oscillator. Thus, there is provided an arrangement for suppressing undesirable frequency components in an angle modulated phase-locked loop frequency synthesizer comprising means for applying a common modulating signal simultaneously to two different elements of the phase-locked loop in order to maintain the phase ratio of the signals developed at the phase . detector input ports constant. In the " preferred embodiment, I'IG. z ] a conver oscillator is modulated with the same modulating signal applied to input 40 of voltage controlled oscillator 30. Output signals of voltage controlled oscillator 30 and the converting oscillator are mixed to cancel modulation deviation. The output of the mixer is coupled through variable ratio divider 36 to the B signal input of loop phase detector 24. In essence, there is provided a phase-locked loop circuit comprising: - a reference oscillator for providing a reference signal having a fixed predetermined frequency; - a loop phase detector having a first input coupled to the output of said reference oscillator, a second input and an output for providing a signal related to the phase difference between the signals coupled to its first and second inputs; - a low pass filter having an input coupled to the output of said loop phase detector and having an output; - a voltage controlled oscillator having a first control input for controlling the operating frequency thereof coupled to the output of said low pass filter, a second control input for coupling to a modulating signal and an output; - a converting, oscillator having a first- input for coupling to the modulating signal and an output, for providing a signal having the same deviation as the deviation of the output signal of said voltage controlled oscillator resulting from the modulating signal; - a mixer having a first-signal input coupled to the output of said voltage controlled oscillator, a second signal input coupled to the output of the converting oscillator and an ou-tput for providing a difference signal free of modulation deviation; and - a variable divider having a ratio control input for receiving a ratio control signal, a signal input coupled to the output of said mixer, and a signal output coupled to the second input of said loop phase detector. in a second arrangement, FIG. 3, a control signal is generated that is responsive to the modulating signal applied to input 40 of voltage controlled oscillator 30. This control signal is used to control an incremental change of the ratio N of variable ratio divider so as to eliminate any modulation deviation from its output. In this second arrangement, there is provided a phase-locked loop circuit comprising: - - a reference oscillator for providing a reference signal having a fixed predetermined frequency; - a loop phase detector having a first input coupled to output ' -of said reference oscillator, a second input, and an output for providing a signal related to the phase difference between the signals a said first and second inputs; - a low pass filter coupled to the output of said loop phase detector; - a voltage controlled oscillator having its control input coupled to the output of said low pass filter, a second control input for coupling to a modulating signal and an output; - a variable ratio divider having a signal input for coupling to the output of said .voltage controlled oscillato , a signal output and a control input for incrementally controlling its ratio; and - means for generating a signal related to the modulation deviation of said voltage controlled oscillator coupled to the control input of said variable ratio divider for causing an incremental change of the ratio of said variable ratio divider thereby adjusting the phase of the signal at said second input of said loop phase detector such that said loop phase detector sees no phase difference between its first and second inputs due to the modulating signal. Thus, there is defined a method for eliminating the effects of modulation deviation within an angle modulated phase-locked loop frequency synthesizer comprising the steps of: - angle modulating the voltage control oscillator with a modulating signal; - angle mdoulating a converting oscillator so that the output of the converting oscillator has the same modualtion deviation as the output of the voltage controlled oscillator; - mixing the output of the voltage controlled oscillator and the converting oscillator so as to - eliminate all modulation deviation resulting from the modulating signal; and - coupling the output of the mixer to a signal input of the phase detector of the phase-locked loo . There is also defined a method for eliminating the modulation deviation from an angle modulated phase-locked loop frequency synthesizer of the type including a reference oscillator, a loop phase detector having a first input coupled to the output of the reference oscillator, a low pass filter coupling the output of the loop phase detector to a voltage control oscillator, and