TRANSCEIVER STAGE FOR A MOBILE TELECOMMUNICATIONS UNIT

Abstract: A transceiver stage for a mobile telecommunications unit, preferably a handheld telephone unit, utilising CDMA modulation technique comprises an RF receiving signal input (Rx) for receiving an RF signal from an antenna (12); a first amplifier (14) for amplifying said received RF signal; and a first attenuation means (18) for attenuating the RF signal. The first attenuation means (18, 40) is provided at or after the first amplifier (14).

Description: Transceiver stage for a mobile telecommunications unit Description The present invention is related to a transceiver stage for a mobile telecommunications unit, preferably a handheld telephone utilising CDMA modulation technique. One problem related to handheld mobile telephone units utilising CDMA modulation technique is the fact that such CDMA telephone systems are co-existing with analogue cellular systems in the same frequency band. In case of interferences caused by analogue cellular systems, the CDMA mobile telephone drops the telephone call. The reasons for this are intermodulation distortions produced by two or more analogue signals causing and in-band (received pass band) signal which extinguishes the wanted received CDMA signal. Moreover, a single analogue signal at +/- 900 kHz offset from the wanted received CDMA signal can crossmodulate with the signal transmitted by the handheld mobile telephone unit. An article in RF DESIGN, December 1996, p. 46 - 53 by Dick Bain, "Reducing IM distortion in CDMA cellular telephones" suggests to provide an attenuator between the RF input and the input of a subsequent low noise amplifier. However, this approach has the significant disadvantage that noise is introduced into the RF signal by the attenuator. Subsequently, not only the RF signal, but also the noise is amplified by the low noise amplifier. Moreover and more important, the sensitivity of the RF input path is drastically reduced by this approach. Finally, the second of the two problems mentioned above is not solved by this approach. The present invention suggests a solution to overcome the above problems. In particular, the invention teaches a transceiver stage for a mobile telecommunications unit, preferably a handheld telephone unit, utilising CDMA modulation technique; said transceiver stage comprising: a RF receiving signal input for receiving an RF signal from an antenna; a first amplifier for amplifying said received RF signal; and an attenuation means for attenuating the RF signal; wherein said attenuation means is provided at or after the first amplifier. Surprisingly, the fact that the attenuation means is provided at or after the first amplifier not only solves both of the above problems. Rather, it also significantly reduces the introduction of noise while at the same time not effecting the sensitivity of the RF signal path. In other words, the invention provides for an attenuation of the received signal either in the first amplifier by means describes more specifically hereinafter, or the received signal is attenuated by suitable means downstream of the first amplifier. For this alternative solution, suitable means are also described hereinafter. The concept underlying the invention is that the (controlled) reduction of the gain or the (controlled) attenuation of the received signal reduces or eliminates intermodulation products. The reduction of the gain or the attenuation is also carried out to maintain the frame error rate in the receiver below a predetermined level. An important aspect of the invention is to utilize a low noise amplifier (LNA) having a high linearity. This means also, that the low noise amplifier (LNA) has a high third- order intercept point (IP 3 ) . The higher the intercept point, the lower the level of intermodulation distortion products generated for a given level of input tones. An increase of the third-order intercept point to further reduce the distortion results also in a higher supply current. Therefore, it is not possible to raise the third-order intercept point into a region, where no intermodulation distortion products would not create problems in a CDMA system. An important aspect of the invention is the fact that said first attenuation means comprises a variable attenuator. The attenuation factor of the variable attenuator is controlloed in a manner described in greater detail hereinafter. A first way to implement the invention is to have said first attenuation means comprising a switch means to control the operation of said first amplifier. This switch means can either be provided to allow the RF signal to bypass said first amplifier. Or, said switch means is provided to control the power supplied to said first amplifier. According to one aspect of the invention, said first attenuation means is controlled by a signal indicative of the signal strength of the received RF signal. This signal is generated by a circuit already present in the receiver circuit. Hence, this does not increase the overall complexity. As an alternative, said first attenuation means can be controlled by