Method and apparatus for selecting optimum receiving frequency in a multichannel receiver

Abstract: A receiver and a method of setting the frequency of the reference signal in the receiver, the receiver comprising a voltage controlled crystal oscillator which provides the reference frequency for a synthesizer. The synthesizer in turn produces the local oscillator frequency signal in the receiver. Temperature fluctuations in the crystal are compensated for by microprocessor control of the oscillator. The reference frequency is accurately set by incrementaly scanning the signal channel until a signal of magnitude greater than a predetermined level is detected, whereupon the receiver locks onto said detected signal and instructs the oscillator to produce a reference frequency signal of a certain frequency according to the frequency of said detected signal.

Description: The present invention relates to a method of and apparatus for setting the reference frequency in a receiver. When a phone call is placed from a radio telephone mobile station (MS) to a base station (BS), or when the best calling channel is searched for, in known cellular telephone systems presently in use the mobile station has to search a great number of radio channels according to acceptance criteria defined in the respective system specifications. For example in the NMT-900 system the number of channels to be searched is between 1 and 1999. The channel search process in a mobile phone is described below using the NMT system as an example. Figure 1 shows the blocks required for the channel search in an NMT telephone. The received signal is shifted to the intermediate frequency f IF in a mixer and then supplied to the intermediate frequency circuit 1 separating from the carrier the alternating speech signal and the signalling, which is supplied to the data demodulator 3. Here the signalling is converted into a binary signal and supplied to the microprocessor 5 of the telephone. When the data detector 4 discovers that there is signalling on the data line from the intermediate frequency circuit 1 it examines the activating pulse DDC. The intermediate frequency circuit 1 also supplies a d.c. voltage RSSI, which is proportional to the strength of the received high frequency signal. This d.c. voltage is sampled with the analog-to-digital converter 2 each time the converter receives a control pulse ADC. All information between the microprocessor 5 and the functional blocks is transmitted on the data.bus. The channel search proceeds according to the timing diagram shown in figure 2, in which the operating times are shown as time related pulses. t 0 is marked as the moment when the synthesizers receive a command to tune onto a new channel n. In the topmost timing diagram the time T 0 shown as a pulse illustrates the time required to supply a new divisor to the synthesizer. This time is less than 1 ms. After the synthesizer settling time T1, about 10 - 15 ms, a short ADC pulse is supplied to the A/D-converter 2, whereby information about the prevailing strength of the received signal is obtained from the RSSI voltage. If the signal is below a given level, then a jump according to arrow A is made directly to a phase where the synthesizer is programmed to the next channel n+1. If the signal strength is above this given level, it is followed by an examination to see whether signalling is received from the intermediate frequency circuit 1. This is performed when an activating pulse DDC is supplied to the data detector 4. This examination takes the time T 2 or about 2 to 3 ms. If no signalling is received, the synthesizer is immediately programmed to the next channel n+1. This jump is illustrated by arrow B. If received signalling is detected, there follows a waiting time T 3 , 0 - 10 ms, and then the received signal strength RSSI is again read by supplying the activating pulse to the A/D-converter 2. If the signal is too weak, then the synthesizer receives a command to change to the next channel n+1, jump C, but if the signal is acceptable, then the reception and interpretation of the signalling received from the base station is started. The time T4 allowed for the reception is less than or equal to 277 ms. This channel is kept, if according to the interpretation it is the channel searched for, otherwise the synthesizer is programmed to the next channel n+1. This process is repeated in sequence for all channels in a given channel band, until an acceptable channel is found. There may be several search cycles before an acceptable channel is found. The channel is accepted on three conditions: first, the received signal must be above a given level. There are three levels: level A (20 dBmyV), level B (10 dBmyV), and level C (-2 dBmyV). First signals above the highest level are looked for, and if there are no such signals, a step to the next level is taken, and so on. Second, signalling must be received from the channel, and third, based on the interpretation of this signalling, the channel must be the channel looked for. In present mobile phones a voltage controlled temperature compensated crystal oscillator is used as a frequency reference with an accuracy of the order 2 ppm, or at least of the order 10 ppm. For example in the GSM system the transmitter's carrier frequency stability must be below ± 100 Hz or 0,1 ppm. Due to this frequency stability requirement the frequency must be controlled by an automatic frequency controller (AFC, Automatic Frequency Control). In multichannel radio telephones such as in modern mobile phones the mixer frequencies of the receiver and the transmitter, i.e. the first and second local oscillator frequency of the receiver and the first and second shift oscillator frequency of the transmitter, are often formed by synthesizers from the radio telephone's own reference frequency. There may be one or more synthesizers to generate these signals. In a known way the synthesizers comprise a phase locked loop having an output frequency, which is locked to a reference frequency, whereby this reference frequency is another input of the phase comparator in the loop. The reference frequency again is generated by a voltage controlled, temperature compensated crystal oscillator, since without voltage control the frequency of a crystal oscillator manufactured with today's technique is not sufficiently accurate and the frequency stability is not sufficient, whereby the frequencies generated by the synthesizers would differ too much from