a variable ratio divider coupling the output of the voltage control oscillator to a second input of the phase detector comprising the steps of: - angle modulating the voltage control oscillator with a modulating signal; and - varying the ratio of the variable ratio divider so as to eliminate phase shifts in the signal coupled to the second input of the loop phase detector resulting from deviation in the output of the voltage controlled oscillator. BRIEF DESCRIPTION OF THE DRAWINGS Many of the attendant advantages of the present invention will be readily apparent as the invention becomes better understood by reference to the following detailed description with the appended claims, when considered in conjunction with the following drawings, wherein: FIGURE 1 is a block diagram of a conventional phase-locked loop frequency synthesizer; FIGURE 2 is a block diagram of a phase-locked loop frequency synthesizer according to the present invention; and FIGURE 3 is a block diagram of an alternate arrangement for the phase-locked loop frequency synthesizer according to the present invention; BEST MODE FOR CARRYING OUT THE INVENTION Referring now to the drawings wherein like reference, numerals designate like or corresponding parts throughout the several views, and specifically referring to FIGURE 2, there is shown a block diagram of the presently preferred exemplary embodiment of the arrangement according to the present invention for eliminating the undesirable effects of modulation within a phase-locked loop frequency synthesizer. The conventional synthesizer arrangement shown in FIGURE 1 is modified by the addition of a level control 50, a converting oscillator 52, and a mixer 54. The modulating signal which is coupled to input 40 of voltage controlled oscillator 30 is also coupled through level control " 50 to a converting oscillator 52. Level control 50, suitably a variable attenuator or- variable gain amplifier is adjusted such that OMPI converting oscillator 52 is modulated to produce an output signal deviation identical to the deviation of the signal at output 34 of voltage controlled oscillator 30. The output of converting oscillator 52 is coupled to one input of mixer 54. The other input of mixer 54 is coupled to output 34 of voltage controlled oscillator 30. The output of mixer 54 is coupled to the input of variable ratio divider 36, suitably a programmable divider. The purpose of mixer 54 is to "cancel" the modulation deviation effects of the modulating signal. The output 34 of voltage controlled oscillator 30 includes a deviation resulting from the modulating signal applied to input 40 thereof. Converting oscillator 52 is modulated so as to produce identical deviation and thus identical phase shift to the deviation of voltage controlled oscillator 30. Thus, the output of mixer 54 representing the difference between its two inputs, does not include any deviation resulting from modulating signal. Level control 50 is utilized to set the level of modulating signal coupled to the input of converting oscillator 52 so that the deviation of the output signal of the converting oscillator will be identical to the deviation at output 34 of voltage control oscillator 30. Typically, converting oscillator 52 operates at a different output frequency than the frequency of the voltage controlled oscillator 30. Mixer 54 provides at its output the difference between the frequencies of signals at its two inputs. Of course, the frequencies of reference oscillator 20, converting oscillator 52, and the divide ratios of divider 22 and the range of ratio of variable ratio divider 36 are selected to be compatible with the desired loop response and output frequency range. Utilizing this circuit arrangement, no deviation resulting from the modulating signal appears at the output, of mixer 54 and thus there is no need to filter these deviation products at low pass filter 26. ■ Therefore, the cut off frequency of low pass filter 26 can be moved to a higher frequency, giving faster loop response and wider bandwidth. There is an alternate arrangements set forth below for accomplishing this "cancellation" effect. It is intended that the scope of this invention not be limited to the presently preferred exemplary embodiment shown in FIGURE 2 or- in the alternate embodiment set forth thereafter, but rather that the scope of invention include further alternate arrangements within the spirit of the arrangements set forth herein ' and suggested thereby to one of ordinary skill in the art. Referring now to FIGURE 3 there is shown an alternate arrangement for cancelling the modulation deviation within the loop of a phase-locked loop frequency synthesizer. In this particular embodiment, the variable ratio divider 36 shown in FIGURE 2 now includes a second control input 62. Variable ratio divider 36 is capable of incrementally changing its ' divide ratio in response to a signal coupled to this second control input 62. The modulating signal applied to