a signal indicative of the error rate of the received signal, preferably the frame error rate of the baseband information contained in the received signal. This signal is also generated by a circuit already present in the receiver circuit. Yet another solution according to the invention teaches that said first attenuation means is controlled by a signal being comprised of the signal strength of the received RF signal and the error rate of the received signal, preferably the frame error rate of the baseband information contained in the received signal. Using the combination of these two signals to control the attenuation can avoid losses of calls in situations where the use of only one of said two signals might not be sufficient or safe. If, for example, the signal strength of the received RF signal is used to control the attenuator, the signal strength of the received signal does not change if the mobile unit is not moving relative to the base station. Hence, the presence of a jamming transmitter can not be compensated by using the signal strength of the received RF signal. In one embodiment, the transceiver stage comprises a first filter means, the input of said first filter means being connected to the output of said attenuation means, and the output of said first amplifier being connected to the input of said attenuation means. In this embodiment, a second amplifier can be present, the input of said second amplifier being connected to the output of said first filter means. In a further embodiment of the invention, a first filter means can be present, the input of said first filter means being connected to the output of said first amplifier; said attenuation means is provided downstream of an output of said first filter means; and said first filter means is preferably a surface acoustical wave filter (SAW) . In this embodiment, a second amplifier can be provided, the input of said second amplifier being connected to the output of said attenuation means, and said second amplifier is preferably a low noise amplifier. Further, the transceiver stage according to the invention can comprise a second attenuator, the input of said second attenuator being connected to the output of said second amplifier. Said second attenuator is preferably a fixed attenuator. Further, the transceiver stage according to the invention can comprise a second filter means, the input of said second filter means being connected to the output of said second attenuator. Said second filter means is preferably a surface acoustical wave filter (SAW) . Further, the transceiver stage according to the invention can comprise a signal mixer, having two inputs, a first input of said signal mixer being connected to the output of said second filter means, a second input of said signal mixer being connected to the output of a local oscillator. The output of said mixer is preferably a symmetrical output. Moreover, the transceiver stage according to the invention can comprise a third filter means, the input of said third filter means being connected to the output of said signal mixer. Said third filter means is preferably a surface acoustical wave filter (SAW) . Furthermore, the transceiver stage according to the invention can comprise a fourth amplifier, the input of said fourth amplifier being connected to the output of said third filter means. Said fourth amplifier is preferably a gain control amplifier. Moreover, the transceiver stage according to the invention can comprise a fourth filter means, the input of said fourth filter means being connected to the output of said fourth amplifier. Said fourth filter means is preferably a passive network filter. Furthermore, the transceiver stage according to the invention can comprise a demodulator to obtain the baseband information contained in the received signal, the input of said demodulator being connected to the output of said fourth filter means. The demodulator provides a first output signal (Q) being a quadrature-phase (90°) signal and a second output signal (I) being an in-phase (0°) signal. The transceiver stage according to the present invention comprises a signal processor to process the baseband information obtained by said demodulator, the inputs of said signal processor are connected to the outputs of said demodulator. Said signal processor provides a first control signal indicative of the signal strength of the received RF signal, and a second signal indicative of the error rate of the received signal, preferably the frame error rate of the baseband information contained in the received signal. Finally, the present invention is related to a cellular mobile telephone unit comprising a transceiver stage having one or more of the above described features. Especially, the cellular mobile telephone unit according to the present invention has the (variable) attenuator at or after the first (low noise) amplifier. Moreover, the cellular mobile telephone unit according to the present invention has a second (low noise) amplifier. Further features, characteristics, advantages and variations of the present invention become appearent