the allowed, so that the frequency deviation from the carrier would be too large. E.g. the NMT-900 network allows a frequency deviation of ± 0,8 ppm from the nominal frequency. Generally different automatic frequency control means (AFC) are used in order to control the frequency of the reference oscillator. The basic idea in the AFC methods is to lock the reference frequency in some way to the received carrier frequency, since the carrier frequency transmitted by a base station in cellular networks is very stable. According to the general principle the own first local oscillator frequency is compared with the received signal frequency, and the frequency of the own oscillator generating the reference frequency is controlled in accordance with the obtained frequency difference. A modern AFC method is disclosed in the patent FI-79636, Nokia Matkapuhelimet Oy and the corresponding German Patent Application DE 3738124.5. There a constant time is generated with the aid of the frequency of the crystal oscillator connected to the processor, during which time the periods of a signal received and shifted to the second intermediate frequency are counted in a counter. The counting result is in digital form, and then after a D/A-conversion the counting result is used to control the reference oscillator. This mode, as well as many other modes utilizing the carrier frequency, are substantially based on the use of a processor performing the control and the calculations. Therefore the telephone's RF-parts require for the AFC only a buffer for the intermediate frequency signal, an output line for the intermediate frequency signal, and an input control line for the temperature compensated crystal oscillator. The AFC facility itself, or the calculation and the control, is performed by the processor and its peripheral equipment in the audio/logic parts of the telephone. The AFC facility moreover requires information about the received signal strength and a memory, which stores the correct control data or the frequency information for the temperature compensated crystal oscillator. It is necessary to store the frequency information regarding those situations where the signal received from the base station is momentarily lost (the maximum waiting time in the NMT system is about 4 minutes). In the NMT the AFC facility must not offset the output frequency more than 4 kHz from the nominal frequency, regardless of the frequency received from the base station. However, the control must be able to correct a possible error of 2,5 kHz, caused by temperature drift in the crystal oscillator. The advantage of known systems of this kind is that the frequency reference limits can be set by a program, and that the last control voltage value is left in the memory when the received signal fades out. On the other hand these known systems have a disadvantage in that they use the processor capacity by utilizing its program memory capacity. On the other hand this load is periodic. Further they require counters and at least the control lines mentioned above, as well as a plurality of connections between the RF-parts and the logic parts. The UK patent application No 9203597.1 discloses an invention, with which it is possible to obviate the above mentioned disadvantages and to provide means to realize the whole AFC facility without programming, using only components (in hardware). Thus it is possible to realize all of the AFC facility in the radio frequency parts, whereby it is possible, considering the manufacture, to integrate the AFC facility on the same silicon chips as the other RF-parts, and thus to reduce the space requirements. Due to the frequency jumps and the monitoring of the neighbour base station service area both the transmitter and the receiver must tune to a new frequency in less than one millisecond. This poses considerable requirements on the setting time of the synthesizer. Temperature variations and e.g. ageing will cause frequency drift. It has been tried to compensate frequency changes due to temperature changes by using an expensive voltage controlled temperature compensated crystal oscillator. On the other hand it is not so easy to compensate for frequency changes caused by ageing. The European patent application no. 890111143.7 describes an AFC method operating according to the counter principle, with which it is possible to compensate for the frequency drift caused by ageing. Neither is the compensation possible with the method described in the publication, if the frequency provided by the reference oscillator differs largely from the correct value of the reference frequency. GB-A-2040622 relates to a radio receiver arrangement, the purpose of which is to enable proper runing of the receiver to broadcast stations in a broadcast frequency spectrum. The arrangement in GB-A-2040622 comprises a phase locked loop synthesiser 14, 21, 22, 22' and a reference oscillator 23, wherein the phase locked loop synthesiser is controlled by adjusting the dividing ratio N to enable tuning to the proper channels which contain the broadcast station signals. The reference frequency itself reviews a constant frequency. The invention is defined by the appended claims. According to a first aspect of the present invention there is provided a method of setting the reference frequency in a radio receiver in which an oscillator supplies a reference signal to a synthesizer, the synthesizer adjusting the receiver to a frequency channel in response to said reference signal, the receiver incrementally scanning said channel, determining at each increment whether or not a signal indicative of the received signal strength is greater than a predetermined threshold, and locking on to said signal, adjusting the frequency of said reference signal in accordance with the frequency of said signal, and maintaining the frequency of said reference signal substantially constant. According to a second aspect of the present invention there is provided a receiver comprising an oscillator coupled to a synthesizer for providing a reference signal to the synthesizer, the synthesizer being coupled to a mixer providing a local oscillator frequency signal to an input of the mixer, another input of the mixer receiving as an input a radio frequency signal received by the receiver, the mixer producing an intermediate frequency output signal which is coupled to detection means which both determine the magnitude of the received signal and determine whether or not an information