Full duplex transceiver having a method for immunizing itself against self-jamming

A full-duplex transceiver using a method immunizing itself against self-jamming. The transceiver includes a receiver and a transmitter. The receiver includes a frequency immunization converter and a high pass IF filter. The transmitter transmits a TX signal. The receiver receives an RX signal and simultaneously receives a portion of the power of the TX signal as an undesired TX jamming signal. The frequency immunization converter uses the center frequency of the TX signal for downconverting the RX signal to an IF signal and simultaneously downconverting the TX jamming signal to near zero frequency. The high pass IF filter passes the IF signal and blocks the signal at near zero frequency. As a consequence of the downconversion using the TX frequency, a second LO frequency is controlled for avoiding image frequencies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to full duplex transceivers and more particularly to a full-duplex transceiver having a method for immunizing itself against self-jamming.

2. Description of the Prior Art

A transceiver can be designed as full duplex or half duplex. A full duplex transceiver receives and transmits signals simultaneously. In contrast, a half duplex transceiver receives signals during different time periods than it transmits signals. In a communication system, the simultaneous transmission and reception of signals results in a full duplex transceiver having a higher information rate than a half-duplex transceiver. However, a full duplex receiver is susceptible to a problem called “self-jamming”.

Self-jamming occurs when a portion of the power of the transmit signal crosses over into the receiver. Because the power level in the transmit signal is so much greater than the power level in the receive signal, even a small portion of the transmit signal power can overwhelm the receive signal and jam the receiver so that the receiver cannot properly receive the receive signal.

Existing full duplex transceivers attempt to resolve the self-jamming problem by transmitting and receiving signals in different frequency bands and then filtering the input to the receiver to pass signal frequencies in the receive frequency band while suppressing signal frequencies in the transmit frequency band. However, practical filters can never suppress 100% of the transmit signals and filters designed to approach 100% suppression are difficult and costly to produce. Moreover the transmit and receive frequency bands may be adjacent or nearly adjacent in order to meet licensing regulations, thereby further increasing the costliness and difficulty of producing the filters. Surface acoustic wave (SAW) filters are used in order to provide the very sharp transitions required for filtering adjacent or nearly adjacent frequency bands. However, SAW filters have a disadvantage that they cannot be integrated into an integrated circuit with standard integrated circuit technology.

Another approach for resolving the self-jamming problem involves the use of separate antennas for transmitting and receiving signals. However, this approach is not entirely successful because it increases cost, and the receive antenna inevitably receives some portion of the high power transmit signals through the air from the transmit antenna. Other approaches involve the use of directional devices, such as isolators, circulators, hybrid couplers and the like, for separating the transmit signals from the receive signals in a shared antenna. However, these approaches are only partially successful because limitations in the directivity of the directional devices and impedance mismatches in the directional device and antenna cause some portion of the transmit signal to cross over into the receiver. Typically, a combination of these approaches is used. However, none of the approaches or combinations tried to date has been entirely successful at a reasonable cost.

There continues to be a need for resolving the problem of self-jamming in a full duplex transceiver.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method and apparatus for suppressing self-jamming in a full duplex transceiver by using information for the center frequency of the transmit (TX) signal for immunizing the receiver.

Briefly, in a preferred embodiment, a full duplex transceiver of the present invention includes a receiver and a transmitter. The receiver receives a desired receive (RX) signal at an RX frequency and simultaneously receives a portion of the power of a transmit (TX) signal from the transmitter as an undesired TX jamming signal. The receiver includes a frequency immunization converter and a high pass filter. The frequency immunization converter uses the center frequency of the TX signal for downconverting the RX signal to an intermediate frequency (IF) signal at an IF frequency and simultaneously downconverting the TX jamming signal to a jamming residual at near to zero frequency. The high pass filter passes the IF signal and blocks the jamming residual. Preferably, the receiver also includes a second local signal generator that is switched between second LO frequencies above and below a first IF frequency; and one or more selectable inverters that are switched for inverting or non-inverting first and/or second IF signals. The second LO frequencies and the inversions are controlled by programming in a microprocessor system according to the RX frequency and the TX center frequency in order to avoid certain undesired RX image frequencies.

