Abstract:
Systems and methods automatically compensate for long-term drift of frequency standards or references used by transmitting and receiver terminals to avoid manual adjustment of the terminals to bring them back within operating tolerances and minimize communications outages caused by inability of the system to otherwise accommodate for such drift. The present invention may provide automatic band selection to maintain a modulated carrier within an operating range of an automatic frequency control or tracking capability of a communication device. This may be accomplished by selecting an appropriate reference signal used to control a center frequency of an automatic frequency control circuit (such as a PLL) or used to convert the carrier frequency of the modulated carrier to fall within the operating range of the automatic frequency control circuit and/or by causing the transmitting station to at least partially correct the frequency discrepancy.

Description:
REFERENCE TO RELATED APPLICATIONS 
   Reference is made to the following and commonly assigned U.S. patent application entitled COMMERCIAL NETWORK BASED ON POINT TO POINT RADIOS, Ser. No. 09/245,701, filed Feb. 6, 1999, and which issued as U.S. Pat. No. 6,366,584 on Apr. 2, 2002, the disclosure of which is incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention relates generally to point-to-point microwave communication systems and, more particularly, to an apparatus and method for compensating for long-term frequency drift between radio sites. 
   BACKGROUND 
   Point-to-point microwave radio systems are widely used to transmit data between and among two nodes of a communication link. The microwave radio system includes microwave receivers and transmitters at both nodes of the system for transmitting and receiving data. Typically, data is received by a modem which modulates the data onto a baseband or intermediate frequency (IF) signal which is then up-converted to microwave frequencies, amplified and transmitted. The baseband or IF carrier is often provided by a phase-locked loop circuit which is locked to an IF oscillator or frequency synthesizer. In turn, the frequency synthesizer is often provided with a reference oscillator which is also used for the up-conversion of the modulated IF signal to the microwave frequencies. 
   The microwave signal is received at a remote terminal station through an appropriate microwave radio front end circuit. The receiving radio circuitry receives a locally generated reference signal from a reference oscillator to down-convert the microwave radio signal to a lower intermediate frequency or baseband signal to be demodulated so as to recover the digital data signal encoded onto the carrier signal. The demodulating modem uses a corresponding phase-locked loop circuit synchronized to the incoming baseband signal to recover the data. As in the transmitter circuit, the receiver&#39;s phase-locked loop circuit is similarly provided with an intermediate frequency signal from an IF oscillator or frequency synthesizer which is also locked to the reference oscillator. 
   Minor signal frequency variations between sites are accommodated by the tracking capability of the receiver&#39;s phase-locked loop circuitry. The phase-locked loop (PLL) circuit generally includes a phase detector receiving a reference signal together with a sample of the output of the phase-locked loop circuit. The output of the phase detector is provided to a loop filter which, in turn, provides an error signal controlling a voltage controlled oscillator (VCO). The VCO uses the error signal to maintain an output signal having a constant frequency defined by the feedback loop. This is accomplished by sampling the output signal provided by the VCO, dividing the frequency by a programmable counter, and then comparing the frequency divided sample with the reference signal input at the phase detector to provide the error signal. 
   While the PLL can accommodate and adjust to some frequency variation of the reference signal, its operating range is still limited by certain design criteria. Thus, the PLL must receive an input signal which is within a predetermined capture or pull in range of its free running frequency prior to “locking in,” i.e., operating in a stable mode whereby the error signal provided by the phase detector and loop filter to the voltage control oscillator is within the range of the VCO&#39;s operating capability. Once locked, the frequency of the reference signal must be maintained within a hold-in range of frequencies in which the PLL will remain locked to the signal. This range is also known as the lock limit of the PLL. The limited frequency range of the PLL provides a corresponding lock range of the receiver station in which changes of the frequency of the received signal in comparison to the local reference clock frequency can be accommodated. If the difference between the local reference frequency and the received signal becomes too great, the PLL will unlock and the modem will be unable to detect the digital data signal contained in the modulated carrier signal. 
   Referring to  FIG. 6 , a radio communications terminal  100  transmits data over a microwave radio frequency link to radio communications terminal  200  which receives, detects and extracts the digital data for processing and/or retransmission to another site. 
