Abstract:
The frequency of a first local oscillator signal (1.LO) is offset in reference to the center frequency of a first bandpass filter (4), and the frequency of a second local oscillator signal (2.LO) is offset in reference to the center frequency of a second bandpass filter (7). The frequency offsets are selected so that only a portion (x, y) of the filter (4, 7) bandwidths (bw, BW) will add to an &#34;imaginary&#34; effective narrow bandwidth (NBW). The amounts of frequency offset are continuously adjustable by programmable controller.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for adjusting the operating bandwidth in a radiotelephone for operation in a plurality of radiotelephone systems. 
     Radiotelephones, and more particularly cellular radiotelephones, typically operate on only one type of cellular telephone system, for instance, the European GSM (Groupe Special Mobile) or the proposed U.S. system. Prior art telephones that are capable of use in different radiotelephone systems have been equipped with multiple transceivers. U.S. Pat. No. 4,972,455 permits use of different systems by using a transceiver having a dual bandwidth receiver with different filters in the intermediate frequency (IF) section. The filters have different bandwidths and are switchably selected, depending on which cellular service is available. This patent discloses two fixed filters which limit the use of the radiotelephone to systems and applications determined by the selected filter characteristics. Switches may also give rise to crosstalk effects. Thus, there is felt a need for a radiotelephone with a more flexible selection of the operating characteristics. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a new method and apparatus allowing the operating bandwidth in a radiotelephone receiver to be easily adjusted for use in a plurality of systems without the drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     In the present invention, the frequency of a first local oscillator signal is offset in reference to the center frequency of a first bandpass filter, and the frequency of a second local oscillator signal is offset in reference to the center frequency of a second bandpass filter, whereby the frequency offsets are selected so that each filter will attenuate one channel adjacent the desired channel. The amount of frequency offset is continuously adjustable by programmable controlling means. Thus, the present invention permits the radiotelephone to operate in a system with wide channel spacing, and in a system with narrower channel spacing by adjusting the operating bandwidth of the radiotelephone appropriately. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the receiving section in a radiotelephone embodying the present invention. 
     FIGS. 2a-2f schematically present frequency spectra, which explain how local oscillator frequencies are offset in order to produce a narrow operating bandwidth in accordance with the invention, with filters that have essentially broader bandwidths. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 there is schematically illustrated a block diagram of the receiving section in a cellular radiotelephone. In the embodiment shown, an RF-signal (RFIN) containing a desired channel signal (fc) and neighboring channel signals (fa, fb) is connected to the input amplifier 1 and then to a first mixer 2. The mixer 2 also receives a local oscillator signal 1.LO from a first local oscillator 10. The mixer 2 outputs a first intermediate frequency signal 1.IF to the bandpass filter 4 and to the second mixer 5. The second mixer 5 also receives a second local oscillator signal 2.LO from a second local oscillator 12. The mixer 5 outputs a second intermediate frequency signal 2.IF to the bandpass filter 7 and to the FM-detector 8, which outputs a signal OUT to further circuits of the radiotelephone. 
     FIG. 2a shows the present invention with wide channel spacing, i.e., 30 kHz. As an example, some channel signals fa, fb and fc with wide channel spacing, i.e., 30 kHz, are shown. The RF-signal (RFIN) is, in this example, 855 MHz. The first oscillator signal 1.LO is 900 MHz. The first IF-signal 1.IF is derived by mixing RFIN with 1.LO, and thus has a center frequency of 45 MHz, e.g., 900 MHz-855 MHz (as discussed below). The bandpass filter 4 has a bandwidth bw=30 kHz (FIG. 2e), which effectively passes only the desired channel signal fc to the next stage. The frequency of the second oscillator is here 44.545 MHz, whereby the center frequency of 2.IF is 0.455 MHz, e.g., 45 MHz-44.545 MHz (as discussed below). In the same manner, the second stage passes only the desired channel FC in the second IF-signal 2.IF. In this operation there is no offset of the local oscillator frequencies. 
     The signal frequencies are related to each other according to the following general expressions: 
     
         1.IF=1.LO-RFIN                                             (1) 
    
     
         2.IF=1.IF-2.LO                                             (2) 
    
