SUPER RX WITH TRANSFERRED-IMPEDANCE FILTERS

Technical field

The present invention relates to a superheterodyne receiver, and to a communication apparatus for multiband operation having such a receiver. Background of the invention

Multiband receivers are nowadays widely used in communication apparatuses for wireless communication to enable the communication apparatus to operate on two or more frequency bands . One way to achieve a multiband receiver is to use direct conversion technology. However, this technology shows some drawbacks, e.g. direct current

(DC) offset issues, and difficulties in implementation in certain semiconductor processes. Another way to achieve a multiband receiver is to use a traditional superheterodyne receiver, wherein the intermediate frequency (IF) need to be optimized and a plurality of local oscillator signals need to be provided to get multiband features. The superheterodyne receiver also need IF filters, which normally are implemented by crystal or surface acoustic wave (SAW) filters. These filters impose relatively high costs in relation to overall costs of the receiver. In addition to this comes that the filters need to have different bandwidths for different frequency bands and/or modulation types, which further increase complexity and/or cost, if not performance is to be jeopardized. Further, these filters need a considerable space. Therefore, there is a need for an improved approach for achieving multiband receivers. Summary of the invention In view of the above, an objective of the invention is to solve or at least reduce the problems discussed above. In particular, an objective is to facilitate multiband operation.

The present invention is based on the understanding that having frequency controllable filters in a superheterodyne receiver enables implementation of a true multiband receiver. The inventors have found that a transferred impedance filter (TIF) , controlled by a local oscillator which also provides local oscillator signals to mixers of the superheterodyne receiver will enable this .

According to a first aspect of the present invention there is provided a superheterodyne receiver comprising a first amplifier arranged to work at a radio frequency of a received radio signal; a local oscillator for generating a local oscillator signal; a first mixer arranged to mix said received radio signal with said local oscillator signal; and a first transferred impedance filter controlled by said local oscillator signal and arranged to filter an output signal of said mixer to provide an intermediate frequency signal.

This structure will provide for change of operating frequency band by changing the local oscillator frequency, which then will affect both mixing and filtering to be adapted to the new frequency. The receiver may further comprise a pulse shaper arranged to convert said local oscillator signal to have a duty cycle of 25% or less before providing the signal to the first transferred impedance filter. This will facilitate direct switching of impedance network of the TIF.

The receiver may further comprise a frequency divider arranged to divide said local oscillator frequency by two in frequency to provide a second local oscillator signal, which second local oscillator signal is the local oscillator signal provided to said first mixer and said first transferred impedance filter. The frequency of the second local oscillator signal may equal

the intermediate frequency. The receiver may comprise a second transferred impedance filter arranged in a signal path between said first amplifier and said first mixer, and being controlled by said local oscillator, wherein the frequency of the local oscillator signal provided to the second transferred impedance filter is double the second local oscillator signal. The receiver may comprise a pulse shaper arranged to convert said local oscillator signal to have a duty cycle of 25% or less before providing the signal to the second transferred impedance filter.

The receiver may comprise a second mixer arranged to mix said intermediate frequency signal with said local oscillator signal to provide a baseband signal. The receiver may comprise a phase shifter arranged to input said local oscillator signal and to output a phase shifted signal; and a third mixer arranged to mix said intermediate frequency signal with said phase shifted signal to provide a quadrature baseband signal. The receiver may comprise a second amplifier arranged to amplify said intermediate frequency signal.

According to a second aspect of the present invention, there is provided a communication apparatus for multiband operation comprising a receiver according to the first aspect of the present invention, and a controller arranging said local oscillator to operate according to a frequency band to be utilized.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc] " are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings . Brief description of the drawings

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

Fig. 1 illustrates the basics of a superheterodyne receiver; Fig. 2 illustrates a superheterodyne receiver according to an embodiment of the present invention;

Fig. 3 illustrates a superheterodyne receiver according to an embodiment of the present invention;

Fig. 4 illustrates a superheterodyne receiver according to an embodiment of the present invention;

Fig. 5 illustrates switching stage and impedance stage of a transferred-impedance filter; and

Fig. 6 schematically illustrates a communication apparatus according to an embodiment of the present invention.

Detailed description of preferred embodiments

Fig. 1 illustrates the basics of a superheterodyne receiver 100. A radio frequency (RF) electromagnetic field affects an antenna 102 wherein a signal is provided from the antenna 102 to an amplifier 104 arranged to work in the radio frequency band or bands being used. The amplifier 104 is usually a low-noise amplifier (LNA) . The radio frequency signal output of the amplifier 104 is provided to a mixer 106, where the radio frequency signal is mixed with a local oscillator (LO) signal from a local oscillator 108, i.e. the signals are multiplied. Thus, an output of the mixer comprises a signal with a difference

frequency between the mixed signals and a signal with a sura frequency of the mixed signals, if the bandwidth of the mixer enables this. The output from the mixer 106 is fed to a filter 110 to filter out the signal with the difference frequency to form an intermediate frequency (IF) signal. The IF signal is preferably amplified by an IF amplifier 112. The receiver 100 also comprises some type of demodulator 114. The type of modulator depends on which kind om modulation that is used for the RF signal, and on what type of use signal that is to be extracted from the RF signal.