   Communications terminal  100  receives digital data at modulator  112  of modem  110 . Modulator  112  further receives an IF carrier signal from phase-locked loop  114  and superimposes thereon the digital data signal to provide a modulated carrier signal to radio circuitry  140 . Radio circuitry  140  up-converts, i.e., translates the modulated baseband or IF signal output provided by modulator  112  of modem  110  to a microwave frequency, amplifies and transmits the signal to a receiving terminal. Phase-locked loop  114  of modem  110  receives, and is locked to, an IF frequency signal provided by frequency synthesizer  120  which, in turn, is locked to a reference frequency signal provided by reference oscillator  130 . 
   Reference oscillator  130  also provides a signal to radio circuitry  140  to be used in up-converting the modulated IF signal to a microwave frequency signal, e.g., 38 GHz. 
   Communications terminal  200  includes radio circuitry  240  amplifying, filtering and down-converting the received microwave frequency signal received from transmitter terminal  100  to provide an IF or baseband output signal to demodulator  212  of modem  210 . Demodulator  212  receives the IF or baseband signal and, using a local oscillator signal provided by phase-locked loop  214 , recovers the digital signal and provides the same as an output signal corresponding to the input signal of communications terminal  100 . Phase-locked loop  214  of modem  210  is locked to an IF signal provided by local oscillator or frequency synthesizer  220  which, in turn, is locked to an output provided by reference oscillator  230 . As in the case of the transmitting terminal, reference oscillator  230  is used both for demodulation and for down-conversion between microwave and IF frequencies. 
   When initially deployed, reference oscillator  130  of transmitting terminal  100  and reference oscillator  230  of receiving terminal  200  are adjusted to provide reference signals having the same nominal frequency or corresponding frequencies. However, the frequencies of the reference oscillators tend to slowly drift over time due to various factors, including component aging. To the extent these oscillators drift at different rates and/or in different directions over time, the nominal frequency of the microwave signal transmitted and the nominal center frequency of the receiving terminal will increasingly differ over time. Within the hold-in range capability of the phase-locked loop in the receiver, such variations can be accommodated by the receiving modem  210 . However, as the frequency drift between the terminals becomes more severe, the ability of the receiving PLL to retain a lock on the IF signal provided by radio circuitry  240  will be exceeded and the communications link will fail. It will then be necessary to manually adjust or replace the reference oscillators in the transmitting and receiver terminals  100  and  200 , respectively, to bring the system back into frequency alignment. During this time, of course, the radio communications link is inoperative. 
   Accordingly, a need exists for a communications system which is immune to or can accommodate long term frequency drift of its internal frequency standard reference. A still further need exists for a modem which can retain a locked condition over a wide range of IF input signals without requiring an automatic frequency control circuit to have a disadvantageously wide capture, acquisition, or hold range capability. A still further need exists for a communications system which does not require expensive, highly stable reference frequency standards to operate properly and avoid loss of signal lock. 
   SUMMARY OF THE INVENTION 
   The present invention provides a communication modem, terminal and/or system which automatically compensates for long-term drift of the frequency standards or references used by the transmitting and receiver terminals so as to avoid manual adjustment of the terminals to bring them back within operating tolerances and minimize communications outages caused by the inability of the system to otherwise accommodate for such drift. The invention is usable over a broad range of media including the microwave region of the electromagnetic spectrum. 
   Briefly, the invention provides for automatic band selection to maintain a modulated carrier within the operating range of an automatic frequency control or tracking capability of a communication device such as a modem or other modulation device. This may be accomplished by selecting an appropriate reference signal used to control a center frequency of an automatic frequency control circuit (such as a phase-locked loop) or used to convert the carrier frequency of the modulated carrier to fall within the operating range of the automatic frequency control circuit and/or by causing the transmitting station to at least partially correct the frequency discrepancy. 
   According to one aspect of the invention, a communications terminal includes a modem receiving a modulated carrier signal. The modem preferably includes a carrier signal tracking circuit for continuously adjusting an actual center frequency of the modem about a nominal center frequency of the modem so as to cause the actual center frequency of the modem to correspond to a center frequency of the modulated carrier signal. The modem also preferably includes a controller which is responsive to the carrier signal tracking circuit for supplying a band select signal. A band selector is responsive to the band select signal for selecting one of a plurality of ranges of signal frequencies so as to cause the center frequency of the modulated carrier signal to be within one of the ranges including the nominal center frequency of the modem. According to a feature of the invention, the band selector provides a signal to the modem which defines the nominal center frequency of the modem. Alternatively, the band selector is operative to cause a frequency of the modulated carrier signal to be converted so as to provide the modulated carrier signal having the center frequency of the carrier signal. 