An advantage of the present invention is that the frequency immunization converter enables a simple and inexpensive filter to be used in place of more complex and costly filters for suppressing a self-jamming signal. Another advantage of the present invention is that the filter for suppressing self-jamming can be integrated into an integrated circuit using standard integrated circuit technology. Another advantage of the present invention is that undesired image frequencies in the RX signal are avoided or suppressed by selectably switching a second LO frequency and inverting or non-inverting IF signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a block diagram of a full duplex transceiver of the of the present invention referred to by a general reference number10. The transceiver10includes an antenna12, a splitter14, a transmitter16, and a receiver20. In a preferred embodiment, a microprocessor system22is shared and included in both the transmitter16and the receiver20.

The transmitter16receives input information24in a baseband signal; modulates the input information24onto a carrier; and issues the modulated carrier in a transmit (TX) signal26to the splitter14. The splitter14passes the TX signal26as a TX signal26A to the antenna12. The antenna12converts the TX signal26A from a conducted to a radiated form and issues a radiated TX signal26B as an airwave. Simultaneously, the antenna12receives a receive (RX) signal30as an airwave and converts the airwave RX signal30into a conducted RX signal30A. The splitter14receives the conducted RX signal30A and passes it to the receiver20as an RX signal30B. The receiver20demodulates the RX signal30B and issues a baseband signal having output information32that is representative of the input information24.

The TX signal26B is transmitted in a TX frequency channel within a TX frequency band. At the same time, the RX signal30is received in an RX frequency channel within an RX frequency band. In some instances the TX signal26B and/or the RX signal30are further confined into subchannels. At any one time the TX and RX frequency channels and/or subchannels are separate. In a preferred embodiment the TX signal26B and the RX signal30are transmitted and received, respectively, in time slots of a time division multiple access (TDMA) communication plan.

The splitter14preferably includes a diplexer or separate filters so that signal frequencies in the TX frequency channel are passed from the transmitter16to the antenna12but are not passed to the receiver20. The splitter14may also include one or more directional devices, such as a isolators, circulators, directional couplers and the like, for preventing the transmit signals26,26A, and26B from coupling into the receiver20. However, limitations in the stop band signal suppression of the filters; limitations of the directivity in the directional devices; impedance mismatches and other imperfections in the splitter14and/or antenna12; and/or crosstalk from the transmitter16to the receiver20inevitably cause some portion of the TX signal26to leak into the receiver20as an undesired TX jamming signal34.

The TX jamming signal34has the same signal frequencies and center frequency as the TX signal26. If the signal level of the undesired TX jamming signal34is too strong with respect to the signal level of the desired RX signal30B, the receiver20is “jammed”, meaning that the receiver20is partially or completely overwhelmed by the TX jamming signal34. When the receiver20is jammed, its ability to provide the output information32that is a correct representation of the input information24is impaired or eliminated.

The transmitter16includes one or more signal generators having frequencies that are controlled by frequency data from the microprocessor system22for setting the center frequency of the TX signal26. This center frequency, termed herein a TX center frequency38, places the TX signal26in a selected TX frequency channel or subchannel. The TX center frequency38may change or hop as the TX signal26changes or hops among channels or subchannels. However, at any one time there is a current frequency for the TX center frequency38. Typically, the TX center frequency38is the carrier frequency, even when the carrier is suppressed, of the TX signal26. However, in the case of single sideband transmission the TX center frequency38is offset by a known frequency from the carrier frequency.

In the present invention, the receiver20uses information for the TX center frequency38for immunizing itself from jamming by the TX jamming signal34even when the TX jamming signal34is relatively large with respect to the RX signal30B. Preferably, the information used by the receiver20for the TX center frequency38is in the form of data from the microprocessor system22. The microprocessor system22computes this data for the TX center frequency38in the same manner as it computes the frequency data that it provides to the transmitter16for controlling the frequencies of the signal generators in the transmitter16. However, in an alternative embodiment the receiver20uses the actual local signals from the one or more signal generators in the transmitter16or a combination of the signals and microprocessor data for the TX center frequency38. In some cases, the use of the actual signals is preferred in order to reduce cost by using some of the same signal generator hardware for both the transmitter16and the receiver20.

FIG. 2is a block diagram of the receiver of the present invention referred with the reference number20. The receiver20includes a bandpass filter52, a low noise amplifier54, a frequency immunization converter60, a high pass filter62, and an intermediate frequency (IF) receiver64.

