Patent Publication Number: US-2005143031-A1

Title: Multi-band receiver

Description:
The invention relates to a multiple band receiver as described in the preamble of claim  1 .  
      Frequency bands for communication networks are defined in international and national standards such as IEEE 802.11a and HIPERLAN. Their frequency bands are [2.4-2.5] GHz according to HIPERLAN and [5.2-5.8] GHz according to IEEE 802.11a. A heterodyne receiver transforms a frequency of an input signal into an intermediate frequency (IF) signal. This transformation is realized in a mixer that combines the input signal with a signal generated by a local oscillator. The result of this combination is an IF signal. The IF signal has a frequency representing either the difference between the oscillator frequency and the frequency of the input signal in so called upper heterodyning mode or the difference between the frequency of the input signal and the oscillator frequency in so called lower heterodyning mode. Normally, a receiver for receiving signals situated in different frequency bands has different oscillators, one for each band or group of bands, if possible. Reducing the number of oscillators has multiple benefits as reducing costs, reducing the size of the receiver, reducing the complexity of the circuits that are used for building the oscillator and the input circuits.  
      Such a solution is known from U.S. Pat. No. 4,132,952. In this patent application a multi-band tuner with fixed broadband input filters is presented. The receiver described in this document is used for receiving broadcasting video frequency signals that are situated in two frequency bands spaced from each other. The IF band is selected such that the image frequency rejection is improved. Furthermore a mixer used in this invention could be either in upper heterodyning mode or in lower heterodyning mode. In this case two selectable band-pass filters are used. A first band-pass filter is used for selecting the frequency resulting when upper heterodyning mode is used. A second band-pass filter for selecting the frequency resulted when lower heterodyning is used. It should be mentioned here that the oscillator must be a variable frequency oscillator having a minimum frequency (f min ) and a maximum frequency (f max ). In the presented embodiments the ratio f max /f min  is greater that 2. It must be observed that the above ratio is hard to be realized for oscillators operating in relatively high frequency ranges e.g. Ghz. The local oscillators are normally voltage controlled and when low voltage operation is necessary, as in relatively high frequency systems, the voltage range is not sufficient for controlling the oscillation frequency. Furthermore, in order to reduce costs it is desirable to use as few as possible components.  
      It is therefore an object of present invention to provide a multiple band receiver having a relatively low cost.  
      In accordance with the invention this is achieved in a device as described in the preamble of claim  1  being characterized in that the central frequency of the IF band-pass filter is substantially independent of a combining mode of the amplified signal and the periodical signal, the combining mode being selected from an upper heterodyning mode and a lower heterodyning mode.  
      In the upper heterodyning mode, the intermediate frequency (IF) signal has a frequency representing the difference between the frequency of the amplified signal and the frequency of the periodical signal. If the amplified signal is included in different bands a carefully chosen IF signal is such that f IF =f RF −f OSC  in upper heterodyning mode and f IF =f OSC −f RF  in lower heterodyning mode. In previous relations fIF is the frequency of the IF signal, f OSC  is the frequency of the periodical signal generated by the oscillator and f RF  is the frequency of the input signal. In this way, a receiver for receiving signals situated in different bands uses only one oscillator. Furthermore, because the frequency of the IF signal does not depend on how the signals are combined in the mixer, only one band-pass filter having a central frequency substantially equal to the frequency of the IF signal is necessary. The band-pass filter could comprise a plurality of image rejection filters for rejecting image frequencies that appear either in upper heterodyning mode or in lower heterodyning mode. It is observed that the tuned frequencies of image rejection filters are controllable using an external signal for indicating whether upper heterodyning mode or lower heterodyning mode is performed. Using only one band-pass filter for the IF signal and only one local oscillator, the multiple band receiver is relatively cheap and easy to be built. 
    
    
      The above and other features and advantages of the invention will be apparent from the following description of the exemplary embodiments of the invention with reference to the accompanying drawings, in which:  
       FIG. 1  depicts a block diagram of a multiple band receiver according to the invention,  
       FIG. 2  depicts a block diagram of a transceiver using the multiple band receiver according to the invention. 
    