   According to another feature of the invention, the communications terminal preferably includes an interface to a remote transmitter terminal providing the modulated carrier signal. The communications terminal provides the remote transmitter terminal with a control signal over the interface to cause the remote transmitter terminal to change a frequency of the modulated carrier signal. According to a related feature of the invention, the communications terminal computes a frequency change value corresponding to a frequency change required to cause the modulated carrier signal to have a center frequency within a range of frequencies included in a median one of the plurality of ranges of signal frequencies. The communications terminal preferably causes the remote transmitter terminal to change the frequency of the modulated carrier signal by approximately one-half of the frequency change value computed. Thus, each terminal changes frequency by approximately one-half of the total change required to bring the frequencies back into alignment. 
   According to another feature of the invention, the controller of the communications terminal preferably causes a frequency reference signal to be generated corresponding to a selected one of the plurality of ranges of signal frequencies including the actual center frequency of the modulated carrier signal. 
   According to another aspect of the invention, a communications terminal preferably includes a converter receiving a microwave radio signal and, in response, provides an intermediate frequency signal. A frequency synthesizer is responsive to a tuning signal for providing a local oscillator signal. The communications terminal further includes a modem receiving the intermediate frequency signal, the modem including a demodulator and a phase-locked loop circuit. The demodulator recovers a digital signal from the intermediate frequency signal, while the phase-locked loop supplies a comparison signal in response to a comparison of a characteristic of the intermediate frequency signal and the local oscillator signal. A controller is responsive to the comparison signal to provide the tuning signal. The characteristic may be a phase relationship or a frequency of the signals. In the latter case, the comparison signal is representative of a frequency difference between the transmitter carrier and the receiver&#39;s center frequency defined by its phase-locked loop. 
   According to another feature of the invention, the tuning signal preferably varies the frequency of the local oscillator signal in a plurality of discrete steps on either side of a nominal center frequency value. The phase-locked loop may be configured to lock to the intermediate frequency signal over a range of signal frequencies which are on the same order of magnitude as a frequency range between ones of the discrete steps. That is, the hold-in or capture range of frequencies for the phase-locked loop is approximately equal to or slightly greater than the step size used to adjust the operating frequency of the phase-locked loop. The steps may be equally spaced, having a frequency difference between steps within a range of 50 to 200 KHz. 
   According to another feature of the invention, the communications terminal further preferably includes an alarm corresponding to a predetermined value of the comparison signal. The controller may be responsive to the alarm for adjusting the tuning signal. 
   According to another feature of the invention, the communications terminal preferably includes a communication interface to a transmitting terminal originating the radio frequency signal. The controller negotiates with the transmitting terminal on the interface to change the frequency of the radio frequency signal by an amount such as that equal to approximately one-half of a frequency change required to bring the frequency of the radio signal within capture range of the phase-locked loop. 
   According to another feature of the invention, the controller preferably provides the tuning signal so as to deterministically affect the comparison signal, e.g., minimize the value of the offset error signal. The phase-locked loop is operable over a predetermined range of signal frequencies on either side of a nominal center frequency while the controller calculates a number of discrete steps required to minimize the offset error signal. 
   According to another feature of the invention, the controller supplies the tuning signal to correspond to the number of discrete steps calculated. Alternatively, according to another feature of the invention, the controller supplies the tuning signal to correspond to a portion of the discrete steps calculated while communicating a remainder portion of the discrete steps calculated to a transmitter terminal originating the radio frequency signal. 
   According to another feature of the invention, the communications terminal further preferably includes a signal reacquisition routine which causes the frequency synthesizer to provide local oscillator signals having a predetermined sequence of frequencies. According to another feature of the invention, a reference frequency oscillator supplies a reference signal, whereby the converter additionally is responsive to the reference signal to provide the intermediate frequency signal. The frequency synthesizer is additionally responsive to the reference signal so as to provide the local oscillator signal. 