The receiver20receives the desired RX signal30B and simultaneously receives the undesired TX jamming signal34. The bandpass filter52at the front end of the receiver20passes signal frequencies within a prescribed RX frequency band for the RX signal30and suppresses signals having frequencies outside this band. In an alternative embodiment, the function of the bandpass filter52is included within the splitter14(FIG. 1). The bandpass filter52passes a filtered RX signal30C to the low noise amplifier54. The low noise amplifier54amplifies the filtered RX signal30C and passes a filtered amplified RX signal30D to the frequency immunization converter60.

The bandpass filter52provides some suppression of the TX jamming signal34with respect to the RX signal30B. However, because the TX jamming signal34may be very strong and because 100% suppression of out of band signals is not possible, a portion of the TX jamming signal34passes through the bandpass filter52as a partially suppressed TX jamming signal34A. The low noise amplifier54amplifies the TX jamming signal34A and passes an amplified TX jamming signal34B to the frequency immunization converter60. It should be noted that the filter52must have sufficient dynamic range for filtering the RX signal30B in the presence of the TX jamming signal34, the low noise amplifier54must have sufficient dynamic range for amplifying the RX signal30C in the presence of the TX jamming signal34A, and the frequency immunization converter60must have sufficient dynamic range for frequency converting the RX signal30D in the presence of the TX jamming signal34B.

The frequency immunization converter60converts the signal frequencies of the RX signal30D to an intermediate frequency (IF) signal66at frequencies equal to the difference between frequencies of the RX signal30D and the TX center frequency38and simultaneously converts the TX jamming signal34B to a residual jamming signal68having a center frequency of approximately zero frequency (DC). Preferably, the IF signal66has a relatively wide frequency range in order to accommodate a relatively wide RX frequency band. The high pass filter62passes the IF signal66as an IF signal66A and blocks the residual jamming signal68.

The high pass filter62must have a sufficiently sharp frequency cutoff to effectively eliminate the residual jamming signal68while passing the IF signal66. It is much less difficult to produce the high pass filter62having this sufficiently sharp cutoff than it would be to produce the bandpass filter52having a sufficiently sharp cutoff because there is a greater ratio of the frequencies of the IF signal66to the residual jamming signal68than the ratio of the frequencies of the RX signal30B to the TX jamming signal34.

In order to mix the TX jamming signal34B to near zero frequency is not required that the TX center frequency38be exactly the center of the TX signal26but only that it be close enough that the ratio is large enough between the frequency of the IF signal66and the near zero frequency of the jamming residual signal68for the high pass filter62to substantially eliminate the residual jamming signal68.

The ratio of the lower edge of the lowest IF signal66to the upper edge of the residual jamming signal68, in a preferred embodiment, should be three (3) or greater. As an example assume the following: a bandwidth of K for the TX signal26; a difference of L between the true center frequency of the TX signal26and the TX center frequency38; and a bandwidth of M for the RX signal30. The center of the near zero frequency of the residual jamming signal68is L. For the ratio of three (3), the lowest allowable center frequency for the IF signal66is calculated by 3×(L+K/2)+M/2. In a numerical example, for K=6 MHz, L=2 MHz and M=6 MHz, the lowest center frequency for the IF signal66is 18 MHz and the high pass filter62should be designed to suppress signal frequencies up to 5 MHz and pass signal frequencies above 15 MHz. However, in order to avoid a need for fine frequency resolution and to include a margin for filter variation, the lowest center frequency of the IF signal66in the numerical example is preferably about 24 MHz for a ratio of (24−3)/(2+3)=4.2.

Alternatively, the frequency immunization converter60can use a TX offset frequency381having a known frequency offset from the TX center frequency38for converting the TX jamming signal34B to a residual jamming signal681having that known frequency offset from zero frequency. In this alternative, the high pass filter62is replaced by a filter621having a series notch or a shunt trap tuned to the known frequency offset for suppressing the residual jamming signal681at the frequency of the offset.

In a simple case, the high pass filter62is a coupling capacitor or a shunt inductor. The coupling capacitor or shunt inductor may be a part of the frequency immunization converter60or the IF receiver64.

