GB2273405A

GB2273405A - A communications device and method of frequency control thereof

Info

Publication number: GB2273405A
Application number: GB9225852A
Authority: GB
Inventors: Doron Shalev; Gadi Shirazi; Yossie Bercovich; Avi Kopleman
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1992-12-10
Filing date: 1992-12-10
Publication date: 1994-06-15
Anticipated expiration: 2012-12-10
Also published as: GB2273405B; GB9225852D0; DE4342248A1

Description

a 2273405 A Communications Device and-- Method of Calibrat on therefor.

Background to the Invention.

This invention relates, in general, to a communications device having a crystal oscillator and is particularly, but not exclusively, applicable to the automatic field calibration of a crystal oscillator of a communications device operative in a trunked radio system.

Summary of the Prior Art.

It is well known that a crystal oscillator, or reference oscillator, in a frequency modulated (FM) radio suffers drift in its output frequency due to variations in the operating characteristics of its crystal. More specifically, the variations in the oscillation frequency of the crystal result from uncompensated temperature changes therein. Furthermore, natural ageing of the crystal in the crystal oscillator causes additional frequency drift. This frequency drift results in channel interference and contravention of regulatory standards, such as those set by the Federal Communications Committee (FCC). For example, the FCC stipulate that, during transmission, a frequency may only drift between a few ppm. It will be appreciated that the amount of allowable drift is dependent upon the operating frequency band, e.g. Ippm for 90OMHz, 2.5ppm for 80OMHz. Moreover, these limits are continually being tightened. Eventually, degradation in the performance of the FM radio (i.e. frequency drift) becomes such that the radio must be sent to a laboratory for the re-calibration thereof.

On the return of a FM radio to a laboratory, tuning (or 'warping') of the crystal of the crystal oscillator, i.e. the adjustment of the frequency thereof, is accomplished by several tuning methodologies. For example, the manual tuning of a trimmer capacitor of a pre-assembled temperature compensated crystal oscillator is possible. This is a time consuming method and takes considerable patience. Alternatively, there is rnanuallautomatic 1 tuning of a memory compensated crystal oscillator which involves incremental microprocessor controlled adjustment of a tuning network coupled to the crystal oscillator. In the latter case, a conventionally tuning network adjusts the capacitance across the crystal and thereby maintains the desired output frequency therefrom. More specifically, the loop output frequency of the crystal oscillator is monitored by an accurate frequency counter whilst data words are serially applied to a port of the tuning network, thereby warping the crystal oscillator's frequency by controlled adjustment of the capacitance of a varactor diode. Moreover, control of the voltage administered to the varactor diodes is regulated by a D/A converter.

In the field of Single Side Band (SSB) radio communication, it is known to use a frequency standard transmission to calibrate the radio, such as disclosed in UK patents GB 2244877 and GB 2220317 and both assigned to Motorola Israel Limited. However, the calibration addresses the problem of short term temperature sensitivity of the reference frequency output. Furthermore, these patents provide a provision for analog warping of a crystal oscillator, which cannot be used on trunked or cellular systems. In addition, use of a frequency standard as a calibration mechanism requires that the radio be left for a significant period of time, at a frequency outside an operational frequency range of the communications device, in order that the operating temperature of the crystal may stabilise and that calibration of the output frequency of the crystal oscillator may subsequently occur.

In (FM) radios, none of the above tuning methodologies address the problems associated long term variation in frequency output of the crystal oscillator arising from ageing of the crystal. It will further be appreciated that the periodic requirement of radio tuning is an inconvenience for a user of the radio. Moreover, expensive equipment, such as an automatic frequency counter, is used in order to achieve accurate tuning. Furthermore, when the oscillator frequency of a radio drifts, the user is not only breaking the law, but is also interfering with adjacent channels and thereby effecting the performance of the communications system in general.

0 1 Therefore, it can be appreciated that there is a requirement within the art to provide a FM radio having a crystal oscillator which is substantially divorced from problems associated with frequency drift and that does not require periodic re-calibration at a 5 specialised laboratory.

Summary of the Invention.

This invention addresses at least some of the deficiencies described in the prior art above. In accordance with the present invention, there is provided a communications device comprising: a receiver for receiving, demodulating and responding to a control channel signal of accurate known frequency; a crystal oscillator having an output frequency that is susceptible to frequency drift from a desired operational output frequency initially generated therefrom; calibration means for calibrating the output frequency of the crystal oscillator; and first comparison means for comparing said output frequency with the known accurate frequency of the control channel signal. The calibration means calibrates said output frequency to substantially said desired operational output frequency in response to the comparison. The first comparison means further comprises: monitoring means for monitoring signal strength of a received signal; and second comparison means, coupled to said monitoring means, for comparing the signal strength for said received signal with a predetermined threshold level for signal strength. The calibration means only calibrates said output frequency to substantially said desired operational output frequency when the signal strength of said received control channel signal falls within a predetermined range.

