CIRCUIT FOR REDUCTION OF CROSS MODULATION AND INCREASED BATTERY LIFE IN A BATTERY POWERED TRANSCEIVER

Abstract: A battery powered transceiver including a circuit (52) for increasing the bias current of a low noise amplifier (LNA) (50), in the receiver section of the transceiver, during the time that the transmitter of the transceiver is transmitting, and for reducing the bias current when the transmitter is not transmitting. The increased bias current causes the LNA (50) to operate in a highly linear mode and it is thus less susceptible to cross modulation when transmitting at high power in the presence of a high power "jammer". Since the high bias current level only exists when the transmitter is on (transmitting), current drain on the battery is reduced thus increasing standby time and talk time. In a preferred embodiment, a signal indicative of the state of the transceiver's transmitter causes a switch (Q24) to close only when the transmitter is transmitting, thereby inserting a resister (R12) in parallel with a biasing resister (R11) and raising the bias current of the low noise amplifier.

Description: CIRCUIT FOR REDUCTION OF CROSS MODULATION AND INCREASED BATTERY LIFE IN A BATTERY POWERED TRANSCEIVER BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to battery powered radios and is of particular advantage for reducing cross modulation in battery powered full duplex radio telephone transceivers. II. Description of the Prior Art The primary contributor to cross modulation distortion in the receiver section of a radio telephone is the transmitter of the same radio telephone. A full-duplex radio telephone is capable of transmitting and receiving at the same time. Because radio telephones are so small, the transmitter and the receiver are of necessity located very close to each other. It is unavoidable that energy, particularly high power level energy, transmitted by the radio telephone will leak into, and cause distortion of signals received by, the receiver portion of the radio telephone. In particular, the distortion will find its way to the input of the receiver section low noise amplifier (LNA) and the distortion will be amplified and will corrupt the desirable signals and information received by the receiver. Because of the increase in popularity and availability of portable radio telephones, it is now not uncommon to find competing systems and technologies sharing the same geographic territory. This gives rise to a situation of particular concern as illustrated in FIG. 1. A portable radio telephone 20 using CDMA (code division multiple access) technology may be located a relatively great distance D from its CDMA compatible base station (BTS) 10 and may at the same time be significantly closer (distance d) to a base station 30 of a competing non-CDMA technology. The CDMA base station 10, phone 20 and competing technology base station 30 each transmit at their respective frequencies fl, f2 and f3 via respective antennas 12, 22 and 32. Because the telephone 20 is relatively far from its base station 10 the signal transmitted by the CDMA base station will be very weak when received by the telephone. The phone 20 will detect this weak signal level and determine that in order to communicate with the base station 10 the phone will have to transmit (at f2) at a relatively high power. Due to the phone's proximity to the competing technology base station 30, the signal transmitted (at f3) by the competing technology base station will be received by the phone 20 at a relatively high power level. These two high power signals will cross modulate and generate distortion of relatively high power at frequencies that are within the frequency range of the receiver section of the phone. To illustrate, f2 which may be 1908.75 MHz, and f3 which may be 1987.5 MHz, will cross modulate to produce distortion at frequencies which lie within the frequency range 1985 MHz to 1990 MHz of the receiver portion of phone 20. This will produce unwanted distortion of the desirable received information as it is processed in the low noise amplifier of the phone 20. One approach to reducing cross modulation distortion is to bias the LNA to an operating point (a fixed point) that produces highly linear operation of the amplifier. This gives satisfactory performance (i.e. low distortion) even in the presence of high transmitted output power at f2 and f3. This approach may be implemented by use of a classic active bias circuit similar to that described in section 3.9 of Guillermo Gonzalez's book "Microwave Transistor Amplifiers", Prentice-Hall Inc., 1984 (ISBN 0-13- 581646-7). According to Gonzalez, the bias point (quiescent point) can be selected according to the expected operating conditions. Gonzalez suggests different fixed quiescent operating points depending on the anticipated operating conditions. For example, a first point (point A) is recommended