The IF receiver64includes filters, amplifiers, oscillators, mixers, demodulators, local oscillators, summers, analog-to-digital converters and/or other like electronic elements for processing the high passed IF signal66A for providing the output information32.

The frequency immunization converter60includes one, two, or more frequency conversion stages for frequency converting the RX signal30D and the TX jamming signal34B to the IF signal66and the residual jamming signal68. One or more of the frequency conversion stages may use complex frequency conversions having in-phase (I) and quadrature phase (Q) component signals. In a simple case, the frequency immunization converter60has a single frequency conversion stage having a local signal generator72and a mixer74. The local signal generator72uses information for the TX center frequency38in the form of data from microprocessor system22and/or actual signals from signal generators in the transmitter16for recreating the center frequency of the TX signal in an LO signal. The mixer74multiplies the LO signal times the RX signal30D for providing the IF signal66. At the same time the mixer74uses the same LO signal for mixing the TX jamming signal34B to the residual jamming signal68having signal frequencies relatively near to zero frequency.

There are several ways that are well known by those skilled in the art of generating and synthesizing signals by which the data for the TX center frequency38can be used for controlling the frequency of the local signal generator72. For example, the data can be converted to an analog voltage and then the analog voltage used to control the frequency of a voltage controlled oscillator (VCO). For another example, the data can be used for setting one or more divide numbers in one or more loops that lock a VCO into a frequency relationship with the frequency of a reference oscillator. In another preferred embodiment the local signal generator72mixes, frequency divides, and/or frequency multiplies the actual signals from the transmitter16for recreating the center frequency of the TX signal26. In another example, the local signal generator72uses a combination of data and actual signals from the transmitter16in order to recreate the TX center frequency38.

It is understood by those skilled in the art that mixers typically produce many mixing products. These mixing products have frequencies of the sum and difference of the frequencies of the signals and harmonics of the signals received by the mixer. For example, an RX frequency of 2510 MHz and an LO frequency of 2603 MHz would typically result in mixer output signals of 93 MHz (LO−RX), −93 MHz (RX−LO), 5113 MHz (LO+RX), 2696 MHz (2nd harmonic of LO−RX), and many more combinations. For a mixer used as a frequency downconverter, the desired mixing product is separated from several undesired mixing products with a low pass filter. For the present invention a low pass filter is placed between or within the frequency immunization converter60and the front end of the IF receiver64for passing the desired mixing products at ±93 MHz and suppressing the unwanted mixing products at the other frequencies.

FIG. 3is a block diagram of the frequency immunization converter60having several frequency conversion stages in series represented by first through Nth frequency converters60A–N, respectively. The first frequency converter60A includes a local signal generator72A, a mixer74A, and a filter76A. The local signal generator72A generates an LO signal A. The mixer74A mixes the LO signal A with the RX signal30D and the TX jamming signal34B for providing mixing products A to the filter76A. The filter76A suppresses the unwanted products of mixing products A and issues an IF signal A and an IF jamming signal A. The IF signal A can be higher or lower in frequency than the RX signal30D. This process continues until the last frequency converter60N.

The Nth frequency converter60N includes a local signal generator72N, a mixer74N, and a filter76N. The local signal generator72N generates and LO signal N. The mixer74N mixes the LO signal N with an IF signal N−1 and a TX jamming signal N−1 for providing mixing products N to the filter76N. The filter76N suppresses the unwanted products of mixing products N and issues the IF signal66and the residual jamming signal68described above. When only two frequency conversion stages are used, the IF signal N−1 is the IF signal A and the IF jamming signal N−1 is the IF jamming signal A.

When three frequency conversion stages are used, a frequency converter60B including a local signal generator72B, a mixer74B, and a filter76B is disposed between the first frequency converter60A and the Nth frequency converter60N. The local signal generator72B generates an LO signal B. The mixer74B mixes the LO signal B with the IF signal A and the IF jamming signal A for providing mixing products A to the filter76B. The filter76B suppresses the unwanted products of mixing products A and issues an IF signal B and an IF jamming signal B. In this case, the Nth frequency converter60N receives the IF signal B and the IF jamming signal B as the IF signal N−1 and the IF jamming signal N−1, respectively.