In the preferred embodiment, the communications device further comprises alert means, such as a speaker or a visual display, for generating an alert to inform a user of the communications device that the output frequency of the crystal oscillator has drifted outside a predetermined tolerance of the desired operational output frequency. Furthermore, the first comparison means is activated when the receiver is receiving the control channel signal. In addition, the communications device further comprising request means for allowing a user of the communications device to request that the calibration means calibrate the output frequency. The preferred embodiment is applicable to a trunked radio system.

In an alternative aspect of the present invention, there is provided a method of calibrating an output frequency of a reference oscillator of a mobile communications device, comprising the steps of: transmitting, to the communications device, a control channel signal having a known accurate frequency; receiving said control channel signal at the communications device; demodulating said control channel signal and responding thereto; comparing the known accurate frequency of the control channel signal with the output frequency of said reference oscillator; and calibrating said output frequency of said reference oscillator to substantially that of the known accurate frequency. Moreover, the step of calibrating occurs when the output frequency of the crystal oscillator has drifted outside a predetermined tolerance of the desired operational output frequency. In addition, the step of calibrating occurs when the communications device is receiving signals transmitted thereto. In this alternative aspect, the step of calibrating is preceded by the step of alerting a user of the communications device that the signal strength of the received control channel signal has fallen outside a predetermined range of received signal strengths. Furthermore, a user of the communications device may request calibration of the output frequency of the reference oscillator.

In a further aspect of this invention, there is provided a communications system comprising: a base station for controlling the operation of a communications device through the transmission of a control channel signal; and a communications device operationally responsive to said control channel signal, comprising: a crystal oscillator having an output frequency that is susceptible to frequency drift from a desired operational output frequency initially generated therefrom; and calibration means for calibrating the output frequency of the crystal oscillator. The calibration means of the communications device further comprises comparison means for comparing said output frequency with an accurate frequency source A 1 0 provided by said control channel signal, whereby the calibration means calibrates said output frequency to substantially said desired operational output frequency in response to the comparison.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings.

Fig. 1, is a conventional tuning network for a crystal oscillator 10 suitable for implementation in the present invention.

Fig. 2, is a block diagram of a trunked radio system in accordance with the present invention.

Fig. 3, is an alternative embodiment for a crystal oscillator suitable for implementation in the present invention.

Detailed Description of a Preferred Embodiment.

The present invention overcomes the at least some of deficiencies set forth above by providing a closed loop adjustment of the frequency output generated by a crystal in a crystal oscillator. A conventional prior art tuning circuit for a crystal oscillator, comprising varactor and temperature sensing diodes, is shown in Fig. 1. A temperature sensing zener diode circuit 10 is coupled between a microprocessor 11, through an A/D converter 12, and ground. It will be appreciated that the zener diode circuit 10 will typically comprise biasing and amplification circuitry associated therewith. The microprocessor 11 provides a tuning signal 13, through DIA converter 14, to a circuit node 15 located between a cathode of a varactor diode 16 and a capacitor 17. The anode of the varactor diode 16 is coupled to ground. The capacitor 17 is further coupled to a reference oscillator 18. A crystal 19 is coupled in parallel across the varactor-capacitor series combination. The temperature sensing zener diode circuit 10 generates a voltage in response to the sensed temperature. This voltage is communicated to the microprocessor 11 through the A/D converter 12. The microprocessor 11 then correlates the voltage with a suitable correction factor, and outputs the correction factor as a tuning signal 13, through D/A converter 14, to the varactor diode 16, thereby warping the crystal (reference) oscillator's frequency by controlled adjustment of the capacitance of the varactor diode. It will be appreciated that the D/A and A/D converters and the microprocessor 11 are located within the FM radio.

With reference to Fig. 2, there is shown a preferred embodiment of a trunked radio system 20 in accordance with the present invention. A trunking site, or base station, 22 administers, through use of a control channel, control of the trunked radio system 20 in a specific area and continuously transmits, through an antenna 24, an accurate high frequency signal on the control channel 26 of the trunked radio system 20. The system transmits and receives, on the control channel, commands such as channel request and channel grant commands, together with identification numbers or group identifiers, and synchronisation words. The accuracy of the high frequency signal 26 is, typically, 0.5ppm or better. Furthermore, the high frequency signal of the control channel 26 has a relatively clean spectrum with low interference It will be appreciated by one skilled in the art that the accurate high frequency signal can be either a modulated or an unmodulated signal.