   According to another aspect of the invention, a communications terminal preferably includes a frequency synthesizer responsive to a tuning signal to provide a local oscillator signal. A modem includes a modulator which encodes a digital data signal onto the intermediate frequency signal as received. Also preferably included as part of the modem is a phase-locked loop circuit supplying an offset error signal in response to a difference between a frequency of the intermediate frequency signal and the frequency of the local oscillator signal. An up-converter receives the modulated intermediate frequency signal and, in response, provides a radio frequency signal. The communications terminal further includes a controller which is responsive to a frequency shift command signal from a remote terminal receiving the radio frequency signal to provide the tuning signal. 
   According to a feature of the invention, the terminal also preferably includes a reference frequency oscillator supplying a reference signal. The converter is responsive to the reference signal in addition to the intermediate frequency signal to provide the radio frequency signal. The frequency synthesizer is also responsive to the reference signal to provide the local oscillator signal. 
   According to another aspect of the invention, a preferred method of operating a communications terminal includes the steps of receiving a radio frequency signal, converting the radio frequency signal to an intermediate frequency signal, synthesizing a local oscillator signal in response to a tuning signal, comparing a frequency of the intermediate frequency signal and a frequency of a local oscillator signal to supply an offset error signal, providing the tuning signal in response to the offset error signal, and recovering a digital data signal from the intermediate frequency signal. 
   According to a feature of the method, the step of providing the tuning signal preferably varies the frequency of the local oscillator signal in a plurality of discrete steps on either side of a nominal center frequency value. The method may additionally include a step of providing an alarm corresponding to a predetermined value of the offset signal and, in response, providing a tuning signal. 
   According to another feature of the invention, the preferred method further includes a step of negotiating with a transmitting terminal to change a frequency of the radio frequency signal by an amount equal to approximately one-half of a frequency change required to bring the frequency of the radio frequency signal within a predetermined capture range. 
   According to another aspect of the invention, a communications system includes transmitter and receiver terminals. The transmitter terminal preferably includes a first frequency synthesizer responsive to a first timing signal to provide a first local oscillator signal. A first modem receives the first local oscillator signal and a data signal. The first modem preferably includes a first phase-locked loop circuit supplying a first carrier signal in response to the first local oscillator signal and a modulator encoding a digital data signal onto the first intermediate frequency signal. The transmitter terminal also preferably includes an up-converter receiving the modulated carrier signal and, in response, providing a radio frequency signal. A first controller is responsive to an externally applied frequency shift command signal to provide the first tuning signal. The receiver terminal preferably includes a down-converter receiving the microwave signal to provide a second modulated carrier signal. A second frequency synthesizer is responsive to a second tuning signal to provide a second local oscillator signal used in the demodulation process. In particular, a second modem receives the second intermediate frequency signal which is applied to a demodulator for recovering the digital data signal therefrom. A second phase-locked loop circuit is preferably included as part of the second modem to supply an offset error signal in response to a difference between a frequency of the second intermediate frequency signal and the frequency of the second local oscillator signal. Finally, a second controller is preferably responsive to the second offset error signal to supply the second tuning signal and the frequency shift command signal. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a block diagram of a point-to-point microwave radio communications system according to the invention; 
       FIG. 2  is a process flow diagram of a method for performing frequency compensation IF tuning; 
       FIG. 3  is a process flow diagram of a method for performing transmitter IF tuning; 
       FIG. 4  is a process flow diagram of a method for assisting signal acquisition; 
       FIG. 5  is a block diagram of a bidirectional point-to-point microwave link according to the invention; and 
       FIG. 6  is a block diagram of a point-to-point microwave system according to the prior art. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of a point-to-point microwave radio system according to a preferred embodiment of the invention for transmitting information in the form of a data signal from a transmitter site A to a receiver site B. Typically the data signal is in the form of a digital data stream. The system preferably works in the 38 GHz range but is equally applicable to other frequencies, modes, and media of data transmission wherein receiver and transmitter facilities use distinct clock or frequency references which may drift with respect to each other, degrading or entirely inhibiting link performance. For simplicity of explanation, the embodiment of  FIG. 1  is shown as a unidirectional system for the transmission of data from transmitter site A to receiver site B, although, as will be detailed later, the invention is equally applicable to bidirectional, full duplex radio links and similar media having the aforementioned requirement to maintain a precise carrier frequency between and among transmitter and receiver stations. 