for low noise and low power amplification. A second point (point B), reflecting a higher bias current, is recommended for operation at low noise and higher power gain. This second condition (point B) is substantially the situation illustrated in FIG. 1, i.e. a high power intercept point is required. Once the operating point is selected as taught by Gonzalez, it is fixed and the bias circuit is designed accordingly. This approach provides for relatively high bias current to be supplied to the amplifier. However, high bias current decreases the battery life. Consumers require longer battery life and consequent longer standby times and longer talk times. It would thus be an advantage to provide a method and apparatus that reduces the cross modulation distortion but at the same time provides for increased battery life. SUMMARY OF THE INVENTION The present invention provides a means for increasing the bias current of a LNA in the receiver portion of a transceiver when the transmitter portion of the transceiver is transmitting, and returning the bias current to a lower operating level when the transmitter is not transmitting. The level of the increased bias current is chosen such that the LNA operates in a highly linear fashion, thereby minimizing cross modulation, even in the presence of high transmit power. In a preferred embodiment of the invention, a trigger signal is chosen that is representative of the state of the transmitter, i.e. it indicates when the transmitter is transmitting and when it is not. When the transmitter is transmitting, the trigger signal activates a switch which places a resister in parallel with a resister that sets the bias current for the LNA. The effective resistance is thus lowered, which increases the bias current. In order to reduce the impact of rapid changes (i.e. pulsations at a frequency of about 10 Hz or higher) in the state of the transmitter, the trigger signal may be integrated before it is provided to the switch. By increasing the bias current to the LNA only when the transmitter is transmitting, the current drawn from the battery is reduced and battery life is prolonged, thereby increasing standby times and talk time. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the circumstance when cross modulation poses a severe problem for a portable radio telephone. FIG. 2 is a circuit schematic of a Low Noise Amplifier illustrating implementation of a preferred embodiment of the invention. FIG. 3 is a schematic of an alternate switching circuit for adjusting the bias current provided to the Low Noise Amplifier. DETAILED DESCRIPTION OF THE INVENTION In compact transceivers such as portable radio telephones with simultaneous transmit and receive capability (i.e. full duplex), the transmit circuitry and the receiver Low Noise Amplifier (LNA) are, of necessity, located proximate one another. This proximity causes unwanted cross modulation distortion. The high power energy emitted by the transmitter cross modulates with power from a nearby competing base station (sometimes called a "jammer") and couples into the LNA and is amplified, thus interfering with and degrading the quality of the received signal which is also amplified by the receiver LNA. A simplified circuit schematic diagram of a LNA, showing implementation of an embodiment of the invention in the form of bias switching circuit 70, is shown in FIG. 2. The LNA consists of a first stage LNA 50 and its associated bias circuit 52, a second stage LNA 56 and its associated bias circuit 58 and a mixer /downconverter 60 including a Local Oscillator (LO) 62 coupled to the output of the second stage LNA. The first stage LNA is connected to the second stage LNA through a filter 54 which is configured to pass signals in the receiver frequency range and to reject signals in the transmitter frequency range. The second stage LNA and its bias circuit are substantially similar to the first stage LNA and its bias circuit and therefore the details of the second stage LNA and its bias circuit are not shown. This allows for simplification of FIG. 2. Previous design approaches for reducing cross modulation distortion configured the bias circuit to set the bias current of Ql to a level which ensured satisfactory (i.e. linear) operation even in the most challenging conditions which occur when the transmitter is transmitting at high power. This resulted in the first stage LNA being continuously biased at a relatively high current level. The resultant drain on the transceiver battery significantly shortens battery life, reducing standby time and talk time. These previous design approaches resulted in selection of a fixed bias point which produced a continuous and relatively high current drain. In contrast, the approach of the present invention is to configure the bias circuit to provide as low a bias current as possible (i.e. on the order of 10 mA) that will give