A combination of sums and/or differences of the frequencies of the local signal generators72A–72N is set to the TX center frequency38. For example, for two frequency conversion stages the frequency of the LO signal A minus the frequency of the LO signal N; or the frequency of the LO signal A plus the frequency of the LO signal N; or the frequency of the LO signal N minus the frequency of the LO signal A can be set to the TX center frequency38. For another example, for three frequency conversion stages the frequency of the LO signal A minus the frequency of the LO signal B minus the frequency of the LO signal N can be set to the TX center frequency38. Importantly, the frequencies of the signals in the frequency conversion stages may be selected and the filters76A–N may have pass and stop bands for avoiding and suppressing unwanted image frequencies.

FIG. 4is a block diagram of a preferred embodiment of the present invention having a complex version of the frequency immunization converter60referred to with a reference identifier60X and a complex version of the IF receiver64referred to with a reference identifier64X.

The frequency immunization converter60X includes the local signal generator72and a complex version of the mixer74referred to with a reference identifier74X. The complex mixer74X includes a 90 degree splitter78, and in-phase (I) and quadrature phase (Q) mixers74I and74Q. The local signal generator72generates the LO signal at the LO frequency approximately equal to the TX center frequency38. The 90 degree splitter78splits the LO signal into I and Q signal components and passes the I and Q signal components to the I and Q mixers74I and74Q, respectively.

The RX signal30D and the TX jamming signal34B are both received by both the I and Q mixers74I and74Q. The I mixer74I multiplies the I component of the LO signal times both the RX signal30D and the TX jamming signal34B; and issues an I component IF signal66I and an I component residual jamming signal68I to a high pass filter62I. The high pass filter62I passes the IF signal66I as an I component first IF signal66AI and effectively eliminates the residual jamming signal68I. Similarly, the Q mixer74Q multiplies the Q component of the LO signal times both the RX signal30D and the TX jamming signal34B; and issues a Q component IF signal66Q and a Q component residual jamming signal68Q to a high pass filter62Q. The high pass filter62Q passes the IF signal66Q as a Q component first IF signal66AQ and effectively eliminates the residual jamming signal68Q. The high pass filters62I and62Q are functionally equivalent to the high pass filter62described above for operating on a complex signal.

The IF receiver64X preferably includes I and Q low pass filters81I and81Q; selectable image filters82I and82Q; mixers84II,841Q,84QQ and84QI; a local signal generator86; I and Q 90 degree splitters88I and88Q; A equivalent, B equivalent and C equivalent selectable inverters92A,92B and92C, respectively; first and second summers94A and94B, respectively; a polyphase filter96, also known as a vector filter; and a second IF receiver98.

The I and Q low pass filters81I and81Q filter unwanted mixing products created by the mixers74I and74Q and pass a complex filtered first IF signal to the image filters82I and82Q. The image filters82I and82Q pass the filtered first IF signal and suppress image signal frequencies. The suppression band of the image filters82I and82Q is selected under control of the microprocessor system22(FIG. 1) as described below as a low pass filter as illustrated inFIG. 4or as a high pass filter. In an alternative embodiment the image filters82I and82Q are bandpass filters having passbands controlled for passing signal frequencies around the first IF signal66AI and66AQ and suppressing other signal frequencies. In another alternative embodiment the image filters82I and82Q are notch filters controlled for trapping an undesired image frequency and passing other signal frequencies.

The mixer841I receives a first I component of the image filtered first IF signal (I11) from the image filter82I. The mixer84IQ receives a second I component of the image filtered first IF signal (I12) from the image filter82I. The mixer84QQ receives a first Q component of the image filtered first IF signal from (Q11) the image filter82Q. The mixer84QI receive a second Q component of the image filtered first IF signal (Q12) from the image filter82Q.

The local signal generator86generates a second LO signal and passes the signal to the 90 degree splitters88I and88Q. The frequency of the second LO signal from the local signal generator86is selected under control of the microprocessor system22(FIG. 1) as described below so that the difference frequency between frequency of the second LO signal and the frequency of the first IF signal is a pre-determined second intermediate frequency. The 90 degree splitter88I splits the second LO signal into I and Q components and passes the I component to the mixer84II and the Q component to the mixer84IQ. Similarly, the 90 degree splitter88Q splits the second LO signal into I and Q components and passes the Q component to the mixer84QQ and the I component to the mixer84QI.

