WO1998011674A1

WO1998011674A1 - Method and apparatus for suppressing transmitter overtones and receiver blocking signals in a radio transceiver

Info

Publication number: WO1998011674A1
Application number: PCT/SE1997/001495
Authority: WO
Inventors: Lars Berggren; Hans Stattin; Christian Andersson; Anders Svensson; Hans Camper; Lars Nord
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1996-09-10
Filing date: 1997-09-05
Publication date: 1998-03-19
Also published as: AU4405397A

Abstract

Method and apparatus for efficiently suppressing receiver blocking signals and transmitter overtones in a radio transceiver. In an exemplary embodiment, a transceiver includes a transmitter for generating information signals to be transmitted by the transceiver, and a receiver for processing information signals received by the transceiver. A power amplifier connected to an output of the transmitter amplifies information signals prior to transmission, and a low-noise amplifier connected to an input of the receiver amplifies received signals prior to processing. Additionally, a band-pass filter connected to an input of the low-noise amplifier filters received signals prior to amplification. Time-division multiplexing of transmitted and received signals is accomplished by a switch alternately connecting a radio antenna of the transceiver to the output of the power amplifier in the transmit path and to the input of the band-pass filter in the receive path. A bi-directional low-pass filter, situated between the antenna and the switch, is used to filter both transmitted and received information signals. The band-pass filter and the low-pass filter attenuate disruptive receiver blocking signals. The low-pass filter also attenuates unwanted transmitter overtones.

Description

METHOD AND APPARATUS FOR SUPPRESSING

TRANSMITTER OVERTONES AND RECEIVER BLOCKING SIGNALS

IN A RADIO TRANSCEIVER

BACKGROUND The present invention relates to radio communications systems, and more particularly to the suppression of undesirable signals arising in the transmit and receive signal processing paths of a radio transceiver.

By definition, the performance of a radio-frequency (RF) transceiver is diminished any time interfering RF energy exists within that portion of the frequency spectrum allocated to the transceiver for transmission and reception. Perhaps less apparent is the fact that such an RF transceiver can also be adversely affected by RF energy existing outside the allocated spectrum. For example, extraneous RF signals radiating at frequencies outside the designated reception bandwidth, but nonetheless picked up at the transceiver antenna, may prevent the transceiver from receiving signals of interest by driving sensitive, high-gain amplifiers within the transceiver into saturation. Additionally, extraneous RF signals lying outside the allocated receiver bandwidth may disrupt transceiver operation by mixing with harmonics produced by local oscillators used in the transceiver. These disruptive, outlying RF signals are often referred to in the art as receiver "blocking" signals.

As described in more detail below, prior art systems have addressed the aforementioned problems by employing various filtering techniques to suppress the unwanted blocking signals. For many applications, however, the known solutions are inappropriate in terms of overall cost, complexity, and number of components required. Thus, there is a need for a more economic method of suppressing receiver blocking signals in RF transceivers. SUMMARY

The present invention fulfills the above-described and other needs by providing a radio transceiver which suppresses receiver blocking signals, as well as unwanted harmonics generated by components used in the transceiver during signal transmission, in an elegant and streamlined manner. The present invention affords many advantages over prior art systems, for example, in terms of system stability, cost, complexity, and total part count. In an exemplary embodiment, the transceiver of the present invention includes a transmitter which generates information signals to be transmitted by the transceiver, and a receiver for processing information signals that are received by the transceiver. A power amplifier connected to an output of the transmitter amplifies the information signals prior to their transmission, and a low-noise amplifier connected to an input of the receiver amplifies the received signals prior to their being processed. Additionally, a band-pass filter connected to an input of the low-noise amplifier filters the received signals prior to their amplification. To achieve time-division multiplexing of the transmitted and received signals using a single antenna, a switch is used to alternately connect a radio antenna of the transceiver to the output of the power amplifier in the transmit path and to the input of the band-pass filter in the receive path. A bidirectional low-pass filter, situated between the antenna and the switch, is used to filter both transmitted and received information signals. The band-pass filter can be constructed to prevent receiver blocking signals from saturating the low-noise amplifier. The bi-directional low-pass filter can be constructed, not only to prevent high-frequency receiver blocking signals from saturating the low-noise amplifier, but also to prevent the high-frequency receiver blocking signals from interfering with an intermediate frequency signal generated within the receiver. Additionally, the low-pass filter can be designed to attenuate harmonics, or overtones, generated during signal transmission. By an innovative combination of filter placement and design, the present invention provides a robust, cost- effective RF transceiver having superior stability and performance characteristics. Additional features and advantages of the present invention are explained hereinafter with reference to illustrative examples shown in the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of a prior art, two-way, time-division multiple-access (TDMA) radio transceiver. Figure 2 is a block diagram of a prior art TDMA radio transceiver incorporating a discrete RF trap to reduce the effects of high-frequency blocking signals received at an antenna of the transceiver.