satisfactory operation when low distortion is not required, and to switch the bias current to a higher level, to a level sufficient (i.e. on the order of 20 mA) to produce satisfactory low distortion performance even in the presence of high transmitter power, only when required to do so, i.e. when the transmitter is transmitting. This approach means that the constant drain on the battery is lower, and is only increased during the time that the transmitter is transmitting. This results in longer battery life and increased standby time and talk time. The details of implementation of a preferred embodiment of the invention will now be described with reference to FIG. 2, which shows a low noise amplifier located within the receiver portion of a battery powered transceiver. The first stage LNA 50 of LNA 40 comprises a transistor Ql. The bias current for Ql is established by the bias circuit 52 and in particular by the values of R15 and Rll. Prior art LNA designs resulted in relatively low values for Rll in order to generate relatively high bias current. (As used in this description "relatively" refers to a comparison between values typical of the prior art and those values which result from implementation of the invention.) The output of the first stage LNA 50 is passed through a filter 54 which passes signals having a frequency within the receiver frequency range and rejects signals having a frequency within the frequency range of the transmitter portion of phone 20. The output of the filter 54 is input to the second stage LNA 56 and output to a mixer/downconverter 60. The resulting intermediate frequency (IF) signal RX_IF is further processed by other circuitry (not shown) within the transceiver. The present invention provides means for detecting the on/ off state of the transmitter of the radio telephone transceiver 20 and for increasing the bias current of the first stage LNA 50 only during the time that the transmitter is transmitting. An embodiment of the invention as illustrated in FIG. 2 uses a trigger signal (MSM_PA_ON) which provides a suitably reliable indication of the state of the transmitter. Thus the trigger signal must consistently indicate a first state whenever the transmitter is transmitting, and must consistently indicate a second state (different from the first state) whenever the transmitter is not transmitting. Any signal available in the transceiver which satisfies these requirements and whose selection does not adversely impact the operation of the transceiver, may be used as the trigger signal and provided as input to the bias switching circuit 70. In the preferred embodiment, the selected trigger signal was a digital signal, and one state was indicated by the voltage exceeding about 1.65 volts and the other state was indicated by the voltage being less than 1.65 volts. The bias switching circuit 70 comprises a transistor Q25 having an input port (its base) for receiving the trigger signal, a switching field effect transistor (FET) Q24 and resister R12. The collector of transistor Q25 is coupled to the gate of FET Q24. The source of FET Q24 is connected to one terminal of Rll and the drain of FET Q24 is connected to one terminal of R12. The other terminal of R12 is connected to the other terminal of Rll. Thus, FET Q24 is connected in series with resister R12, and FET Q24 and R12 together are connected in parallel with Rll. When the trigger signal indicates that the transmitter of phone 20 is transmitting, the transistor Q25 turns on and activates switching FET Q24 which closes and thereby connects resister R12 in parallel with resister Rll. This reduces the effective resistance setting the bias current of Ql, which causes the bias current to rise. The specific values of the circuit elements such as Rll and R12 are chosen so that the bias current is raised to a level which insures satisfactory (i.e. highly linear) operation of the first stage LNA 50 during the time that the transmitter is transmitting. This results in reduced cross modulation distortion. Since the bias current is only increased when the transmitter is transmitting, use of this bias switching circuit also results in decreased current drain from the transceiver battery. This in turn produces longer standby time and longer talk time. Alternatively, the bias current can be switched between two desired values by the use of any functionally equivalent circuit. Oner such functionally equivalent circuit is the circuit shown in Fig. 3. Rll (as shown in Fig. 2) has been replaced by two series connected resisters Ra and Rb where Ra + Rb = Rll. The switch (Q24) is connected in parallel across one of the resisters, e.g. Ra. When Q24 is triggered "on" it shorts Ra, reducing the effective resistance from Ra + Rb to Rb, thereby increasing the bias current provided to Ql. The actual values of Ra and Rb would be chosen to achieve the two different bias