signal is present in the received signal, the detection means being coupled to a microprocessor, when in the oscillator is a voltage controlled crystal oscillator the output of which is controlled by a digital to analog converter which is controlled by said microprocessor in order to compensate for fluctuation in the crystal temperature, temperature information being provided by a temperature sensor coupled to the microprocessor. An advantage of the present invention is that the receiver can utilize a non-temperature compensated crystal oscillator, and can accomodate even a quite large temperature variation, e.g. ± 20 ppm. When the radio telephone power is switched on and the radio telephone with the aid of the synthesizer tunes to the channel, the channel is examined in increments of a given size by changing the frequency of the synthesizer's frequency reference. In other words, the channel is covered by small increments, or the channel is scanned. Each increment includes checking whether the required d.c. voltage or RSSI level is received, which is required of the base station signal. If a required RSSI level is not found at the frequency in question, the frequency is changed to the next increment. If the required RSSI level is found, then it is checked whether a modulated data signal (FSK) is received. Thereafter the automatic frequency control facility is switched on and a locking to that signal is made. In this way a correction is obtained for the frequency reference at the prevailing temperature, providing a faster channel search. It is possible to lock onto the signal, even if an FSK signal is not found. An embodiment of the invention is described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the functional blocks of the receiver used in the channel search, in accordance with the present invention. Figure 2 shows as a timing diagram the channel search process in a known system; and Figure 3 shows the functional blocks of the receiver used when the frequency of the reference signal is stabilized. Figures 1 and 2 illustrate the channel search in an NMT telephone. They were described in detail above, and reference is made to them, as required. In the method according to the invention the searched channel is scanned in small frequency increments. At each increment a check is made with the A/D-converter 2 (figure 1), whether the required voltage or RSSI level is received. If a required RSSI level is not found at the checked frequency, the frequency of the synthesizer is changed to the next increment. Accordingly, if no required RSSI level is found on the whole channel, the next channel is selected. When the required RSSI level is found, the modem 3 checks (in transmission the modulator 3 and in reception the demodulator 3), whether an FSK signal is received from the intermediate frequency circuit. Then the automatic frequency control facility is switched on and the phone is locked to that signal. It is not necessary to receive an FSK signal in order to lock onto the received signal. Thus a correction is obtained for the frequency reference, which the crystal oscillator 8 supplies to the synthesizer 9 (figure 3) at the prevailing temperature, which provides for a faster channel search. The temperature of the crystal 8a providing the frequency to the voltage controlled crystal oscillator 8 can be measured during the use of the radio telephone with the temperature sensor 6, e.g. an NTC resistor. The AFC facility's D/A-converter 7 receiving commands from the microprocessor 5 is used to control the reference frequency, which is the output provided by the crystal oscillator 8. The temperature diagram of the crystal 8a is compensated by the processor 5 so that a mean value of crystals is stored in the memory, and with this it is possible to make a coarse compensation, so that the frequency provided by the crystal oscillator 8 already would be close to the correct value. The accuracy of this compensation depends on the tuning made in the production. Based on the temperature of the crystal 8a measured by the temperature sensor 6 and on the frequency of the received signal the processor 5 calculates a correction word used to control via the D/A-converter 7 the reference frequency supplied by the crystal oscillator 8. The processor 5 can update the temperature diagram in the memory in accordance with the temperature measurements and the frequencies of the received signals. When the radio telephone power is switched on and the channels are scanned, and when the reference frequency is set according to the invention, it is not necessary to scan the channels again in a way according to the invention, but the telephone operates reliably at the prevailing temperature, and the channel search is fast as the reference frequency is set to the correct value very accurately. The accuracy or the frequency stability obtained with the method according to the invention depends on the accuracies of the frequency measurement 1 - 4 and of the D/A-converter 7. When the phone is locked to a signal, the exact frequency is searched with the automatic frequency controller AFC, in which the control accuracy of the D/A-converter 7 determines the smallest possible frequency increment obtained with the oscillator 8. The smaller the frequency increment, the better frequency stability is obtained. The crystal oscillator 8 controlled by the D/A-converter 7 is used to control the frequency increment. Alternatively the channel scan with small frequency increments can be realized with an interpolating frequency synthesizer, which can generate frequencies between the integer factor frequencies by using the internal logic of the synthesizer for division and multiplication. Then it is not necessary to control the crystal frequency itself. The realization and the function of an interpolating synthesizer is presented in more detail in the European patent application no. 91300361.2. The method according to the present invention provides a good stability of the reference frequency in a radio telephone and the possibility to use a voltage controlled crystal oscillator, which is more economical and of a smaller size than a voltage controlled temperature compensated crystal oscillator. The method according to the invention can be used in any radio telephone system, such as in NMT, TACS/AMPS, GSM and TIA. Modifications of the present invention may be incorporated without departing from the scope of the present invention as defined by the claims.