    
       FIG. 1  depicts a block diagram of a multiple band receiver according to the invention. The receiver comprises an input I for receiving a relatively high frequency input signal RFin having a frequency f RF  situated either in a first frequency band e.g. [2.4-2.5] Ghz or in second frequency band e.g. [5.2-5.8] Ghz. The input signal could be received via an antenna or via a transducer such as an opto-electrical transducer. The input signal is inputted to a first band-pass filter BPF 1  and in a second band-pass filter BPF 2 . A first central frequency of the BPF 1  is situated in the first frequency band and a second central frequency of the BPF 2  is situated in the second frequency band. Both filters are linear filters i.e. a signal at their outputs has the frequency of the input signal f RF . The output signals of BPF 1  and BPF 2  are inputted to a multiplexer (MUX)  30 . The multiplexer  30  is controlled by a control signal BS. The control signal BS determines which of the output signals from the multiplexer  30  is further transmitted to the receiver  1  i.e. either the output signal of BPF 1  or the output signal of BPF 2 . It is observed that the multiplexer  30  selects the frequency band of the receiver  1 . At the output of the multiplexer  30  a signal having the frequency f RF  is obtained. Usually an amplitude of the input signal RFin is relatively small and an amplification of the signal is necessary. The signal obtained at the output of the multiplexer is linearly amplified in a low noise amplifier (LNA)  40 . An output signal obtained at the output of the LNA  40  has the same frequency as the input frequency i.e. f RF  and an amplitude that is proportional to the input signal, having a higher amplitude. The amplified signal obtained at the output of LNA  40  is inputted to a first input of a mixer  50 , said mixer being coupled to the LNA  40 . A local oscillator (OSC)  70  is coupled to a second input of the mixer  50 . The local oscillator  70  generates a periodical signal having a frequency f OSC . The periodical signal is combined with the signal generated by the LNA  40 . The mixer  50  generates a signal IF. The frequency of signal IF i.e. f IF  is either f IF =f RF −f OSC  in upper heterodyning mode or f IF =f OSC −f RF  in lower heterodyning mode. Besides the intermediate frequency signal, parasitic signals called image signals are also generated. The mixer  50  is coupled to a IF band-pass filter  60  having a central frequency substantially equal to the intermediate frequency f IF . The IF band-pass filter  60  further comprises image-rejection filters that attenuate an amplitude of the image signals. The image-rejection filters are tuned to the image frequencies, said image frequencies depending on the input signal frequency f RF  and on the frequency of the IF signal f IF . The image rejection filters are normally elliptic filters, notch or band-reject filters, preferably realized using passive components. The IF band-pass filter  60  further amplifies the intermediate frequency signal IF for compensating inherent losses obtained during the filtering process. The control signal BS controls the IF band-pass filter  60  such that at the output of the IF band-pass filter  60  a signal having relatively constant amplitude and a frequency substantially equal to f IF  is obtained. Said amplitude and frequency of the output signal of the IF band-pass filter  60  are substantially independent of the mode i.e upper heterodyning mode and lower heterodyning mode. If the input signal RF in  is situated either in the band [2.4-2.5] GHz or in the band [5.2-5.8] GHz a suitable intermediate frequency could be fF=1.5 GHz. A local oscillator  70  generating a periodical signal f OSC  situated in [5.2-5.4] GHz band is chosen. The frequency f OSC  is used to be combined with the signal f RF  in the mixer  50  such that the frequency of the IF signal is independent with respect to the band of the input signal RF in . When the frequency of the input signal is situated in the [2.4-2.5] GHz band the upper heterodyning mode is used and when the frequency of the input signal is situated in [3.7-4.3] GHz band the lower heterodyning mode is used, respectively. It is observed that the tuning ratio of the local oscillator i.e. the ratio between the maximum oscillation frequency and the minimum oscillation frequency is relatively low e.g. 1.16. This tuning ratio is relatively easy to be realized even when relatively high frequencies are used. Furthermore the receiver  1  comprises only one local oscillator and only one IF band-pass filter resulting a cheaper receiver. Modern communication networks use quadrature signals and therefore a quadrature local oscillator could be used.  
      It is observed that the input signal RFin could be generated by an antenna in a wireless communication system, could be a signal generated by a transducer e.g. a photo-detector in an optical network or could be obtained using a mutual coupling e.g. magnetic coupling or charge coupling.  
      It is further observed that if the input signal Rf in  corresponds to the standard IEEE 802.11a e.g. f RF =5.2 GHz then the receiver  1  could be used as it is. So, it results that the receiver  1  could be used for receiving signals corresponding to three standards i.e. HIPERLAN, IEEE 802.11a,b.  
       FIG. 2  depicts a block diagram of a transceiver  100  using the multiple band receiver  1  according to the invention. The transceiver  100  comprises the multiple band receiver  1  coupled to a transmitter  2  via a controllable switch  3 . A control signal MODE determines whether the transceiver  100  is used in a receiving mode or in a transmitting mode. Normally, the control signal MODE is a binary signal. In receiving mode the control signal MODE determines an input signal received at an input/output I/O terminal to be inputted to the input terminal I of the receiver i.e. the switch  3  couples the I/O terminal to a terminal R of the switch. In transmitting mode the control signal MODE determines an output signal  0  transmitted by the transmitter  2  to be inputted to the I/O terminal i.e. the switch  3  couples the I/O terminal to a terminal T of the switch. The control signal MODE could be an electrical signal e.g. a voltage, a current, a charge or a non electrical signal i.e. an intensity of light, temperature, pressure.  
      The transeiver  100  is adapted to transmit signals corresponding to the above mentioned standards being relatively cheap and relatively easy to be practically implemented.  
      It is remarked that the scope of protection of the invention is not restricted to the embodiments described herein. Neither is the scope of protection of the invention restricted by the reference numerals in the claims. The word ‘comprising’ does not exclude other parts than those mentioned in the claims. The word ‘a(n)’ preceding an element does not exclude a plurality of those elements. Means forming part of the invention may both be implemented in the form of dedicated hardware or in the form of a programmed purpose processor. The invention resides in each new feature or combination of features.