A communications device 28, responsive to the trunked system 20, receives the high frequency signal of the control channel 26 through an antenna 30 thereof. The received high frequency signal acts as an input signal to a front-end 32 of the radio 28. As will be understood by one skilled in the art, the front-end 32 comprises, amongst other components, a receiver, filters and amplifiers. A filtered amplified signal 34 emanates from the front-end 32 and provides a first input to a mixer 36. The radio 28 further comprises control circuitry 38. The control circuitry typically includes a microprocessor 40, a D/A converter 42, memory 44 coupled to the microprocessor 40 and, optionally, a visual display unit (VDU) 47 responsive to the microprocessor 40. The control circuitry 38 provides a modulation signal 41 and a frequency control b signal 45 to a synthesiser 49. The frequency control signal 45 subsequently selects a particular synthesiser channel. The microprocessor 40 is coupled to a crystal oscillator 18, through the D/A converter 42, and provides a calibration signal 43 thereto. The 5 crystal oscillator 18 provides a reference frequency signal to the synthesiser 49.

The synthesiser 49 modulates the reference frequency signal with the modulation signal 41 and provides a modulated synthesiser signal 46 to a second input of the mixer 36. The mixer 36 mixes the modulated synthesiser signal 46 with the filtered amplified signal 34 to provide a modulated intermediate frequency (IF) signal 48. It will of course be appreciated that only one of the two inputs to the mixer 36 can be modulated at any one time, i.e. if the control channel is a modulated signal, the input from the syntheziser must be unmodulated, and vice versa. The intermediate frequency signal 48 provides an input to an intermediate frequency bandpass filter 50 as will be appreciated by one skilled in the art. Furthermore, it will be appreciated that the intermediate frequency bandpass filter (BPF) 50 may be a first filter in a succession of bandpass filters and, as such, references to a intermediate frequency bandpass filter 50 and subsequent references to circuitry coupled thereto should be construed accordingly. The intermediate frequency bandpass filter (BPF) 50 is coupled to a back-end 52 of the radio 28. The back-end 52 typically comprises a demodulator, received signal strength indicator (RSSI) circuitry and filters. The back-end 52 provides a received signal strength indicator (RSSI) signal 54 to the control circuitry 38 and receives a back-end control signal 56 therefrom. The back-end 52 further provides an audio signal 58 to audio apparatus, such as an external speaker 60. The audio signal 58 is coupled the microprocessor 40 through an automatic frequency control (AFQ circuit 62. An output from the AFC circuit 62 is coupled to the microprocessor 40. In addition, the audio signal 58 may provide a signal to other external circuitry, such as external monitoring or tuning circuitry, e.g. a SINAD meter. Decoding of information contained in the a received control channel signal, e.g. channel request or channel grant commands, is achieved, as will be appreciated, through control channel decoding logic 57, coupled to the microprocessor 40, contained in the radio control circuitry 38. Specifically, received control channel signals are obtained from the audio signal 58 output and are sent to the control channel decoding logic 57. The microprocessor 40 reacts to these control channel signals and controls the operation of the communications device 28 in response thereto.

In accordance with the preferred embodiment of the present invention, recalibration of the output frequency of the crystal oscillator 18 is based on the comparison between an actual frequency output of the crystal oscillator 18 and the accurate frequency of the control channel signal 26. During operation of the communications device 28 in a receive mode, the output from the AFC circuit 62 is monitored by the microprocessor 40. Moreover, as long as the output from the AFC circuit is non-zero, the microprocessor 40 adjusts the voltage drop across the crystal oscillator, in accordance with the aforementioned prior art tuning methodology and Fig. 1, by applying data words to a tuning network associated with the crystal oscillator 18, thereby warping the output of the crystal oscillator 18 to substantially an exact output frequency by controlled adjustment of the capacitance of a varactor diode 16 located in the tuning network. Therefore, the entire re-calibration of the crystal oscillator by the AFC circuit 62 takes a matter of seconds. In summary, the AFC circuit 62 is optimised in a receive mode to tune the transmission frequency of a communications device.

In an alternative embodiment of the present invention, the crystal oscillator may be replaced by a more sophisticated crystal oscillator, such as the PENDULUM', manufactured by Motorola Inc.