Figure 3 is a block diagram of an improved TDMA radio transceiver constructed in accordance with the teachings of the present invention.

Figure 4 is a block diagram of an exemplary cellular mobile radiotelephone system constructed in accordance with the teachings of the present invention.

DETAILED DESCRIPTION

Figure 1 depicts a conventional TDMA radio transceiver 100. Such a transceiver might be used, for example, in a cellular radio communications system. As shown in Figure 1, the conventional TDMA transceiver 100 includes an antenna 110 which is connected, by way of a standard TDMA switch 120, to an output of a transmit signal processing path and to an input of a receive signal processing path. In the transmit signal processing path, a power amplifier 160 is coupled to a low-pass filter 140 which is in turn coupled to one contact of the TDMA switch 120. In the receive signal processing path, a second contact of the TDMA switch 120 is coupled to a band-pass filter 130 which is in turn coupled to a low-noise amplifier 150. The low-noise amplifier 150 is in turn coupled to a heterodyne mixer 170.

In operation, the TDMA switch 120 alternately connects the transmit and receive signal processing paths to the antenna 110 in order to separate, and time-division multiplex, the transmission and reception processes. During transmission, the switch 120 is positioned such that the antenna 110 is isolated from the receive signal path and coupled to an output of the low-pass filter 140. An information signal (TX) generated by a standard RF transmitter (not shown) is input to the power amplifier 160. An amplified information signal, output from the power amplifier 160, is then input to the low-pass filter 140. The low-pass filter 140 is used to attenuate unwanted harmonics, or overtones, generated by the transmitter and the power amplifier 160. As is well known in the art, such overtones arise at integer multiples of the transmit information signal (TX) carrier frequency. Thus, the filtering characteristic of the low-pass filter 140 is set such that the amplified information signal may pass through the filter, while the higher-frequency overtones are attenuated. An amplified, filtered information signal, output from the low-pass filter 140, is then coupled through the switch 120 to the antenna 110 for transmission.

During reception, the switch 120 is positioned such that the antenna 110 is isolated from the transmit signal path and coupled to an input of the band-pass filter 130. An information signal received at the antenna is thus input to the band-pass filter 130 and then to the low- noise amplifier 150. An amplified and filtered information signal, output from the low-noise amplifier 150, is input to the heterodyne mixer 170 and mixed with a local oscillator signal (LO) , as is well known in the art, in order to downconvert the received information signal carrier to an intermediate level . An intermediate- frequency information signal (IF) is then passed to a standard RF receiver (not shown) for processing. Because information signals received at the antenna may be small in amplitude, the low-noise amplifier 150 must have a relatively high gain characteristic in order to provide the mixer 170 with input signals of sufficient strength. As a result, excess RF energy applied to the input of low-noise amplifier 150 may drive the amplifier into saturation, and thereby compress, or block, received information signals. To avoid such undesirable signal compression, the band-pass filter 130 is used to prevent extraneous, unwanted RF signals received at the antenna 110 from reaching the low-noise amplifier 150. In other words, the filter characteristic of the band-pass filter 130 is set such that desired information signals may pass through the filter, while signals outside the allocated receiver bandwidth are attenuated. As is described in more detail below, however, the real-world filters used to realize the band-pass filter 130 do not, in many cases, adequately attenuate the harmful receiver blocking signals .