Claims: A method of setting a reference frequency signal in a radio receiver, the radio receiver comprising an oscillator (8) for supplying the reference frequency signal to a synthesiser (9), the synthesiser (9) adjusting the radio receiver to a frequency channel in dependence on the reference frequency signal, the method comprising the steps of: a) scanning the frequency channel in frequency increments smaller than the bandwidth of the frequency channel, b) determining at each increment whether or not a signal indicative of the received signal strength is greater than a predetermined threshold, and if greater, locking on to the received signal frequency, c) adjusting the frequency of the reference frequency signal in accordance with the frequency of the received signal, and d) maintaining the frequency of the reference frequency signal substantially constant. A method as claimed in claim 1, wherein the presence of modulated data on the received signal frequency is determined after the received signal frequency is detected and before locking onto the received signal frequency. A method as claimed in claim 1 or claim 2, wherein the frequency channel is scanned by changing the reference frequency signal of the oscillator (8). A method as claimed in any preceding claim, wherein the presence of a signal indicative of the received signal strength is detected by an analogue to digital converter (2). A method as claimed in any preceding claim, wherein information on the received signal frequency is supplied to a receiver microprocessor (5), which also receives from a temperature sensor (6) information about the temperature of a crystal (8a) of the oscillator (8), and based on said information, the receiver microprocessor (5) supplies a correction word to a receiver automatic frequency control facility for controlling the reference frequency signal supplied by the oscillator (8). A method as claimed in claim 5, wherein the microprocessor (5) receives from a memory, information about a mean temperature response of the crystal (8a), according to which, the reference frequency signal of the oscillator (8) is set to an initial value before a scanning action begins. A method as claimed in claim 6, wherein the receiver microprocessor (5) updates mean temperature response data in the memory in accordance with a temperature measurement provided by the temperature sensor and in accordance with the received signal frequency. A method as claimed in any preceding claim, wherein the oscillator is a voltage controlled crystal oscillator, and temperature information is provided from a temperature sensor to a microprocessor, the microprocessor controlling a digital to analogue converter for generating a voltage which is supplied to the voltage controlled oscillator to compensate for fluctuations in the crystal temperature. A radio receiver comprising an oscillator (8) for supplying a reference frequency signal to a synthesiser (9), the synthesiser (9) adjusting the radio receiver to a frequency channel in dependence on the reference frequency signal, the radio receiver further comprising: a) means for scanning the frequency channel in frequency increments smaller than the bandwidth of the frequency channel, b) means for determining at each increment whether or not a signal indicative of the received signal strength is greater than a predetermined threshold, and if greater, locking on to the received signal frequency, c) means for adjusting the frequency of the reference frequency signal in accordance with the frequency of the received signal, and d) means for maintaining the frequency of the reference frequency signal substantially constant. A receiver as claimed in claim 9, wherein the receiver comprises an automatic frequency control facility comprising a D/A-converter (7) which is controlled by a microprocessor (5), the D/A-converter in turn controlling the reference frequency supplied by the oscillator (8) to the synthesiser (9). A receiver as claimed in claim 9 or claim 10, wherein the synthesiser (9) is an interpolating synthesiser.

Inventors: KORHONEN VEIJO, OJANPERA TERO, KORHONEN,VEIJO, OJANPERA,TERO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1993
Main classification: H03J1
IPC codes: H04B1, H03L7, H03L1, H03J1