[PENDULUM is a trade mark of Motorola Inc.]. In such an instance, block 70 of Fig. 2 is substituted for the block of Fig. 3. Certain components within Fig. 3 are identical to those in block 70 and are coupled together in a similar manner. Therefore, for the sake of brevity, only a description of the differences will be given. With reference to Fig. 3, a digital calibration signal 80 is input directly from the microprocessor 40 to a dedicated port on the PENDULUM' crystal oscillator. The facility provided by this dedicated port removes the requirement of the D/A converter 42 of Fig. 2. The PENDULUM' crystal oscillator further comprises an E2PROM in which is stored a look-up table for the correction of changes in the crystal output frequency arising from short term temperature changes. More specifically, data stored in the look-up table represents correction factors that warp the output frequency of the PENDULUM' crystal oscillator to the correct frequency for a range of operating temperatures.

As with block 70, the PENDULUM' crystal oscillator provides a reference frequency signal to the synthesizer 49. A second output from the PENDULUM' crystal oscillator is coupled directly to the microprocessor 40 and provides a temperature measurement signal 84 thereto. The temperature signal 84 represents a measurement of the operating temperature of the crystal oscillator. It will be appreciated by one skilled in the art that a PENDULUM' crystal oscillator has an in-built temperature measurement capability. In the event that the frequency drift is greater than a predetermined level, the microprocessor warps the output frequency of the PENDULUM' crystal oscillator subject to the correction factors contained in the look-up table and through the application of the digital calibration signal 80. The digital calibration signal 80, in combination with a correction factor, is analogous to the application of a data word to the tuning network of the prior art, with warping of the output frequency of the PENDULUM' crystal oscillator being controlled therethrough.

It will further be appreciated that the present invention is also applicable to other types of temperature compensated crystal oscillator having look-up tables for the correction of changes in the crystal output frequency arising from short term temperature changes. Furthermore, it will be understood by one skilled in the art that the look-up table may be stored within the crystal oscillator, such as in the E2PROM 82 of the PENDULUM' or, alternatively, in memory, such as a code- plug, external to the crystal oscillator, e.g.

the memory 44 coupled to the microprocessor 40.

-lo- The method by which warping of the crystal oscillator is achieved is set out below. A user activates a tune function for the communications device. This activation may be response to an alert, initiated by the communications device, indicating that the 5 frequency drift has exceeded the predetermined level. Alternatively, the user may wish to periodically tune his communications device to always ensure optimum performance. The communications device 28 responds to the request to tune by issuing a message instructing the user that the tune procedure is operative and, as such, the communications device must not be moved from its present location. The response can be an audio tone generated from the external speaker 60 and/or a visual display on the VDU 47. The communications device 28 searches for a control channel signal that has an input signal level, measured by the RSSI, that typically satisfies the criterion:

-90dBm < signal level < -40dBm.

If no such control channel signal is available, the communications device 28 sends an audio and/or visual signal to the user indicating that warping is not presently possible. It will be appreciated that this criterion may vary according to the type of communications device used.

In the event that a satisfactory signal is received, the output frequency crystal oscillator is warped up or down in frequency,' according to the automatic frequency control (AFQ circuit 62, until the crystal oscillator frequency is locked to the control channel frequency. If the communications device 28 contains a temperature compensated crystal oscillator or a PENDULUM' crystal oscillator, the appropriate information, i.e. the association of the current state of the varactor diode 16, achieved through warping, corresponds to an exact output frequency, is programmed into a memory (code-plug) associated therewith, e.g. 112PROM 82. Finally, the communications device 28 sends a audio and/or visual message to the user that warping of the crystal oscillator has been completed.

1 r - 11 The communications device 28 can then resume normal reception and transmission.

With specific regard to the warping (tuning) procedure for the PENDULUM' crystal oscillator, two distinct conditions must be satisfied to ensure that an accurate frequency warp is achieved. First, the operating temperature recorded by the PENDULUM' crystal oscillator and sent to the microprocessor 40 must be in the range between ITC and 35"C. Second, the communications device must be in a steady state condition, i.e. at least ten seconds must have elapsed since power to the communications device was supplied. This ten second requirement is necessary because the frequency output of the PENDULUM' crystal oscillator is subject to fluctuation during this period.

It will be appreciated that an invention so designed and described produces the novel advantages of a communications device having a crystal oscillator capable of automatically re-calibrating its output frequency and thereby substantially remaining within defined regulatory standards for frequency drift during transmission. Furthermore, this invention eliminates the need for the communications device to be periodically returned to a specialised service centre for re-calibration of its crystal oscillator. Additionally, the preferred embodiment alerts a user to the fact that they are in contravention of regulatory standards. Moreover, since the need for re-calibration of the crystal oscillator is continually monitored, a communications system, comprising devices configured to automatically re-calibrate their crystal oscillators, would experience an increase in operational performance arising from a decrease in adjacent channel splatter (interference).