Another problem often created by receiver blocking signals relates to the heterodyne mixing process that is carried out in the receiver signal processing path shown in Figure 1. As described above, the heterodyne mixer 170 mixes received information signals with a local oscillator signal (LO) to produce an intermediate-frequency signal (IF) which is input to the receiver. As is well known in the art, a local oscillator (not shown) used to produce the local oscillator signal (LO) generates unwanted harmonics at integer multiples of the local oscillator frequency. Thus, extraneous RF signals which are picked up at the transceiver antenna, and which happen to be radiating at an integer multiple of the local oscillator frequency plus or minus the intermediate frequency, will be inadvertently downconverted to the intermediate frequency and will thereby block the desired information signals. Therefore, the band-pass filter 130 is used to prevent such blocking signals from reaching the heterodyne mixer 170. Again, however, the parts commonly used to implement the band-pass filter 130 provide inadequate blocking signal suppression in many instances. Those skilled in the art will realize that it is possible to reduce the harmful effects of receiver blocking signals by implementing the band-pass filter 130 using a complex RF filter having superior filtering characteristics (e.g., very sharp cut-off from pass -band to stop-band, as well as extreme stop-band attenuation) . However, those skilled in the art will also realize that such a solution is not practical for many applications. This results from the fact that the real-world filters used to implement the band-pass filter 130 are often standardized components, designed specifically to work well with other related transceiver components (e.g., the low-noise amplifier 150) . To redesign a transceiver using an idealized, complex band-pass filter would therefore significantly, and undesirably, increase the overall cost of the transceiver. For this reason, systems using a single RF filter to achieve all blocking signal attenuation is considered undesirable by Applicants.

See, for example, U.S. Patent No. 5,355,524 to Higgins, Jr. This patent, in order to achieve maximum integration of transceiver components, teaches a system in which all RF filtering is accomplished using a single complex filter located at the transceiver antenna. Thus, with respect to signal reception, the disclosed system is equivalent to that of Figure 1 of the present application and, as such, does not address the problems herein identified. More specifically, the Higgins patent does not achieve blocking signal attenuation in a cost effective manner using existing, standardized components. Other prior art systems have utilized discrete RF filters, or traps, to accomplish blocking signal attenuation without requiring an overly complex band-pass filter. See, for example, the prior art system depicted in Figure 2. As shown, the system of Figure 2 is virtually identical to that of Figure 1. However, a capacitor 180 is shown connected between the input of the band-pass filter 130 and circuit ground. During reception, the capacitor 180 serves to filter, or trap, high-frequency blocking signals received at the transceiver antenna 110. Though Figure 2 depicts an RF trap comprising a single capacitor 180, those skilled in the art will appreciate that such an RF trap may include a combination of any number of passive components.

While the system of Figure 2 may accomplish adequate blocking signal attenuation without requiring a complex RF band-pass filter, such a system creates other problems. For example, including an RF trap increases the total part count, and therefore the complexity, of the transceiver. Also, the precise filtering characteristic of a discrete RF trap can be difficult to control as component values tend to drift with time and temperature variations. In sum, undue complexity and relative instability, render a system such as that depicted in Figure 2 inappropriate for many applications.

Figure 3 depicts an exemplary TDMA transceiver 300 constructed in accordance with the teachings of the present invention. Such a transceiver achieves improved blocking signal attenuation while avoiding the difficulties associated with the prior art systems. As shown, the TDMA transceiver 300 includes an antenna 310 coupled to one port of a bi-directional low-pass filter 390. A second port of the low-pass filter 390 is connected, by way of a TDMA switch 320, to an output of a transmit signal processing path and to an input of a receive signal processing path. In the transmit signal processing path, a power amplifier 360 is coupled to one contact of the TDMA switch 320. In the receive signal processing path, a second contact of the TDMA switch 320 is coupled to a band-pass filter 330 which is in turn coupled to a low-noise amplifier 350. The low-noise amplifier 350 is in turn coupled to a heterodyne mixer 370.

In operation, the TDMA switch 320 alternately connects the transmit and receive signal processing paths to the low-pass filter 390 in order to separate, and time- division multiplex, the transmission and reception processes. During transmission, the switch 320 is positioned such that the low-pass filter 390 is isolated from the receive signal path and coupled to an output of the power amplifier 360. An information signal (TX) generated by a standard RF transmitter (not shown) is input to the power amplifier 360. An amplified information signal, output from the power amplifier 360, is then coupled through the switch 320 to the low-pass filter 390. The low-pass filter 390 is used during transmission to attenuate the unwanted overtones generated by the power amplifier 360. An amplified, filtered information signal, output from the low-pass filter 390, is then coupled to the antenna 310 for transmission.

