BACKGROUND OF THE INVENTION: This invention relates to a two-way radio communication system, in general, and particularly to a system in which the frequency of radios utilized in the system can be remotely adjusted.

In radio communication systems, the operating frequency of the radios must be maintained within specified limits. Radios are adjusted for correct operating frequencies at the time of manufacture. However, the aging of components can result in changes in the operating frequency of the radio. While many two- way radios are now utilizing a frequency synthesizer rather than discrete channel elements for each frequency of operation, it is necessary to maintain the proper reference frequency for the frequency synthesizer. It has conventionally been necessary to remove a radio from service in order for the radio to be tested and adjusted as required on a periodic basis. This approach is undesirable for a number of reasons. Not only is a radio unavailable for use when it is in the shop being adjusted, the

process is also expensive as it requires a trained technician to make the necessary adjustments. It is therefore desirable that the radio be adjusted without removing it from operation and without the intervention of a trained technician.

BRIEF SUMMARY OF THE INVENTION

One object of the present invention is to provide a means for a request from the remote unit to generate a frequency error command from the controller to correct the frequency of the remote unit.

Another object of the present invention is to provide a means to compensate for aging, fine tuning, etc. each time the remote unit asks for system access.

Another object of the invention is to provide a means to avoid adjacent channel interference due to radios transmitting "off frequency". In one embodiment of the invention, an oscillator is provided which includes an electronic frequency adjustment network plus a digital memory element such as an EEPROM which is used to digitally set the oscillator frequency upon receiving a correction command in response to a request to the controller.

The features of the invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a two-way radio system in accordance with the present invention.

FIG. 2 is a block diagram of a remote two-way radio of FIG. 1.

FIG. 3 is a schematic of a reference oscillator of the two-way radio of FIG. 2.

FIG. 4 is a block diagram of a control station of FIG. 1. FIG. 5 is a block diagram of a frequency calibration source for use by the central controller in a trunking system.

FIG.s 6a-b are simplified flow diagrams of the signal processing in the receiver site controller.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now by characters of reference to the drawings and first to FIG. 1 , a two-way radio system in accordance with the present invention is illustrated. A control station 10 is utilized in conjunction with a plurality of remote two-way radios 20. The control station 10 can be a base station, a repeater, or as is discussed below, can be part of a trunked radio communication system. The remote radios 20 can be any combination of portable, mobile or base stations capable of communication with the control station 10.

Referring to FIG. 2, a detailed block diagram of a remote two- way radio 20 in accordance with the present invention is illustrated. Each radio 20 includes an antenna 22 operatively coupled, via an antenna switch 24, to either a receiver section 25 or a transmitter section 26. As is known, the antenna switch 22 may be replaced by a duplexer. A demodulator 28, coupled to the output of the receiver section 25, has its output coupled to audio output circuits 30 in a conventional manner. The output of audio circuits 30 is applied to a transducer such as a speaker 32.

The output of demodulator 28 is also applied to a decoder 34 which, in turn, is coupled to a control logic 36. In the preferred embodiment, the decoder 34 and control logic 36 are preferably implemented in a microprocessor, but can be discrete circuitry. The control logic 36 communicates with a memory 40, which in a preferred embodiment includes an EEPROM that serves as the radio's code plug (non-volatile memory). As is conventional, the memory 40 would also include RAM and ROM. An output of the control logic 36 is connected to a digital to analog (D/A) converter 42 which provides a bias voltage for controlling the frequency of a reference oscillator 44. The oscillator 44 provides the reference frequency signal for a synthesizer 46. The local oscillator signals for both the receiver section 25 and transmitter section 26 are provided by synthesizer 46 in a conventional manner. A

microphone 35 is connected via audio input circuits 33 to the transmitter section 26. As is conventional, the control logic 36 also has control lines, not shown, which connect to the audio circuits 30 and 33, the synthesizer 46, the receiver section 25, the transmitter section 26, the antenna switch 24, and the microphone 35.

Referring now to FIG. 3, a schematic diagram of the reference oscillator 44 of the two-way radio 20 is illustrated. This reference oscillator 44 may commonly be recognized as a voltage controlled oscillator. An amplifier 50 and a resistor 52 are both connected in parallel across a crystal 54. A capacitor 56 is connected between one side of the crystal 54 and ground while a capacitor 58 is connected between the other side of the crystal 54 to ground. A B+ voltage is supplied via an RF choke 60 to the junction of the crystal 54 and the capacitor 58. An anode of a varactor diode 62 is also connected to this junction of the RF choke 60 and the crystal 54. A capacitor 64 is connected between the cathode of varactor 62 and ground. The cathode of varactor 62 is connected to the output of the D/A converter 42 of FIG.2.