input- 40 of voltage controlled oscillator 30 is also coupled to the input of an analog to digital converter 64. Analog to digital converter 64 generates a signal at its output 66 related to the deviation of voltage controlled oscillator 30 induced by the- modulating signal. A variable attenuator or variable gain amplifier can be used to couple to the modulating signal to analog to digital converter 64 so that its output level can be controlled. It is contemplated that control input 62 of variable ratio divider 36 is digital and thus an analog to digital converter 64. is utilized to generate a digital signal at its output 66 related to modulation deviation. However, for use in a synthesizer having a variable ratio divider 36 with analog control, suitable modification can be made such that an analog signal coupled to control input 62 would alter the divide ratio incrementally. Output 34 of voltage control oscillator 30 is coupled to the signal input of variable ratio divider 36 and is operated upon in accordance with the divide ratio .of the variable ratio divider. The output of variable ratio divider 36 is coupled to the B input of loop phase detector 24. There is a nominal value of N for a locked condition of the phase-lock loop based upon the frequencies of reference oscillator 20, voltage controlled oscillator 30 and the divide ratios of dividers 22 and 36. If analog to digital converter 64 and its associated input and output were disconnected, modulation applied to input 40 of voltage control oscillator would cause a modulation deviation to occur at its output 34 which would be divided by variable ratio divider 36 and coupled to the B input of loop phase detector 24. However,, by utilizing a control signal generated by analog to digital converter 64 , the ratio of variable ratio divider 36 is incrementally adjusted so that the output of the divider coupled to the B input of phase detector 24 is free of deviations resulting from modulating signal. In other words, the nominal value of N for a stable locked condition in the absence of modulating signal is altered incrementally as modulation is applied. In this manner, the divide, ratio of variable ratio ■ divider 36 varies with the modulating signal, constantly adjusting the phase of the signal at its output coupled to the B input of loop phase detector 24. This phase adjustment is specifically designed to 5 cancel out phase changes resulting from the modulation - deviation appearing at output 34 of voltage, control oscillator 30 : . The two arrangements shown in FIGURES 2 and 3 relate to-schemes for cancelling or eliminating the 10 effect f modulation deviation within a phase-locked loop frequency synthesizer that is angle modulated. Other embodiments and modifications of the present invention will be apparent to those of ordinary skill in the art having the . benefit of the 15 teachings presented in the foregoing description and drawings. It is therefore to be understood that this invention is not to be unduly limited and such . modifications are intended to be included within the " scope of the appended claims. 20 INDUSTRIAL APPLICABILITY The phase-locked loop frequency synthesizer arrangements presented herein are particularly useful for communications equipment applications wherein modulation must be applied to a synthesized signal. 2.5 By using these arrangements, a designer of such equipment is given a greater degree of freedom in selecting the cut-off frequency of the loop filter of the phase-locked loop to achieve a desired bandwidth and ' dynamic response. These arrangements also have two important improvements useful for communicatio equipment, that is, low modulation distortion near loop cross over and low adjacent channel side band powe f O

Claims: WHAT IS CLAIMED IS : 1. An arrangement for suppressing undesirable frequency components in an angle modulated phase-locked loop frequency synthesizer comprising means for aplying a common modulating signal simultaneously to at least two different elements of the phase-locked loop in order to maintain the phase ratio of the signals developed at the phase detector input ports constant, said means for applying comprising means for applying a common modulating signal to both a voltage controlled oscillator and to a converting oscillator. 