During reception, the switch 320 is positioned such that the low-pass filter 390 is isolated from the transmit signal path and coupled to an input of the band-pass filter 330. An information signal received at the antenna 310 is thus input to the low-pass filter 390, then to the band-pass filter 330 and the low-noise amplifier 350. The low-pass filter 390 and the band-pass filter 330 work together to eliminate the receiver blocking signals described above. An amplified and filtered information signal, output from the low-noise amplifier 350, is input to the heterodyne mixer 370 and mixed with a local oscillator signal (LO) for downconversion. An intermediate-frequency information signal (IF) , output from the mixer 370, is then passed to a standard RF receiver (not shown) for processing.

Note that the low-pass filter 390 is used to eliminate the harmonics generated by the power amplifier 360 during signal transmission and to attenuate high- frequency blocking signals during signal reception. A ceramic RF filter can be used to implement the low-pass filter 390 of Figure 3. Because allocated RF transmit and receive bands are typically adjacent one another in the frequency spectrum, the cut-off frequency of the low-pass filter 390 can be set to pass the transmitted and received information signals while at the same time attenuating the higher-frequency receiver blocking signals and transmitter overtones. Thus, the cut-off frequency of the low-pass filter 390 will differ from that of low-pass filter 140.

Also note that the combination of low-pass filter 390 and band-pass filter 330 together achieve ' adequate receiver blocking signal attenuation without requiring that a complex RF filter, or a discrete RF trap, be used.

In other words, because additional filtering in the receiver signal processing path is obtained via the low- pass filter 390, the band-pass filter 330 can be implemented using an already existing RF filter such as that used to implement the band-pass filter 130 of Figure 1. Also, by strategically using the low-pass filter 390 to serve dual roles in the transmit and receive signal processing paths, the total part count of the transceiver is not increased relative to the prior art system of Figure 1.

Figure 4 is a block diagram of an exemplary cellular mobile radiotelephone system in which a transceiver constructed in accordance with the teachings of the present invention can be used. The system shows an exemplary base station 410 and a mobile 420. The base station 410 includes a control and processing unit 430 which is connected to a mobile switching center (MSC) 440 which in turn is connected to the public switched telephone network (not shown) . The base station 410 also includes a voice channel transceiver 450 and a control channel transceiver 460. The mobile 420 includes a voice and control channel transceiver 470 and a processing unit 480. The base station transceivers 450, 460 and the mobile transceiver 470 can be constructed in accordance with the teachings of the present invention.

In operation, the base station 410 for a cell includes a plurality of voice channels handled by voice channel transceiver 450 which is controlled by the control and processing unit 430. The control channel transceiver 460 is also controlled by the control and processing unit 430 and may be capable of handling more than one control channel. The control channel transceiver 460 broadcasts control information over the control channel of the base station or cell to mobiles locked to that control channel . The voice channel transceiver broadcasts the traffic or voice channels which can include digital control channel location information.

The mobile 420 periodically scans the control channels of base stations like base station 410 to determine which cell to lock on or camp to. The mobile 420 receives absolute and relative information broadcast on a control channel at its voice and control channel transceiver 470. Then, the processing unit 480 evaluates the received control channel information which includes characteristics of the candidate cells and determines which cell the mobile should lock to. The received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated. These adjacent cells are periodically scanned by the mobile 420 while it is monitoring the primary control channel to determine if there is a more suitable candidate. Additional information relating to specifics of mobile and base station implementations can be found in copending U.S. Patent Application Serial No. 07/967,027 entitled "Multi- Mode Signal Processing" filed on October 27, 1992 to P. Dent and B. Ekelund, which disclosure is incorporated herein by reference.

In sum, the present invention teaches efficient, controllable techniques for eliminating receiver blocking signals using standard transceiver components. Overly complex RF filters and unreliable passive components are not required. Nor is an increase in the overall number of transceiver components required. It will be appreciated that the present invention is not limited to the specific illustrative embodiments described herein. The scope of the invention, therefore, is defined by the claims which are appended hereto, rather than the foregoing description. All equivalents which are consistent with the meaning of the claims are intended to be embraced therein.