currents, e.g. 10 mA and 20 mA. Various other such equivalent circuits could readily be devised. Under certain circumstances the transmitter will turn on and off rapidly, i.e. at a frequency of about 50 Hz or more. If the bias current to the first stage LNA (Ql) is allowed to change at the same frequency or higher, undesirable transient effects will occur. To eliminate the transient effects, Q25 may be operated as an integrator circuit. Thus R10, CH and Q25 are configured as an integrator, having a time constant of about 0.47 seconds, to smooth out the trigger signal and hold Q24 on (i.e. in the closed state) if the trigger signal is pulsating at a frequency of about 10 Hz or higher. At lower frequencies, the pulsating trigger signal will be allowed to cause the switching FET Q24 to also turn on and off at the same pulsating rate. In a specific implementation of the embodiment of the invention shown in Fig. 2: Ql = BFP420 (Siemens) Q2 = UMT1NTN (ROHM) Q24 = IRLML 2803 (International Rectifier)or 182-18323-0000 Q25 = DTC143EE (ROHM) CH = 4.7 μF RIO = 100KΩ Rll = 39 KΩ R12 = 62 KΩ R15 = 33 KΩ There has thus been described apparatus for increasing the bias current of a low noise amplifier of the receiver section in a transceiver only during the time that the transmitter of the transceiver is transmitting. The amplifier is thus less susceptible to cross modulation and the current drain on the battery is reduced. A preferred embodiment has been described in sufficient detail to enable one skilled in the art to make and use the invention. However, it should be understood that various changes and modifications may be made by a person skilled in the art without exercise of the inventive faculty, and such modifications and changes are intended to be included within the scope of the invention which is set forth in the appended claims. WHAT IS CLAIMED IS:

Claims: CLAIMS 1. A transceiver including a transmit portion and a receive portion, said receive portion including an amplifier; said transceiver comprising: means for providing a first indication when said transmit portion is transmitting, and a second indication, different from said first indication, when said transmit portion is not transmitting; and switch means responsive to said means for providing, said switch means for increasing a bias current to said amplifier when said means for providing indicates that said transmit portion is transmitting, and for reducing said bias current to said amplifier when said means for providing indicates that said transmit portion is not transmitting. 2. The transceiver according to claim 1 wherein said means for providing comprises: a trigger signal having a first amplitude when said transmit portion is transmitting and a second amplitude when said transmit portion is not transmitting. 3. The transceiver according to claim 2 wherein said means for providing further comprises an integrator circuit having an input port for receiving said trigger signal, said integrator circuit providing an output signal which is a smoothed representation of said trigger signal. 4. The transceiver according to claim 1 wherein said amplifier includes a first resister for setting the bias current of said amplifier and wherein said switch means comprises: a switch in series connection with a second resister, said switch and second resister being connected in parallel with said first resister whereby upon closure of said switch said second resister is connected in parallel with said first resister. 5. The transceiver according to claim 4 wherein said switch is a transistor. 6. The transceiver according to claim 5 wherein said transistor is a field effect transistor having its gate coupled to the output of the integrator, its source coupled to said first resister and its drain coupled to said second resister. - 7. The transceiver according to claim 1 wherein said amplifier includes a first resister in series connection with a second resister for setting the bias current of said amplifier and said switch means comprises: a switch connected in parallel across one of said first and second resisters whereby closure of said switch shorts said one resister. 8. A method for reducing cross modulation and increasing battery life of a battery powered transceiver including an amplifier, a transmitter portion and a receiver portion, comprising the steps of: providing a first indication when said transmitter portion is transmitting; providing a second indication, different from said first indication, when said transmitter is not transmitting; and increasing bias current to said amplifier in response to said first indication, and decreasing bias current to said amplifier in response to said second indication.

Inventors: CAREY JOSEPH M, TAN CHENG FONG, CAREY, JOSEPH, M., TAN, CHENG, FONG
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 1999
Main classification: H04B1
IPC codes: H04B1