to a person skilled in the art from the following detailed description referring to the drawings. Fig. 1 shows a schematic drawing of a first embodiment of a transceiver stage for a mobile telecommunications unit, utilising CDMA modulation technique, according to the present invention. Fig. 2 shows a schematic drawing of a detail of a second embodiment of the transceiver stage according to the present invention. Fig. 3 shows a schematic drawing of a detail of a third embodiment of the transceiver stage according to the present invention. In the drawings, Fig. 1 shows a transceiver stage for a handheld mobile telecommunications unit utilising Code Division Multiple Access (CDMA) modulation technique. The transceiver stage comprises duplexer 10 being connected to an antenna 12 and feeding a RF receiving signal input Rx received from the antenna 12. The input of a first amplifier 14 connected to the RF receiving signal input Rx and is provided for amplifying the received RF signal. The first amplifier 14 is devised as a low noise amplifier (LNA) . The input of a first band-pass filter means 16 is connected to the output of the first (low noise) amplifier 14. The first filter means 16 is a surface acoustical wave filter (SAW) having a bandwidth of 25 MHz. The input of a first attenuation means 18 is connected to the output of the first band-pass filter 16. The first attenuation means 18 is provided to attenuate the RF signal. The first attenuation means 18 is devised to be a variable attenuator. A more detailed description thereof is given hereinafter with reference to Fig. 2, 3. The first attenuator 18 is controlled by a control signal 20 being generated from a first signal which is indicative of the signal strength of the received RF signal and from a second signal which is indicative of the frame error rate of the baseband information contained in the received RF signal. Alternatively, the control signal 20 can be indicative of the signal strength of the received RF signal only. Further alternatively, the control signal 20 can be indicative of the error rate of the received signal only. Preferably this is the frame error rate of the baseband information contained in the received RF signal. In an alternative embodiment (not shown in greater detail here) , the input of the first filter means 16 can be connected to the output of attenuation means 18, and the output of the first amplifier 14 is then connected to the input of attenuation means 18. The input of a second amplifier 22 is connected to the output of the vairable attenuator 18. The second amplifier 22 is also a low noise amplifier. The input of a second attenuator 24 is connected to the output of the second amplifier 22. This second attenuator 24 is a fixed attenuator adjusted to 8 dB. The input of a second band-pass filter means 26 is connected to the output of the second attenuator 24. This second filter means 26 is also a surface acoustical wave filter (SAW) having a bandwidth of 25 MHz. As such, it is identical to the first band-pass filter 16. A signal mixer 28 following the second filter 26 has two inputs. The first input 28a of the signal mixer 28 is connected to the output of the second filter 26. A second input 28b of the signal mixer is connected to the output of a local oscillator 28c. This local oscillator 28c has a constant frequency equalling the sum of the frequency of the received RF and of an intermediate frequency IF. Hence, the output signal of the signal mixer 28 has the frequncy of the intermediate frequency IF. In present CDMA technology, the intermediate frequency IF is 85.38 MHz. As can be seen from Fig. 1, the output of the signal mixer 28 is a symmetrical output . The input of a third band-pass filter means 30 is connected to the output of the signal mixer 28. The third filter means 30 is also a surface acoustical wave filter (SAW) and has a bandwidth of 1.2 MHz. The input of a fourth amplifier 32 is connected to the output of the third band-pass filter 30. This fourth amplifier is an automatic gain control amplifier (AGC) controlling the signal level of the intermediate frequency (IF) received signal to maintain a constant level of -57 dBm. The input of a fourth band-pass filter means 34 is connected to the output of the fourth (AGC) amplifier 32. The fourth filter 34 is a passive network filter allowing the intermediate frequency signal to pass. Following the filter 34, a demodulator 36 is provided to obtain the baseband information contained in the received signal. The input of the demodulator 36 is connected to the output of the fourth filter 34. The demodulator 36 generates a first output signal (Q) being a quadrature-phase (90°) signal and a second output signal (I) being an in-phase (0°) signal. The inputs of a signal processor 38 are connected to the outputs of the demodulator 36. This signal processor 38 is provided to process the baseband information obtained by said demodulator 36. Amongst other output signals like the actual baseband information (BBI) , the signal processor 38 provides the first control signal 20 used to control the first attenuator 