   Referring to  FIG. 1 , a microwave transmitter terminal  300  of the preferred embodiment includes a transmitter modem  310 , frequency synthesizer  320 , reference oscillator  330 , radio transmitter  340  and computer CPU (central processing unit)  350 . Transmitter modem  310  includes a modulator  312  and phase-locked loop  314 . Computer CPU  350  is connected to frequency synthesizer  320  for providing tuning commands and for receiving status information to/from frequency synthesizer  320 . A frequency or time standard in the form of reference oscillator  330  is connected to frequency synthesizer  320  and radio circuitry  340  to provide a reference signal of a predetermined frequency or clock rate. In turn, frequency synthesizer  320  is connected to phase-locked loop  314  of modem  310 . 
   Frequency synthesizer  320  is tunable, preferably in discrete steps under the control of transmitter computer CPU  350 , e.g., in 100 KHz increments on either side of a predetermined center frequency. For example, frequency synthesizer  320  may be programmable to generate frequencies over a range of plus and minus 500 KHz in 100 KHz steps, the resultant carrier signal being provided to PLL  314 . PLL  314  in combination with modulator  312  receives the inputs from frequency synthesizer  320  and a digital data input signal to provide a modulated carrier signal to transmitter radio circuitry  340 . Transmitter radio circuitry  340  up-converts and amplifies the modulated carrier signal to the desired transmission frequency range (e.g., 38 GHz). The amplified signal of this preferred embodiment is radiated by directional microwave transmitter antenna  342  toward microwave radio receiver site B. 
   Microwave receiver terminal  400  at receiver site B receives the microwave RF transmission from microwave transmitter terminal  300  at directional microwave receiver antenna  442 . Conventionally, a preamplifier and an initial down-converter may be located at or be part of directional microwave receiver antenna  442 . Such circuitry may include portions or all of microwave receiver terminal  400  which receives an output from antenna  442  at receiver radio circuitry  440 . Using a master reference frequency signal from reference oscillator  430 , radio circuitry provides an IF output to receiver modem  410 . Reference oscillator  430  also provides a master reference or master frequency signal to frequency synthesizer  420  which is responsive to tuning commands from receiver computer CPU  450  for providing a signal having a frequency of the nominal IF frequency to PLL  414  of receiver modem  410 . PLL  414  in combination with demodulator  412  receives the IF signal from radio circuitry  440  to recover a baseband or demodulated signal, separated from the IF carrier signal. The demodulated baseband signal is then provided as an output to other terminal equipment at the site (not shown) or for retransmission to a subsequent radio site, such as when deployed as part of a network as shown and described in the above referenced patent application entitled COMMERCIAL NETWORK BASED ON POINT TO POINT RADIOS PLL  414  also preferably provides a carrier frequency offset error signal to receiver computer CPU  450  indicative of the frequency difference between the incoming reference carrier signal provided by receiver radio circuitry  440  and a nominal center operating frequency of PLL as set by frequency synthesizer  420 . Receiver computer CPU  450  is responsive to the error signal generated by PLL  414  to periodically change the frequency of the signal output by frequency synthesizer  420  to minimize the error signal received from PLL  414  and, thereby, recenter its nominal operating frequency or otherwise optimize operation of PLL  414  to compensate for drift in the carrier signal relative to reference oscillator  430 . 
   PLLs  314  and  414  may be conventional phase-locked loops including, for example, a phase detector receiving an input signal such as from frequency synthesizer  420 . Generally, phase detector also receives a portion of the signal output by the PLL which has been divided by a programmable counter. When the PLL is “locked,” the sample has a frequency and phase which is the same as, or in a predetermined constant relationship with, the reference frequency signal. The phase detector provides an output signal to a loop filter corresponding to the phase difference between these two signals. The loop filter provides a control signal analogous to an error signal to a voltage controlled oscillator to provide an output signal having a desired frequency. Because the output signal is sampled and processed to have a frequency which is substantially the same as the reference signal, the PLL uses a feedback loop to lock the frequency and phase of the output signal to that of the input signal to the PLL. When used as part of a demodulator, the PLL acts as a low pass filter to recover a baseband signal from a modulated carrier signal. Similarly, the PLL provides the appropriate IF frequency in a radio transmitter modem for modulation and subsequent up-conversion and transmission by the transmitter station. 