     FIG. 2b shows a second example of the present invention with narrow channel spacing, i.e., 10 kHz. Ordinarily, the three channels fa, fb, fc would pass through the 30 kHz first bandpass filter 4, because they are narrowly spaced. Correspondingly, the three channels FA, FB, FC would pass through the second bandpass filter 7, making correct FM-detection of the desired channel fc/FC impossible. The prior art solution to this problem is to introduce new filters with narrower bandwidths, i.e., about 10 kHz, instead of the broad bandwidth filters 4 and 7. 
     However, the method according to the invention solves this problem. The first local oscillator 10 and mixer 2 perform the channel selection, the first local oscillator frequency 1.LO is adjusted downwards, in this example by 10 kHz, so that 1.LO=899.990 MHz. Adjusting 1.LO 10 kHz downwards causes the center frequency of 1.IF to be 10 kHz lower. FIG. 2c shows the effect of this, whereby the bandwidth of 1.IF is shifted downwards by 10 kHz (e.g., 899.990 MHz-855 MHz=44.990 MHz), and correspondingly, the channels fa, fb and fc are shifted 10 kHz to the left in the figure. Correspondingly, the channels FA, FB and FC are shifted 10 kHz to the left in the second IF-signal 2.IF. The second step is to offset the second local oscillator signal 2.LO, and in this case by 20 kHz, so that 2.LO=44.525 MHz. Adjusting 2.LO 20 kHz downwards causes the center frequency of 2.IF to be 10 kHz higher than if the mixer frequency was not adjusted. This leads to a second IF-signal 2.IF=0.465 MHz (e.g., 44.990 MHz-44.525), whereby the channels FA, FB and FC are shifted 10 kHz to the right in FIG. 2d. 
     The bandwidths of the bandpass filters 4 and 7 are kept the same, i.e., 30 kHz, cf. FIG. 2e. Now it can be seen, comparing FIGS. 2d and 2e, that the local oscillator frequency offsets produce the desired result: In the first filter 4, the channel fa on the left side in FIG. 2d is effectively attenuated, and the two other channels, fc and fb, are passed to the next stage. In other words, the channel fa is now off-band, which is indicated by the asterisk, fa* and only fb and fc are passed. In the second stage, that is in the bandpass filter 7, a corresponding attenuation of channel FB is performed, the channel being out of band, FB*. Thus, only FC is passed (FA does not exist because fa was not passed). The net result is that only the desired channel signal FC is now connected to the FM-detector 8. 
     The local oscillator frequency offsets and the resulting bandwidth reduction are further explained in FIGS. 2e and 2f. In the first filter 4 there remains an effective bandwidth x of the total bandwidth bw, because fa* is off band and fb will be attenuated at the next stage. In a corresponding manner in the second filter 7 there remains an effective bandwidth y of the total bandwidth BW, because FB* is off band. These parts x and y combine to produce an &#34;imaginary&#34; filter, with the narrow bandwidth NBW. 
     In existing radiotelephones, the first local oscillator usually is digitally controlled, with an adjustable output frequency 1.LO. The digital control is realized with programs contained in the radiotelephone controlling microprocessor (memory 14, CPU 16 in FIG. 1) by methods known per se. 
     The present method also requires the second local oscillator frequency 2.LO to be digitally controlled in the same manner as 1.LO. 
     The filter at the higher frequency, i.e., 45 Mhz, requires good selectivity. This is achieved by using crystal filters known per se, which also typically provide assymmetrical bandwidth edge slopes, as is shown in FIG. 2e. The characteristic of the bandwidth of a crystal filter, as shown on the left side in FIG. 2e has the figure a steeper edge than the right side. The 1.IF filter can also be a saw-type filter. The 2.IF filter is typically a ceramic filter having symmetric characteristics. This leads to the narrow net bandwidth NBW in FIG. 2f which has very sharp edges. 
     In certain cases it would be conceivable to provide a receiver with three IF stages, a so called &#34;triple super&#34;, in order to obtain better channel selectivity. In the triple super, the inventive method would be realized by the second and the third IF-states where the 2 and 3.IF filters are ceramic filters. 
     In the table below, there is shown different frequency combinations, whereby the examples 1 and 2 were discussed above. 
     
                       TABLE 1______________________________________Examples of frequency combinations(frequencies in MHz)        Desired   Neighboring                             Neighboring        channel   channel    channelExample 1    Example 2 Example 3  Example 4(wide band)  (narrow)  (narrow)   (narrow)______________________________________RFIN  855        855       854.990  855.0101.LO  900        899.990   899.990  899.9901.IF  45         44.990    45.000   44.980*2.LO  44.545     44.525    45.525   44.5252.IF  0.455      0.465     0.475*   0.445______________________________________ *off band 
    
     It is understood that the frequencies could be selected in many other combinations, in accordance with equations 1) and 2) above. 
     The main advantage of the present inventive method is that it requires no switches to change the effective channel bandwidth of the receiver. Thereby crosstalk effects introduced by the extra switches are also avoided. By using digitally controlled oscillators, it is easy to programmably adjust the predetermined local oscillator frequency offsets.