18. This first control signal 20 is generated to control the first attenuator 18 in the above-described manner. As already mentioned, the signal on control line 20 can either be indicative of the received signal strength or of the frame error rate or be a combination of both. Moreover, the signal processor 38 provides a second control signal RSS (received signal strength) used to control the fourth (AGC) amplifier 32 to maintain the required IF signal level. The signal processor 38 also provides the base band information BBI to be further processed by subsequent circuitry of the telephone unit. The variable attenuation of the received signal protects the second amplifier 22, the mixer stage 28 and the automatic gain control amplifier 32 from high signal levels. This effect reduced the level of intermodulation products. Thus, it is (only) essential that the first amplifier has a high third order interception point. Due to the variable attenuation of the received signal in dependance from the received signal strength and/or the frame error rate, a dynamic control of the received signal is provided. A main advantage of the present invention lies in the fact that calls received by CDMA telephone units endowed with the present invention can be maintained despite the presence of strong analogue signals. In Fig. 2, a detail of a transceiver stage according to the invention is shown. In this embodiment, the first attenuation means comprises a switch means 40 to control the operation of the first amplifier 14. In other words, the signal on line 20 controls whether the received signal at the input of the first amplifier 14 is fed into the first amplifier 14 and thereby amplified (switch 40 is in it open position) , or whether the received RF signal actually bypasses the first amplifier 14 through the closed switch 40. In addition the the switch 40, there can be an attenuator 41 in the bypass. In Fig. 3, a detail of another embodiment of the transceiver stage according to the invention is shown. In this embodiment, the first attenuation means also comprises a switch means 40 to control the operation of the first amplifier 14. In other words, Here however, the switch means 40 is provided to control the power supplied to the first amplifier 14. In the shown position of the switch means 40, the first amplifier 14 is connected to the power supply V+. Therefore, the first amplifier 14 operated normally. In the other position of the switch means 40, controlled by the signal on line 20, the first amplifier 14 is cut off from the power supply. At the same time, the attenuator means formed by a PIN diode 42 is activated. This PIN diode 42 is connected to the output of the first amplifier 14. A resistor 44 connected in series with the PIN diode 42 defines the current flowing through the PIN diode 42. Thus, the resistance of the PIN diode 42 as seen by the RF signal at the output of the first amplifier 14 is determined. The coil 46 is provided to avoid spurious RF signals. In the embodiments of Figs. 2, 3, the control signal on line 20 is used in a "digital" mode. That is, the control signal switches a certain attenuation on or off. As an alternative thereto, the control signal can also be used in an "analogue" or "continuous" mode, as is shown in Fig. 1. That is, instead of simply switching the attenuator (or the amplifier) on or off, the degree of attenuation could be varied in accordance with the level of the control signal on line 20. Both, the received signal strength and the frame error rate vary continuously. Hence, also the control signal on line 20 can vary accordingly. This alternative is also shown in Fig. 4. Here, the amplifier 14 is powered constantly. However, the resistor 44 is not connected to a constant voltage (as in Fig. 3) . Rather, the control signal on line 20 controls the current through the PIN diode 42 and hence the resistance of the PIN diode 42 as seen by the RF signal at the output of the first amplifier 14.

Claims: CLAIMS 1. Transceiver stage for a mobile telecommunications unit, preferably a handheld telephone unit, utilising CDMA modulation technique; said transceiver stage comprising: - a RF receiving signal input (Rx) for receiving a RF signal from an antenna (12); - a first amplifier (14) for amplifying said received RF signal; and - a first attenuation means (18) for attenuating the RF signal; wherein - said first attenuation means (18, 40) is provided at or after the first amplifier (14) . 2. Transceiver stage according to claim 1 wherein said first attenuation means comprises a variable attenuator (18) . 3. Transceiver stage according to claim 1 wherein said first attenuation means comprises a switch means (40) to control the operation of said first amplifier (14). 4. Transceiver stage according to claim 3 wherein said switch means (40) is provided to allow the RF signal to bypass said first amplifier (14). 5. Transceiver stage according to claim 3 wherein said switch means (40) is provided to control the power supplied to said first amplifier (14). 