   In addition to providing an output for controlling frequency synthesizer  420 , receiver computer CPU  450  also preferably provides control signaling to transmitter computer CPU  350  over communications link  452 . Communications link  452  may be a dedicated network maintenance channel, a discrete RF back or control channel, or an overhead channel used by and/or available to the system as are described in the above referenced patent application entitled COMMERCIAL NETWORK BASED ON POINT TO POINT RADIOS. Using this link, receiver computer CPU  450 , preferably in cooperation with compute CPU  350 , can control both frequency synthesizers  320  and  420  to bring microwave transmitter terminal  300  and microwave receiver terminal  400  back into frequency alignment and, particularly, within the capture and frequency hold ranges of phase-locked loop  414  to maintain reception and demodulation of the digital data signals transported over the link. In a preferred embodiment, receiver computer CPU  450  monitors the carrier frequency offset from PLL  414  to periodically or continuously select an appropriate offset frequency for frequency synthesizer  420 . Alternatively, CPU  450  may await making adjustments to the output from synthesizer  420  until a predetermined maintenance period or in response to certain other network conditions. For example, CPU  450  may defer frequency adjustments until a period of low network use to avoid losing the communications link and creating a network outage which may result from reprogramming synthesizer  420 . 
   Upon frequency synthesizer  420  reaching or approaching a maximum offset signal, or other predetermined threshold condition, or receiver computer CPU  450  requiring an offset amount greater than achievable by frequency synthesizer  420  alone, receiver computer CPU  450 , preferably negotiates with transmitter computer CPU  350  to accomplish a desired adjustment. As a result, microwave transmitter terminal  300  and microwave receiver terminal  400  may each change frequency by approximately one-half of the total required to minimize the magnitude of frequency offset experienced and compensated by PLL  414 . 
   Receiver modem  410  provides computer CPU  450  with the following variables in a most preferred embodiment to implement the frequency compensation method according to the invention, including:
         1. Carrier frequency offset (FO): A signed scalar quantity indicating the frequency difference between the IF signal of the transmitter and that of the receiver.   2. Transmitter IF frequency (TF): The frequency of the baseband IF signal output by PLL  314  at the transmitter site A.   3. Receiver IF frequency (RF): The frequency of the signal provided by PLL  414  to demodulator  412 .   4. Maximum transmitter IF frequency (MT): The maximum frequency offset effect from a predetermined nominal operating frequency achievable by corresponding changes to the output of frequency synthesizer  320  of microwave transmitter terminal  300 .   5. Maximum receiver IF frequency (MR): The maximum offset frequency from a PLL nominal center frequency achievable by programming frequency synthesizer  420 .   6. Carrier phase-locked indication (FLAG): A binary indicator of the locked/unlocked condition of PLL  414 .       

   A portion of the processing performed to accomplish the frequency compensation method according to a preferred embodiment of the invention is shown in  FIG. 2 . This process can be performed continuously, initiated in response to a maintenance feature, performed automatically when a predetermined high carrier frequency offset value is determined, performed at predetermined intervals, or the like. For example, receiver CPU  450  may initiate the frequency compensation procedure when the modem carrier offset exceeds 75% of the PLL lock range as indicated by a carrier frequency offset output FO from the modem. Alternatively, the procedure may be delayed until authorization is received from a network manager system so that any possible outage caused by the frequency compensation procedure, although unlikely depending on the speed of the circuitry instigating the change and that locking on the signal and, if occurring, only likely to be very brief, will be scheduled to minimize impact on network availability. Thus, the frequency compensation may be scheduled for a time period when network usage is minimal, non-critical, or backup systems are available and are operating to compensate for any system outage or interruption during the frequency compensation procedure. Upon initiation of the procedure of  FIG. 2  at entry point  500 , flow continues to determine if the IF carrier signal received by modem  410  is phase-locked by PLL  414 . If PLL  414  is unlocked, then the variable RF frequency is set to the last known value and processing continues at step  514  to initiate reacquisition of the carrier and associated IF signals. 
   If PLL  414  is operating in a locked mode, i.e., the IF frequency received from radio circuitry  440  is within the hold-in range of PLL  414  so that the output signal provided by PLL  414  is locked to the signal provided by frequency synthesizer  420 , the process continues at step  504  where the carrier frequency offset value FO is obtained from receiver modem  410 . The carrier frequency offset FO represents the frequency difference between the receiver and transmitter input and output signals or the difference between their respective IF frequencies. 
