SYNTHESIZED SELECTIVE CALL RECEIVER HAVING VARIABLE CHARACTERISTICS

Abstract: A paging receiver has a synthesizer for governing the receive frequency. The paging receiver further has characteristics which are varied in response to the receive frequency. These characteristics include varying the bandwidth (52) of a loop filter (26) within a phase lock loop (16) within the synthesizer as well as varying the time in which a detector circuit used to extract a DC level from a recovered audio signal is disabled. Furthermore, the bandwidth of the loop filter is varied in response to switching from a first receive frequency to a second receive frequency in order to provide for either a uniform frequency lock time (54) or for a rapid frequency lock time. Furthermore, the time in which the detector circuit is disabled is correspondingly changed.

Description: Synth.SJZS-, Selec ive Call Receiver Having Variable Characteristics Background of the Invention This invention relates in general to a selective call receiver having a frequency synthesizer for changing receive frequencies in order to receive a paging signal. This invention more specifically relates to varying characteristics of the receiver in response to the receive frequency. A paging receiver which receives a paging signal on one of a plurality of receive frequency channels, needs to switch between receive frequency channels. However, the lock time of a frequency synthesizer having a phase lock loop (PLL) may vary depending upon the differences in frequency between two channels. It is desirable to provide a constant lock time when switching channels regardless of the differences in frequency. Furthermore, it is desirable to lock to another channel very rapidly in one instance while the lock time may not be critical in another instance. Thus it is desirable to vary the characteristics of a selective call receiver in order to provide for a desired lock time. The loop filter of the PLL also effects the noise level of the synthesized frequency used within a superheterodyne receiver, the noise level effects the selectivity of the receiver. Different applications of paging receivers or different channels upon which paging receivers receive paging signals may require differing selectivity performance. Thus it is desirable to adjust the loop bandwidth of a PLL in order to provide a desired level of selectivity performance. During the reception process, a demodulated signal has a DC component and an AC component upon which the paging signal is included. The DC component is removed in order to digitize the AC component. A complex circuit is used to determine and extract the DC component, such a circuit is shown in U.S. patent 4,631,737 to Davis et al. It is desirable to disable this circuit while the PLL is acquiring lock. Summary of the Invention It is therefore an object of the invention to provide for the aforementioned desirable ends. In accordance with the present invention, a selective call receiver comprises a receiving means for receiving and demodulating a selective call signal, said receiving means including a phase lock loop (PLL) for programmably generating an injection frequency used to select a receive frequency for receiving the selective call signal, said receiving means further having a variable characteristic, and a decoding means for processing the demodulated selective call signal and for programming the frequency of the PLL and for varying the characteristic of the receiving means in response to the programmed frequency. Brief Description of the Drawings Figure 1 shows a paging receiver operating in accordance with the present invention. Figure 2 shows a flow chart of a decoder operating in accordance with the present invention. Description of a Preferred Embodiment. Figure 1 shows a paging receiver operating in accordance with the present invention. RF modulated paging signals are received on antenna 10, amplified by amplifier 12 mixed by mixer 14 with an injection frequency from synthesizer 16 in order to create an intermediate frequency signal. Demodulator 18 generates an audio signal from the intermediate frequency signal. The audio signal is digitized by the peak and valley detector 20 which generates a digital signal from the audio signal. Peak and valley detector 20 is described in U.S. patent 4,631,737 to Davis et al. which is hereby- incorporated by reference. Synthesizer 16 comprises a phase lock loop (PLL) and is driven by a reference frequency from oscillator 22. The reference frequency is compared by phase comparator 24. The output of the phase comparator is filtered by a programmable loop filter 26 which is described in U.S. patent application 07/345,809 to Herold et al. and assigned to the assignee of the present invention which provides for a variable filter characteristic. .The output of the loop filter is coupled to voltage controlled oscillator 28 which generates the injection frequency. The injection frequency is further coupled to programmable divider 30 the output of which is coupled to phase comparator 24, thus closing the loop of the. PLL. Decoder 30 receives and processes the digitized signal from peak and valley detector 20 and also enables and disables the peak and valley detector. Programmable filter 26 and programmable divider 30 are also programmed by decoder 40. Decoder 40 includes a microcomputer for performing it's functions. Use of a microcomputer or control functions of a paging receiver is known in the art. A preferred microcomputer is the MC68HC05C4 microcomputer manufactured by Motorola- Inc. Some elements of the invention described herein are implemented in software operating within the microcomputer. Codeplug 50 includes several variables used to vary the characteristics of the receiver in response to the receive frequency. A first portion 52 contains frequency information and corresponding loop bandwidth