6. Transceiver stage according to claim 1 wherein said first attenuation means (18) is controlled by a control signal (20) indicative of the signal strength of the received RF signal. 7. Transceiver stage according to claim 1 wherein said first attenuation means (19) is controlled by a signal indicative of the error rate of the received signal, preferably the frame error rate of the baseband information contained in the received RF signal. 8. Transceiver stage according to claim 1 wherein said first attenuation means (18) is controlled by a signal being comprised of the signal strength of the received RF signal and the error rate of the received signal, preferably the frame error rate of the baseband information contained in the received signal. 9. Transceiver stage according to any of the previous claims, further comprising: - a first filter means (16), the input of said first filter means (16) being connected to the output of said attenuation means (18), and the output of said first amplifier (14) being connected to the input of said attenuation means (18) . 10. Transceiver stage according to any of claims 1 - 9, comprising: - a first filter means (16), the input of said first filter means (16) being connected to the output of said first amplifier (14) ; wherein - said attenuation means (18) is provided downstream of an output of said first filter means (16); and wherein - said first filter means (16) is preferably a surface acoustical wave filter (SAW) . 11. Transceiver stage according to claim 10, comprising: - a second amplifier (22) , the input of said second amplifier (22) being connected to the output of said attenuation means (18) , and wherein said second amplifier (22) is preferably a low noise amplifier (LNA) . 12. Transceiver stage according to claim 9, comprising: - a second amplifier (22) , the input of said second amplifier (22) being connected to the output of said first filter means (16) . 13. Transceiver stage according to claim 11 or 12, comprising: a second attenuator (24) , the input of said second attenuator (24) being connected to the output of said second amplifier (22) , and wherein said second attenuator (24) is preferably a fixed attenuator (PAD) . 14. Transceiver stage according to claim 13, comprising: - a second filter means (26) , the input of said second filter means (26) being connected to the output of said second attenuator (24), and wherein said second filter means (26) is preferably a surface acoustical wave filter (SAW) . 15. Transceiver stage according to claim 14, comprising: - a signal mixer (28) , having two inputs (28a, 28b) , a first input of said signal mixer (28a) being connected to the output of said second filter means (26) , a second input (28b) of said signal mixer (28) being connected to the output of a local oscillator (LO) , and wherein the output of said mixer (28) is preferably a symmetrical output. 16. Transceiver stage according to claim 15, comprising: - a third filter means (30) , the input of said third filter means (30) being connected to the output of said signal mixer (28) , and wherein said third filter means (30) is preferably a surface acoustical wave filter (SAW) . 17. Transceiver stage according to claim 16, comprising: - a fourth amplifier (32) , the input of said fourth amplifier (32) being connected to the output of said third filter means (30) , and wherein said fourth amplifier (32) is preferably a gain control amplifier (AGC) . 18. Transceiver stage according to claim 17, comprising: - a fourth filter means (34), the input of said fourth filter means (34) being connected to the output of said fourth amplifier (32) , and wherein said fourth filter means (34) is preferably a passive network filter. 19. Transceiver stage according to claim 18, comprising: a demodulator (36) to obtain the baseband information contained in the received signal, the input of said demodulator (36) being connected to the output of said fourth filter means (34) , and wherein the demodulator (36) provides a first output signal (Q) being a quadrature-phase (90°) signal and a second output signal (I) being an in-phase (0°) signal containing said base band information. 20. Transceiver stage according to claim 19, comprising: a signal processor (38) to process the baseband information obtained by said demodulator (36) , the inputs of said signal processor (38) being connected to the outputs of said demodulator (36) , and wherein said signal processor (38) provides - a first control signal indicative of the signal strength of the received RF signal (RSS) , and a second signal indicative of the error rate of the received signal, preferably the frame error rate (FER) of the baseband information contained in the received signal. 21. Cellular mobile telecommunications unit utilising CDMA technology, comprising a transceiver stage having the features of one or more of the previous claims.

Inventors: BOCCHETTA MARCO, MILLS STUART, BOCCHETTA, MARCO, MILLS, STUART
Applicants: BOCCHETTA MARCO, MAXON SYSTEMS INC LONDON LTD, MILLS STUART, BOCCHETTA, MARCO, MAXON SYSTEMS INC. (LONDON) LTD., MILLS, STUART
Application year: 1997
Main classification: H03G3
IPC codes: H03G3, H04B1