   As previously discussed, prior art modems require that the receiver modem PLL be the sole resource for automatically compensating for deviation between transmitter and receiver frequencies. In contrast the invention (as shown at process  506 ) computes a new receiver IF frequency RF as being equal to the nominal receiver center frequency plus the carrier frequency offset FO rounded or truncated to the nearest 100 KHz (i.e., the step size used by frequency synthesizer  420  to adjust its output on either side of the nominal center frequency.) For example, according to one embodiment, PLL  414  may be controlled to achieve a maximum obtainable IF synthesizer frequency offset of plus or minus 500 KHz in 100 KHz increment steps by corresponding stepped changes to the output of frequency synthesizer  420  on either side of its nominal center frequency. Thus, the receiver IF frequency required to minimize PLL  414  offset is computed at step  506  and, at step  510 , is compared with the maximum receiver IF frequency shift obtainable using frequency synthesizer  420 . If the proposed offset falls within the capability of frequency synthesizer  420  (i.e., the absolute value of the computed radio IF frequency is less than or equal to the maximum receiver IF frequency MR), the processing continues at the start acquisition assistance flow diagram of  FIG. 4 . Alternatively, if RF is greater than MR, then processing continues at step  512  which is expanded in the flow diagram of  FIG. 3 . Thus, if the required frequency change falls outside the receiver&#39;s capabilities or desired operating range, then microwave receiver terminal  400  will, according to a preferred embodiment of the invention, negotiate with microwave transmitter terminal  300  to split the required offset by varying both the transmitter and receiver IF frequencies toward each other to minimize carrier frequency offset FO. 
   Of course, other divisions of the desired offset may be employed, if desired. For example, as a preferred embodiment utilizes a maximum amount of available adjustment at a receiver location before exploiting communication resources and transmitter adjustment resources to distribute the desired offset, or alternative embodiment may operate to initially provide all or most offset at a transmitter location to lengthen the interval until a next such negotiation occurs. Moreover, such negotiations may consider information in addition to an amount of offset desired. For example, historical information, such as a direction of drift (i.e., increase or decrease in frequencies) of either or both ends of the link, a speed or rate at which drift has been experienced, or the like may be considered in order to better delegate the desired offset, such as to minimize a number of such further negotiations, etc. 
   A flow chart of a preferred embodiment of the IF tuning method coordinating adjustment of the transmitter frequency is shown in  FIG. 3 . If the IF synthesizer in the receiver has exceeded its tuning capability, the required frequency compensation may be achieved by tuning the IF synthesizer in the transmitter in conjunction with the IF synthesizer  10  in the receiver. If the required IF tuning cannot be accommodated by a proper adjustment of both the transmit and receive IF synthesizers, a fault is preferably indicated. 
   In particular, adjustment of the transmitter IF frequency is initiated at step  520 , the previously computed receiver IF frequency RF being read at step  522  and the corresponding transmitter frequency representing half of the required frequency shift is computed at step  524 . That is, the transmitter IF frequency is determined to be its nominal transmitter IF frequency TF minus one-half of the receiver IF frequency previously computed. The remainder of the frequency adjustment required is computed at step  526  by setting the receiver IF frequency equal to its nominal value plus one-half of the computed value. A check is performed at step  528  to determine if the computed transmitter IF frequency is within the maximum transmitter IF frequency for the system. If the computed transmitter IF frequency falls outside of this range so that frequency synthesizer  320  cannot accommodate the frequency adjustment, then an error message is generated at step  530 . Otherwise, processing continues with the start acquisition process at step  532  as will be more fully explained with reference to  FIG. 4 . 
     FIG. 4  is a flow chart of the acquisition assist process of a preferred embodiment of the present invention. When acquisition assistance is initiated, receiver modem  410  begins by trying for a predetermined time, such as up to 0.5 seconds, to lock to the received carrier. If signal lock is not achieved, the receive modem attempts to lock by repeatedly tuning the IF frequency synthesizer  420  further and further from the current center frequency until lock is attained, the acquisition assist process times out, or no lock is attained and further IF frequency tuning is not possible. If a lock is achieved, the link will return to service. If a lock is not achieved, the process continues to attempt a lock, but a critical alarm due to a link out of service condition will persist until lock-up is achieved. Such an alarm may operate to cause synthesizer  320  to also begin tuning to various IF frequencies in order to assist modem  410  in locking to the received carrier. 