information. A second portion contains the change in frequency informatiαn, ΔF and corresponding bandwidth and lock time information. For example, if substantial selectivity was required at a first" frequency of 131.000 MHz, nominal selectivity was required at a second frequency of 132.000 MHz and selectivity was not critical at 133.000 MHz, table 52 for the paging receiver of this example could read: Fl = 131.000 MHz B 1 = 50 Hz F2 = 132.000 MHz BW2 = 75 Hz F3 = 133.000 MHz B 3 = 120 Hz The frequencies above indicate receive frequencies, alternately the injection frequency, which differs from the receive frequency by the intermediate frequency, could be used, while maintaining the same bandwidths. Table 52 governs the loop bandwidth while the PLL is locked. Table 54 governs the loop bandwidth during lock acquisition and table 54 further governs the lock time provided by the decoder. A relationship exists between the loop bandwidth and the lock time. The relationship is characterized and stored in the codeplug for use by the decoder. The following example uses the above frequencies for table 54. The first three rows show progression from Fl - F3 and back to Fl, much like a scanning receiver would operate while scanning channels Fl, F2, and F3. The values provide for maintaining a constant lock time independent of the distance in MHz of the frequencies. In order to provide for switching from Fl to F2 or from F2 to F3 with a 10 mS lock time, a 330 Hz bandwidth is used for the loop filter. However, when switching from F3 to Fl, a frequency jump twice as far as the prior two, the bandwidth is widened to 660 Hz in order to maintain the constant 10 mS lock time. The fourth and fifth rows show changing the bandwidth in order to provide for faster or slower lock times respectively. These rows may be used in response to commands within the paging signal causing the pager to switch from Fl to F2. The final three rows show starting from a starting frequency FS and bandwidth required to lock on to Fl through F3 respectively in 10 mS. The starting frequency, FS is indicative of the free running frequency of the VCO when it is initially powered on. Thus these rows are used when power is initially applied to the receiver which corresponds to the pager being brought out of the battery saving state. Figure 2 shows a flowchart of a decoder operating in accordance with the present invention. The flowchart begins in step 70 with the receiver in the battery save state, thereby disabled. Step 70 is executed until it is time to turn the receiver on. Then in step 72 the receiver is activated or turned on, thereby exiting the battery saving state. Then in step 74 the peak valley detector is reset. Step 76 determines the receiver frequency which may be made by one of several known algorithms. One such algorithm is shown in U.S. patent 4,644,347 Feb. 17, 1987 to Lucas et al. which is hereby incorporated by reference. Step 78 determines the loop bandwidth with which to program loop filter 30. For example say from step 76, the determined frequency was F3, since the VCO was just previously enabled, it is free running thus the decoder selects the bandwidth corresponding to FS-F3 from the table above (570 Hz) . The divider 30 is then programmed in order to receive frequency F3. Then a delay is executed by the software as determined by the FS-F3 lock time entry in the codeplug (lOmS from the above table) . Then in step 84, the bandwidth of the loop filter 26 is changed to correspond to the desired bandwidth of frequency F3 (120 Hz from the table above) . After the delay of step 82, the peak and valley detector 20 is enabled. Thereafter the receiver detects valid digitized information and . in step 88 the decoder processes the detected signal. Step 90 checks if the frequency is to be changed, which could be in response to a signal received by the receiver. If the frequency is not to be changed, step 92 checks if it is time to turn off the receiver, a decision which may be made after completing reception of a paging signal. If it is not time to turn the receiver off, step 80 is returned ' to and signal processing continues. If it is time to turn the receiver off, step 94 deactivates the receiver thereby entering the battery save mode and step 70 is executed. If in step 90, it was decided it was time to change to a new receive frequency, step 74 would be executed. The peak and valley detector would be reset and the next frequency would be determined. If for example, the pager was in the scanning mode, currently receiving F3, and Fl was the next frequency to be scanned, information from the F3-F1 row of the above table would be used in steps 78-86. If for example, the receiver was receiving Fl and detected a paging signal directing the pager to rapidly switch to F2, the receiver would use the 5mS lock time setting from the F1-F2 row of the table above in steps 78-86. Note that the programmable loop filter draws higher current when operating in the wider bandwidth, so it is generally desirable to use narrower bandwidths when possible. Furthermore, narrower bandwidths provides for reduced phase noise which may be desirable. Although the invention has been described by way of example, it should be clear that numerous alternate applications of the principles described herein may be implemented while remaining within the scope of the invention which is defined by the following claims. What is claimed is:

Claims: CLAIMS: 1. A selective call receiver comprising: receiving means for receiving and demodulating a selective call signal, said receiving means including a phase lock loop (PLL) for programmably generating an injection frequency used to select a receive frequency for receiving the selective call signal, said receiving means further liaving a variable characteristic; and decoding means for processing the demodulated selective call signal and for programming the frequency of the PLL and for varying the characteristic of the receiving means in response to the programmed frequency. 2. The selective call receiver of claim 1 wherein the PLL has a loop filter having the variable characteristic of a multiplicity of selectable bandwidths and further comprising: memory means for storing at least one predetermined injection frequency and at least one bandwidth corresponding to the predetermined injection frequency, wherein said dec'Qding means programs the injection frequency and selects the bandwidth of the loop filter in response to said memory means. 3. The selective call receiver of claim 1 wherein the PLL has a loop filter having the variable characteristic of a multiplicity of selectable bandwidths and wherein a bandwidth is selected by said decoding means in response to the programmed injection frequency. 4. The selective call receiver of claim 1 wherein the PLL has the variable characteristic of a loop filter having a multiplicity of selectable bandwidths and the selective call receiver further capable of switching from a first receive frequercy to a second receive frequency, the selective call receiver further comprising: means for determining the difference between a first injection frequency corresponding to the first receive frequency and a second injection frequency corresponding to the second receive frequency, the determining means generating a difference signal in response thereof; and memory means for storing at least one delta bandwidth signal corresponding to the difference signal, wherein said decoding means selects the bandwidth of the loop filter corresponding to the delta bandwidth signal and switches the PLL from the first to the second injection frequencies . 5. The selective call receiver of claim 4 further comprising: delay means for generating a completion signal in response to the completion of a programmed delay, wherein said memory means further stores a lock time delay signal associated with the difference signal, and said decoding means having hitherto programmed the first injection frequency and selected a first bandwidth, selects the bandwidth of the PLL to correspond to the delta bandwidth and programs the delay means in response to the lock time delay signal and further programs the second injection frequency, and further selects a second PLL bandwidth in response to the completion signal. 6. The selective call receiver of claim 1 wherein said receiving means produces a demodulated signal having DC and AC components and further wherein a variable characteristic of said receiving means includes: detecting means for removing the DC component; and converting means for converting the AC component into a digital signal including the selective call signal for processing by the decoding means, the removing means being enabled in order to determine the DC component of the demodulated signal, and the selective call receiver further comprises : delay means for generating a completion signal in response to the completion of a programmed delay, wherein said decoding means programs said delay means in response to and in coincidence with the programming of the injection frequency and enables the detecting means in response to the completion signal wherein the programmable delay is selected in response to the injection frequency. 7. The selective call receiver of claim 6 further comprising: memory means for storing at least one lock time delay signal corresponding to the predetermined injection frequency, wherein said decoding means programs said delay means in response to the lock time delay signal within said memory means. 8. The selective call receiver of claim 6 further capable of switching from a first receive frequency to a second receive frequency further comprising: means for determining the difference between a first injection frequency corresponding to the first receive frequency and a second injection frequency corresponding to the second receive frequency, the determining means generating a difference signal in response thereof; and memory means for storing at least one lock time delay signal corresponding to the difference signal, wherein said decoding means having hitherto programmed the first injection frequency disables said detecting means, programs the second injection frequency, and programs said delay means in response to the lock time delay signal said decoding means further enables said detecting means in response to the completion signal. 9. A method of controlling a selective call receiver having variable characteristics and a frequency synthesizer for selecting a receive frequency for receiving radio frequency modulated information, the method comprising the step of: selecting a receive frequency; and adjusting the variable characteristics in response to the selected receive frequency. 10. The method of claim 9 wherein the frequency synthesizer includes a phase lock loop and a variable characteristic includes a variable bandwidth loop filter for the phase lock loop and the step of adjusting includes adjusting the variable bandwidth of the phase lock loop in response to the selected receive frequency. 11. The method of claim 10 wherein the selective call receiver receives information on at least a first and second receive frequency and the the step of adjusting includes adjusting the variable bandwidth of the phase lock loop to a first bandwidth in response to the selection of the first receive frequency and adjusting the variable bandwidth of the phase lock loop to a second bandwidth different from the first bandwidth in response to the selection of the second receive frequency.

Inventors: DAVIS WALTER LEE, HEROLD BARRY WAYNE, TAHERNIA OMID, DAVIS, WALTER, LEE, HEROLD, BARRY, WAYNE, TAHERNIA, OMID
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: H04B1
IPC codes: H03J5, H03L7, H04B1, H04Q7