The amplifier 50 provides the gain for the reference oscillator circuit 44. The feedback portion of the oscillator circuit consists of the resistor 52, the crystal 54, the varactor 62 and the capacitors 56, 58, and 64. The output of the D/A converter 42 along with the B+ bias voltage control the capacitance of varactor 62 in order to warp or change the frequency of the oscillator 44.

Referring to FIG. 4, a block diagram of the control station 10 in accordance with the present invention is illustrated. The control station 10 can be a base station or, as illustrated here, a repeater. An antenna 62 of the repeater 10 is coupled via a duplexer 64 to a receiver section 65 and a transmitter section 66. A master oscillator 68 provides the reference frequency signal for a receiver synthesizer 70 and a transmitter synthesizer 71 which are connected to the receiver section 65 and transmitter section 66, respectively.

A demodulator 72 is coupled to the output of the receiver section 65 and has its output coupled to audio circuits 74. The output of the demodulator 72 is also applied to a low-pass filter 76 and to a control logic 81. The direct line from the demodulator 72 to the control logic 81 provides the signal path for digital signals. An output of the filter 76 is connected to an analog to digital (A/D) converter 78 which provides a digital signal for the control logic 81. In the preferred embodiment, the control logic is implemented in a microprocessor. An output of the control logic 81 is connected to a data filter 82 which, in turn, is coupled to the input of a summer 84. Another input of the summer is coupled to the output of the audio circuits 74. The output of the summer 84 is applied to a modulator 86 and then to the transmitter section 66 for transmission in a conventional manner. In normal operation, the control logic 36 of radio 20 generates a predetermined setting for the reference oscillator 44. This setting is also stored in the memory 40. In order to provide the reference frequency for the synthesizer 46, the D/A converter 42 converts this setting into a bias voltage for controlling the frequency of the reference oscillator 44. According to the , invention, the remote radio 20 transmits a request for frequency correction of the reference oscillator 44 along with its identification address (in what could be an Inbountf Signalling Word (ISW) if the radio is in a trunking environment). The central station 10 receives the transmitted request. After waiting a short time period before measuring the frequency, the control logic 81 can check the response of a signal strength detector (not shown) to determine if the signal is strong and clean enough to measure. Additionally, other checks to ensure a quality correction process can be optionally implemented in the control logic 81.

If the received signal is good, frequency measurement can proceed. By averaging the data stream received, the central station 10 measures the frequency. The analogue digital converter 78 converts the transmitted waveform into data usable by the control logic 81. The filtered demodulate, output voltage,

which corresponds to the average frequency received, is converted into a binary value. A previously stored binary value representing a threshold voltage level (assuming no frequency error) is compared to the binary output of the A/D 78. Based on the comparison, the control logic 81 generates the number of incremental correction steps needed to correct the average frequency received from the known threshold voltage level.

The control logic 81 then determines whether the steps are too small or too large for the remote unit to correct. No correction is necessary if the correction needed is too small. On the other hand, if the frequency error is out-of-spec. (i.e. cannot be corrected), the remote radio maybe commanded to shut down and transmission is inhibited. As a further option, if the control . logic 81 is not busy, it can generate a command for transmission to the remote unit to re-try frequency correction. If the verification is unsuccessful once too often, the control logic 81 provides a disable remote radio command instead of generating the data bits for frequency correction. In addition, an alert signal either audio or visual may be generated along with the disabling command. Otherwise, a correction signal is transmitted. These command signals along with the identification (ID) of the remote radio to be corrected may be incorporated in an Outbound Signalling Word (OSW) for a trunking application

In response, the remote radio, after matching the transmitted ID with its own identification or address, adjusts its reference oscillator. The demodulator 28 recovers the frequency correction from the receiver output and feeds the information into the decoder 34 for decoding the incremental step information. Based on the step information, the control logic 36 changes the value in the memory 40 and outputs a corresponding word for the D/A converter 42 to stepwise increment or decrement the reference oscillator 44 in order to arrive at the correct frequency. As previously described, the voltage generated by the digital to analog converter 42 is placed across the varactor 62 to correct the frequency of the synthesizer 46 by a certain amount of offset.

Referring to FIG. 5, one embodiment of the radio communications system of the present invention is illustrated. A central controller 202 is shown using an external frequency calibration source 108 in a trunking system. The trunked communication system is comprised of a plurality of trunked repeaters 201 , each including a receiver R1-R5 and a transmitter T1-T5. Each repeater has circuitry to defeat local repeater operation and is connected to the external frequency calibration source 108 and the central controller 202. The central controller 202 is coupled to the plurality of trunked repeaters 201 via a receive data bus 112 and a transmit data bus 114. The frequency calibration source is coupled to each of the trunked repeaters 201 upon inputs from the control lines (CCI) and the mute lines.