Fig. 2 illustrates a superheterodyne receiver 200 according to an embodiment of the present invention. It can be seen that many similarities to the receiver 100 demonstrated with reference to Fig. 1 are present, and these will for the sake of conciseness not be further explained. However, a local oscillator 202 not only provides a local oscillator signal to a mixer 204, but also to a filter 206. The filter 206 is a transferred- impedance filter (TIF) which is described in

WO2006/097835 A2, which is hereby incorporated by reference. A TIF transfers a baseband impedance to RF range around a LO frequency that is used in the filter. A TIF generally consists of three parts. (1) The transferred-impedance stage consists of two similar impedances. (2) A switching stage, which transfers the impedances to RF frequencies, consists of two similar stages. In a balanced topology, both stages contain four switches. (3) The LO-generation circuit generates the LO signals that control the switches of the switching stage. Fig. 5 illustrates a switching stage and an example of the impedances of a TIF. The switching stage consists of eight switches and two similar transferred impedances, which in this case are capacitors. A TIF is usually a balanced circuit since balanced topologies are generally used in mixed-mode RFICs to reject the interference from digital circuits, supply voltage, or silicon substrate.

The TIF and the impedance in parallel form a bandpass or bandstop RF filter that depends on the impedance. The center frequency of the filter is determined by the LO signal of the filter, and can thus be changed by changing the LO frequency. This is particularly interesting in the present invention, where a LO signal is present for providing mixing of the RF signal. Thus, returning to Fig. 2, the filter 206 will have a pass frequency range that follows the LO frequency, which also determines the IF signal frequency. This enables for a multiband receiver which is controlled in sense of selected frequency band simply by changing the LO frequency.

A pulse shaper (not shown) can be arranged to provide a control signal for the TIF 206 from the LO signal. The pulse shaper provides a signal having a duty cycle of 25% or less. One way of providing

Fig. 3 illustrates a superheterodyne receiver 300 according to an embodiment of the present invention. It can be seen that many similarities to the receiver 200 demonstrated with reference to Fig. 2 are present, and these will for the sake of conciseness not be further explained. A second mixer 302 is exemplary illustrated. The second mixer 302 will mix the IF signal with a LO signal to translate the IF signal to baseband (BB) signal. This is to be considered only as an example of processing or demodulating the IF signal, but illustrates a further advantage of the structure of the receiver according to the present invention, where we can see that still is only one oscillator 304 needed for the receiver 300.

Fig. 3 also illustrates a second TIF 306 used for filtering of the RF signal from an LNA 308. As discussed above, the passband frequency of a TIF is determined by the control frequency for the switching capacitive network of the TIF. Hence, the LO 304 provides a frequency f, which is the frequency of a desired RF band, both to the second TIF 306, which will have its passband

on the desired RF band, and to a frequency divider 308, which will output a divided frequency f/2. The divided frequency is fed to a first mixer 310, which will mix it with the RF signal (having frequency f) with the result of an IF signal with frequency f/2 (f-f/2), at least after filtering in a first TIF 312 which is controlled by the divided frequency and thus will have its passband on the divided frequency f/2, which happens to be the intermediate frequency. After filtering, the IF signal is amplified by a second amplifier 314 and can for example be mixed with the divided frequency in the second mixer 302 down to baseband, all this since both the IF signal and the divided LO signal have the frequency f/2.

Fig. 4 illustrates a superheterodyne receiver 400 according to an embodiment of the present invention. A local oscillator 402 generates a LO signal with frequency 2f, which is divided in a first frequency divider 404 to a divided LO signal with frequency f. From the LO signal and the divided signal can four pulse trains, each having a duty cycle of 25%, divided in phase by π/2, and having a frequency of f, easily be provided by logic operations for switches of a TIF 406. These pulse trains can be generated by a pulse shaper (not shown) . The divided LO signal with frequency f is further fed to a second frequency divider 408, dividing it to a secondly divided LO signal with a frequency f/2. The secondly divided LO signal is fed to a first mixer 410 for mixing with an RF signal output from the TIF 406 to produce a sum and difference frequency signal, wherein the difference signal with frequency f/2 is filtered out by a second TIF 412 to produce an IF signal. The second TIF 412 is controlled by the secondly divided LO signal, which form basis of four pulse trains, each having a duty cycle of 25%, divided in phase by π/2, and having a frequency of f/2, which can easily be provided by logic operations. These pulse trains affects switches of the second TIF 412.

The frequency divider 408 also provides an in-phase signal with frequency f/2 I(f/2) and a quadrature-phase signal, also with frequency f/2, Q (f/2) for provision to a second mixer 414 and a third mixer 416, respectively. The mixers 414, 416 provide baseband signals in phase and quadrature to a baseband processor 418 for further processing.

With reference to any of the above demonstrated embodiments, there is provided the ability to control frequency band throughout the RF and IF, and where applicable also BB, stages of the receiver by only providing a suitable frequency from one local oscillator. Several exemplary features provided with this concept are provided in each of the embodiments demonstrated above. It is further possible to control the bandwidth of the filters with the above described approach. It should be emphasized that these features can be used in any combination or sole in any of the embodiments.

The above demonstrated receivers are particularly suitable for a communication apparatus 600 having such a receiver 602, wherein processor 604 of the apparatus 600 receives a baseband signal from a baseband processor 606 for further processing of received information. The processor 604 further provides control signals to a local oscillator 608 of the receiver 602 for controlling the frequency, and thus be able to control which frequency band to operate on. The communication apparatus can comprise numerous of other elements, such as transmitter, user interface, other communication interfaces, etc. However, details on this are omitted not to obscure the invention .

Further, not to obscure the features of the present invention, details regarding practical implementation, such as matching, filtering, component values, signal strengths, etc. that is not directly and essentially affected by the novel and inventive features of the invention, have been omitted. In the disclosure above,

mixing of signals is described where the frequency component comprising the difference in frequencies is used, and the frequency component comprising the sum in frequencies is filtered out. However, it is readily understood by a person skilled in the art taking part of the disclosure above that, for the disclosed invention, the frequency component comprising the sum of frequencies can be used, and the frequency component comprising the difference in frequencies is filtered out. The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.