Claims

Claims / 1. A communications device (28) comprising:

a) a receiver (30, 32) for receiving, demodulating and responding to a control channel signal (26, 55) of accurate known frequency; b) a crystal oscillator (18) having an output frequency that is susceptible to frequency drift from a desired operational output frequency initially generated therefrom; c) calibration means (70) for calibrating the output frequency (46) of the crystal oscillator (18); and is d) first comparison means (36, 49, 62) for comparing said output frequency with the known accurate frequency of the control channel signal (26); wherein the calibration means (70) calibrates said output frequency to substantially said desired operational output frequency in response to the comparison.
2. A communications device (28) in accordance with claim 1, wherein said first comparison means further comprises:

signal; and monitoring means for monitoring signal strength of a received second comparison means (40), coupled to said monitoring means, for comparing the signal strength for said received signal with a predetermined threshold -level for signal strength; wherein said calibration means (70) only calibrates said output frequency (46) to substantially said desired operational output frequency when the signal strength of said received control channel signal falls within a predetermined range.
3. A communications device (28) in accordance with claim 2, further comprising alert means (47, 60) for generating an alert to inform a user of the communications device that the output frequency of the crystal oscillator has drifted outside a predetermined tolerance of the desired operational output frequency.

v 4i f
4. A communications device in accordance with any preceding claim, wherein said first comparison means is activated when the receiver is receiving said control channel signal.
5. ' A communications device in accordance with any one of claims 3 or 4, wherein the alert means is a speaker (60) and the alert is an audio alert (60).
6. A communications device in accordance with any one of claims 103, 4 or 5, wherein the alert means is a visual display (47) and the alert is a visual display displayed thereon.
7. A communications device in accordance with any preceding claim, wherein the communications device is a mobile communications device in a trunked radio system.
8. A communications device in accordance with any preceding claim, further comprising request means for allowing a user of the communications device to request that the calibration means (70) calibrate the output frequency.
9. A method of calibrating an output frequency of a reference oscillator of a mobile communications device, comprising the steps of:

a) transmitting, to the communications device, a control channel signal having a known accurate frequency; b) receiving said control channel signal at the communications device; c) demodulating said control channel signal and responding thereto; d) comparing the known accurate frequency of the control channel signal with the output frequency of said reference oscillator; and e) calibrating said output frequency of said reference oscillator 35 to substantially that of the known accurate frequency.
10. The method of calibrating the output frequency of a reference oscillator in accordance with claim 9, wherein the step of calibrating occurs when the output frequency of the crystal oscillator has drifted outside a predetermined tolerance of the desired operational 5 output frequency.
11. The method of calibrating the output frequency of a reference oscillator in accordance with claim 9 or 10, wherein the step of calibrating occurs when the communications device is receiving signals transmitted thereto.
12. The method of calibrating the output frequency of a reference oscillator in accordance with claim 9, 10 or 11, wherein the step of calibrating is preceded by the step of alerting a user of the communications device that the signal strength of the received control channel signal has fallen outside a predetermined range of received signal strengths.
13. The method of calibrating the output frequency of a reference oscillator in accordance with any one of claims 9 to 12, wherein the step of calibrating occurs when a user of the communications device requests calibration of the output frequency of the reference oscillator.
14. The method of calibrating the output frequency of a reference oscillator in accordance with any one of claims 9 to 13, wherein the step of calibrating can only proceed when said control channel signal has a signal strength of -90dBm < signal quality -40dBm f -
15 15. A communications system (20) comprising:

a) a base station (22) for controlling the operation of a communications device (28) through the transmission of a control channel signal (26); and b) a communications device (28) operationally responsive to said control channel signal (26), comprising:

i) a crystal oscillator (18) having an output frequency (46) that is susceptible to frequency drift from a desired operational output frequency initially generated therefrom; and ii) calibration means (70) for calibrating the output frequency (46) of the crystal oscillator (18); characterised in that the calibration means (70) further comprises comparison means (36, 49, 62) for comparing said output frequency (46) with an accurate frequency source provided by said control channel signal (26), whereby the calibration means (70) calibrates said output frequency (46) to substantially said desired operational output frequency in response to the comparison.
16. A communications device substantially as described herein 20 with reference to Figs. 1 to 3 of the accompanying drawings.
17. A method of calibrating an output frequency of a reference oscillator of a communications device substantially as described herein with reference to Figs. 1 to 3 of the accompanying drawings.
18. A communications system (20) substantially as described herein with reference to Figs. 1 to 3 of the accompanying drawings.