Claims

WHAT IS CLAIMED IS;

1. A radio transceiver, comprising: a transmitter for generating information signals to be transmitted by said transceiver; a receiver for processing information signals which are received by said transceiver; a low-noise amplifier connected to an input of said receiver for amplifying received signals prior to processing by said receiver; a band-pass filter connected to an input of said low-noise amplifier for filtering received signals prior to amplification by said low-noise amplifier; an antenna for transmitting and receiving signals generated and processed by said transmitter and said receiver, respectively; a bi-directional low-pass filter having a first port connected to said antenna for filtering signals which are transmitted and received on said antenna,- and a switch connected to a second port of said low- pass filter for alternately connecting said low-pass filter to an output of said power amplifier and to an input of said band-pass filter.

2. The radio transceiver of claim 1, wherein said band-pass filter has a filtering characteristic which prevents blocking signals received at said antenna from saturating said low-noise amplifier.

3. The radio transceiver of claim 1, wherein said low-pass filter has a filtering characteristic which prevents high-frequency blocking signals received at said antenna from saturating said low-noise amplifier.

4. The radio transceiver of claim 1, wherein said low-pass filter has a filtering characteristic which prevents high-frequency blocking signals received at said antenna from interfering with an intermediate frequency signal generated within said receiver.

5. The radio transceiver of claim 1 , f rther comprising a power amplifier disposed between the output of said transmitter and said switch for amplifying signals generated by said transmitter prior to transmission.

6. The radio transceiver of claim 5, wherein said low-pass filter has a filtering characteristic which attenuates harmonics generated by said power amplifier and by said transmitter.

7. A radio transceiver having a transmit signal processing path and a receive signal processing path, comprising: an antenna for transmitting signals generated by said transmit signal processing path and for receiving signals processed by said receive signal processing path; a switch for alternately connecting said antenna to an output of said transmit signal processing path and to an input of said receive signal processing path; a first filter, disposed between said antenna and said switch, for attenuating blocking signals received at said antenna; and a second filter, disposed in said receive signal processing path, for further attenuating said received blocking signals.

8. The radio transceiver of claim 7, wherein said received blocking signals include signals which occur outside a frequency band associated with said receive signal processing path and which compress signals within said receive signal processing path by saturating a low- noise amplifier disposed in said receive signal processing path.

9. The radio transceiver of claim 7, wherein said received blocking signals include signals which occur outside a frequency band associated with said receive signal processing path and which interfere with an intermediate frequency signal generated within said receive signal processing path by mixing with harmonics generated by a local oscillator disposed in said receive signal processing path.

10. The radio transceiver of claim 7, wherein said first filter also attenuates harmonics generated in said transmit signal processing path.

11. A mobile telephone, comprising: a transmitter for generating information signals to be transmitted by said transceiver; a receiver for processing information signals which are received by said transceiver; a low-noise amplifier connected to an input of said receiver for amplifying received signals prior to processing by said receiver; a band-pass filter connected to an input of said low-noise amplifier for filtering received signals prior to amplification by said low-noise amplifier,- an antenna for transmitting and receiving signals generated and processed by said transmitter and said receiver, respectively; a bi-directional low-pass filter having a first port connected to said antenna for filtering signals which are transmitted and received on said antenna; and a switch connected to a second port of said low- pass filter for alternately connecting said low-pass filter to an output of said transmitter and to an input of said band-pass filter.

12. The mobile telephone of claim 11, wherein said band-pass filter has a filtering characteristic which prevents blocking signals received at said antenna from saturating said low-noise amplifier.

13. The mobile telephone of claim 11, wherein said low-pass filter has a filtering characteristic which prevents high-frequency blocking signals received at said antenna from saturating said low-noise amplifier.

14. The mobile telephone of claim 11, wherein said low-pass filter has a filtering characteristic which prevents high-frequency blocking signals received at said antenna from interfering with an intermediate frequency signal generated within said receiver.

15. The mobile telephone of claim 11, further comprising a power amplifier disposed between the output of said transmitter and said switch for amplifying signals generated by said transmitter prior to transmission.

16. The mobile telephone of claim 15, wherein said low-pass filter has a filtering characteristic which attenuates harmonics generated by said power amplifier and by said transmitter.