The central controller 202 is further comprised of an inbound recovery board (IRB) 310, one or more receiver interface boards (RIB) 312, a receiver site controller (RSC) 314, a transmit site controller (TSC) 316, a transmit interface board (TIB) 318, and a central site controller (CSC) 320. To process the received Inbound Signalling Words (ISW)s and channel information from the frequency calibration source, the RSC 314 is coupled to the CSC 320.

The above mentioned modules are shown in U. S. Patent number 4,698,805 and more fully described in Motorola Instruction Manual 68P81066E60-O, entitled "Trunked Radio System Central Controller", which are hereby incorporated by reference. The Motorola manual is available from the Service Publications Department of Motorola, Inc., 1301 East Algonquin Road, Schaumburg, III., 60196.

In operation, a repeater calibration procedure is initiated when the central system controller 320 generates a "Frequency Calibration" command to the receiver site controller 314 to suspend normal ISW decoding of peripheral information such as identification of the remote unit 20. With this command, the control (CCI) and mute lines connect the calibration source 108 to the repeater (R1 -R5) being calibrated. The external calibration source 108 transmits a known modulated frequency which

simulates the remote units' transmission frequency (currently functioning as the control channel).

The calibration procedure is similar to that described already for the remote unit 20 of FIG. 2. As before, the analog to digital converter is used to measure the frequency error from the "zero offset level" (also called the threshold valued already stored in memory) in the receiver site controller 314. The main variation is that now the repeater itself is being measured instead of the remote unit 20. Preferably there is no correction of the repeater if the repeater falls outside the correctable limits. A different repeater would then be selected.

Referring to FIG.s 6a-b, a simplified flow diagram of the signal processing in the receiver site controller 314 is illustrated. A "frequency calibration" command generated by the central site controller 320 is received at a block 802 and passed to a timer block 804 to keep track of the total elapsed time available for calibration. To keep track of the time between retries, the routine proceeds to another counter in a block 805. Upon entry, this retry counter 805 is always initialized to zero. As part of the normal ISW decoding procedure, the middle level A/D value is computed in a block 806, after which, a block 808 suspends the rest of the normal ISW decoding procedure. An A/D "zero frequency offset" level already stored in a memory block 810 is subtracted in a block 812 from the value computed in the block 806. A decision 814 determines whether the difference from the block 812 is greater than a predetermined allowable limit. If the difference is within the allowable limit, the routine proceeds to block 816 to store the measured middle level of block 806 as the new "zero offset level" in block 810. After storage, the routine proceeds to a block 822 to remove the frequency source. Resuming normal ISW decoding in a block 818, the frequency error of the remote unit's ISW is computed as the "zero offset level" subtracted from the measured middle level of A/D value.

On the other hand, if the error difference in the block 814 is greater than the allowable limit, the routine proceeds to a decision 820 which determines whether the total allowable time

for calibration has expired. If time has expired, the routine exits to the block 822 where the frequency source will be removed and normal decoding resumed (818). The routine exits at this point to continue normal communication processing. Otherwise referring to FIG. 6b, the routine proceeds to a decision 824 which determines whether this is a second attempt at calibration. An affirmative decision from the decision block 824 transfers program control to a decision 826 to determine whether the current control channel has just been changed. If the channel is new, the routine proceeds to a block 828 to send an error message to a system manager before removing the external source (822) and resuming normal decoding (818). Otherwise, if the channel is not new, the routine proceeds to a block 830 to change the control channel to another repeater. To re-try at calibration, the routine returns to close to the beginning of the routine via a block 805'. On the other hand, a negative decision from the decision block 824 will enable the routine to proceed to a block 832 where the frequency measurement will be disabled. To determine whether the minimum time between retries has been reached, the routine proceeds to a decision 834. If not enough time has elapsed yet, the routine returns to block 832. Otherwise, the routine returns to the block 805' to retry calibration.

From the above description, it is clear that the invention involves a method of requesting frequency correction including the steps of receiving the request, generating the correction commands and decoding the commands to correct the frequency of the remote unit (or the repeater). Furthermore, this method provides safeguards to guarantee a certain confidence level that the corrected frequency is indeed correct. The foregoing describes a system and method for measuring and keeping routine IS s to an accuracy sufficient to avoid adjacent channel interference.

In operation, a remote radio (or the central system controller) requests a frequency correction. The central station measures the frequency by averaging the data stream received. If necessary, a correction signal is transmitted. In response, the

1 0 remote radio (or the repeater) adjusts its reference oscillator. However, if the frequency error is out-of-spec. (i.e. cannot be corrected), the remote radio (or the repeater) maybe commanded to shut down and transmission is inhibited. 5