   Referring to  FIG. 4 , the start acquisition assistance process is entered at step  540  and the variables used to step through the process are initialized at step  542 . In particular, the receiver IF frequency is set as previously computed by the frequency compensation IF tuning process of  FIG. 2  or the receiver and transmitter IF tuning process of  FIG. 3 . Loop variables k and N are set, k being a binary value indicating the direction of each progressive offset and N indicating the magnitude of each offset from the value RF previously computed. 
   Step  544  is the top of a “while” loop ending with carrier lock at decision box  548 . The while loop is entered at step  544  where the appropriate step value is added to or subtracted from the receiver IF frequency depending on the current value for k. Initially, upon loop entry, N=0 and k=0 so that there is no change to the receiver IF frequency RF. The process then preferably waits for 0.5 seconds at step  546  to provide time for PLL  414  to capture the signal and lock to it. If lock is achieved, then the while loop is exited and the process terminates at step  562 . If, however, PLL  414  is unable to lock to the IF frequency, then the process sequentially steps the value of RF in increasing magnitudes on both sides of the center frequency RF until frequency lock is attained or the combined capability of the transmitter modem and receiver modem to converge is exceeded. Thus, at step  550 , if k=1 (indicating that the current offset had already been used to increment the value of RF), then k is reset to be equal to zero at step  556  so that the value of N is used to decrement the value of RF the next time through process step  544 . Alternatively, if k is not equal to one, i.e., k=0, then processing continues out of the top of decision step  550  to set k equal to one at step  552  and to increment N at  554 . A check is then performed at decision step  558  to determine if the value of RF when incremented by the current value of N would exceed the maximum receiver IF frequency MR. If the value of RF when incremented by N falls within the capability of the receiver, i.e., is less than or equal to the maximum receiver IF frequency, then processing continues at process step  544  to attempt a lock. Otherwise, processing continues out of the top of decision step  558  to initiate transmitter IF tuning to attempt to converge the transmitter frequency to the receiver frequency as previously described in connection with  FIG. 3 . As previously explained, the transmitter IF tuning process preferably has receiver CPU  450  instruct transmitter CPU  350  to adjust the frequency of frequency synthesizer  320  to shift frequency one-half of the total required to bring transmitter modem  310  and receiver modem  410  into frequency alignment to a degree whereby PLL  414  can achieve a locked condition at step  548 . 
     FIG. 5  depicts a bidirectional embodiment of the invention wherein microwave sites  300   a  and  400   a  each include receiver and transmitter capabilities and an existing site-to-site overhead channel may be used to coordinate automatic alignment of the systems to maintain or reacquire lock by PLLs  364  and  464 . In this configuration, one of the sites may be designated as a master and the other a slave. For example, even though microwave radio facility  400   a  has both receive and transmitter capabilities, it would perform the frequency compensation process earlier detailed for the receiver terminal whereas facility  300   a  would perform those functions detailed in connection with the microwave transmitter terminal. Alternatively, each radio site might perform processing to maintain lock on its receive signal independently of the far transmitter station in each case. 
   Although several embodiments of the invention have been described in detail above, it should be clear that the present invention is capable of numerous modifications as would be apparent to one of ordinary skill in the art. Such modifications fall within the purview of the appended claims. For example, the invention is equally applicable to point-to-multipoint radio systems in general and to other media of communication wherein gross frequency alignment between a transmitter and one or more receivers is achieved using one capability such as the programmable frequency synthesizer described in the embodiments above, and real time fine alignment is achieved by alternate means such as the PLLs according to those embodiments. Thus, the invention is applicable to all such tunable communications devices including, for example, an automatic frequency control system of limited range augmented by a gross frequency adjustment facility such that frequency alignment between terminals is maintained and, as necessary, reachieved. Moreover, there is no limitation of the present invention to its use with wireless or even radio frequency communications. For example, the concepts of the present invention are applicable to wired communication systems utilizing modulated signals. Additionally, electro magnetic modulation of signals is not required for the advantages of the present invention to be realized, as these concepts are applicable to other modulation techniques, such as light or optic transmission of data. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.