2. A phase-locked loop circuit comprising: - a reference oscillator for providing a reference signal having a fixed predetermined frequency; - a loop phase detector having a first input coupled to the output of said reference oscillator, a second input and an output for providing a signal related to the phase difference between the signals coupled to its first and second inputs; - a low pass filter having an input coupled to the output of said loop phase detector and having an output; - a voltage controlled oscillator having a first control input for controlling the operating frequency thereof coupled to the output of said low pass filter, a second control input for coupling to a modulating signal and an output; - a converting oscillator having a input for coupling to the modulating signal and an output for providing a signal having the same deviation as the deviation of the output signal of said voltage controlled oscillator resulting from the modulating signal; « - a mixer having a first signal input coupled to the output of said voltage controlled oscillator, a- second signal, input coupled to the outpu of the converting oscillator and an output for providing a - difference signal free of modulation deviation; and - a variable divider having a ratio control input for receiving a ratio control signal, a signal input coupled to the output of said mixer, and a signal output coupled to the second input of said loop phase detector. 3.- A phase-locked loop circuit according to claim 2 further including a divider coupling said reference oscillator to said -loop phase detector. 4. A phase-locked loop circuit according to claim 2 further including a multiplier coupling said reference oscillator to said loop phase detector. 5. A phase-locked loop circuit ' comprising: - a reference oscillator for providing a reference signal having a fixed predetermined f equency; - a loop phase detector having a first input coupled to output of said reference oscillator, a second input, and an output for providing a signal related to the phase difference between the signals at said first and second inputs; OMP - a low pass filter coupled to the output of said loop phase detector; - a voltage controlled oscillator having its control input coupled to the output of said low pass filter, a second control input for coupling to a modulating signal and an output; - a variable ratio divider having a signal input for coupling to the output of said voltage controlled oscillator, a signal output and a control input for incrementally controlling its ratio; and - means for generating a signal related to the modulation deviation of said voltage controlled oscillator coupled to the control input of said variable ratio divider for causing an incremental change of the ratio of said variable ratio divider thereby adjusting the phase of the signal at said second input of said loop phase detector such that said loop phase detector sees- no phase difference between its first and second inputs due to the modulating signal. 6. A phase-locked loop circuit according to claim 5 wherein said means for generating a signal is an analog to digital converter. 7. A phase-locked loop circuit according to either of claims 5 or 6 further including a divider coupling said reference oscillator to the first input of said loop phase detector. 8. A phase-locked loop circuit according to either of claims 5 or 6 further including a multiplier coupling said reference oscillator to the first input of said loop phase detector. 9. A phase-locked loop circuit according to any of claims 5, 6, 7 or 8 further including an attenuator coupled to the input of said analog to digital converter for controlling the level of modulation coupled to said analog to digital converter. 10. A phase-locked loop circuit according to any of claims 5, 6, 7, or 8 further including a variable gain amplifier coupled to the input of said analog to digital converter for controlling the level of modulation coupled to said analog to digital converter. 11. A method for eliminating the eff cts of modulation deviation within an angle modulated phase-locked loop frequency synthesizer comprising the steps of: - angle modulating the voltage control oscillator with a modulating signal; - angle modulating a converting oscillator so that the output of the converting oscillator has the same modulation deviation as the output of the voltage controlled oscillator; - mixing the output of the voltage controlled oscillator and the converting oscillator so as to eliminate all modulation deviation resulting from the modulating signal; and - coupling the output of the mixer to a signal input of the phase detector of the phase- locked loop. l > O P 12. A method for eliminating the modulation deviation from an angle modulated phase-locked loop frequency synthesizer of the type including a reference oscillator, a loop phase detector having a first input coupled to the output of the reference oscillator, a low pass filter coupling the output of the loop phase detector to a voltage control oscillator, and a variable ratio divider coupling the output of the voltage control oscillator to a second input of the phase detector comprising the steps of: - angle modulating the voltage control oscillator with a modulating signal; and - varying the ratio of the variable ratio divider so as to eliminate phase shifts in the signal coupled to the second input of the loop phase detector resulting from deviation in the output of the voltage controlled oscillator.

Inventors: SORENSEN S, TUGCU M
Applicants: GEN ELECTRIC, SORENSEN S, TUGCU M, GENERAL ELECTRIC CO
Application year: 1980
Main classification: H03C3
IPC codes: H03C3, H03L7