The mixer84II multiplies a first component of the I first IF signal I11by the I component from the 90 degree splitter88I and issues a first I component of a second IF signal (I21) to a first input of the summer94B. The mixer84IQ multiplies a second I component of the I first IF signal I12by the Q component from the 90 degree splitter88I and issues a first Q component of the second IF signal (Q21) to a first input of the summer94A through the selectable inverter92B. The inverter92B inverts or non-inverts the Q21under control of the microprocessor system22(FIG. 1). The mixer84QQ multiplies a first Q component of the first IF signal Q11by the Q component from the 90 degree splitter88Q and issues a second I component of the second IF signal (I22) to a second input of the summer94B through the selectable inverter92A. The inverter92A inverts or non-inverts the I22under control of the microprocessor system22(FIG. 1). The mixer84QI multiplies a second Q component of the first IF signal Q12by the I component from the 90 degree splitter88Q and issues a second Q component of the second IF signal (Q22) to a second input of the summer94A.

The summer94B adds the first I21and second I22I components and issues a summation I component of the second IF signal (I2S) to a first input of the polyphase filter96. The summer94A adds the first (Q21) and second (Q22) Q components and issues a summation Q component of the second IF signal (Q2S) through the selectable inverter92C to a second input of the polyphase filter96. The inverter92C inverts or non-inverts the summation Q component Q2Sunder control of the microprocessor system22(FIG. 1). The selections of invert or non-invert in the A equivalent, B equivalent and C equivalent selectable inverters92A–C are controlled by the microprocessor system22(FIG. 1) as described below.

The polyphase filter96suppresses signals having negative frequencies and combines the signals at its first and second inputs having positive frequencies for providing an image filtered second IF signal to the second IF receiver98. The second IF receiver98processes the signal from the polyphase filter96for providing the baseband signal having the output information32. In an alternative embodiment the mixers84IQ and84QI; and the inverters92B and92C are not included and the second IF receiver98processes a filtered summation I component for I21and I22.

It is understood that frequency downconverters convert two input signal frequencies—the desired input signal frequency and an unwanted image input signal frequency—to the same output signal frequency. When the LO signal frequency is higher than the input signal frequency, the unwanted image input signal frequency is the sum of the input signal frequency plus twice the output signal frequency. When the LO signal frequency is lower than the incoming signal frequency, the unwanted image frequency is the input signal frequency minus twice the output signal frequency.

The switching of the second LO signal between frequencies above and below the first IF frequency is controlled in the IF receiver64X by the microprocessor system22(FIG. 1) for avoiding the image signal frequencies that are expected or measured to be the worst (highest level) image signals accompanying the RX signal30D. The image frequencies that not avoided are suppressed by the image filters82I and82Q, the 4 mixer quadrature mixing process of the IF receiver64X, and the polyphase filter96.

There are two cases that must be considered depending upon whether the TX center frequency38(and the LO frequency from the local signal generator72) is greater or less than the frequency of the desired RX signal30. In the first case, when the TX center frequency38is greater than the frequency of the desired RX signal30, one of the selection alternatives101or102(FIG. 5) is used. In the second case, when the TX center frequency38is lower than the frequency for the desired RX signal30, one of the selection alternatives103or104(FIG. 5) is used. It should be noted that the need for the alternative selections101–104in the IF receiver64X for avoiding image signal frequencies is a result of two factors. First, the selection alternatives101–102(or103–104) are needed to set the second LO frequency so that worst of the image frequencies are avoided. Second, the selection alternatives101and104(or102and103) are needed to accommodate the frequency of the TX signal26either above or below the frequency of the RX signal30.

There are several variations for the disposition of the inverters92A–C in the block diagram of the IF receiver64X within the scope of the present invention. In one variation, the A equivalent selectable inverter92A for inverting or non-inverting the I22signal is embodied by a selectable inverter92D disposed for inverting or non-inverting the Q11signal. In another variation, the B equivalent selectable inverter92B for inverting or non-inverting the Q21signal is embodied by a selectable inverter92E disposed for inverting or non-inverting the I12signal. In another variation, the A equivalent selectable inverter92A for inverting or non-inverting the I22signal is embodied by a combination of a selectable inverter92F disposed for inverting or non-inverting the I2Ssignal and one of a selectable inverter92G for inverting or non-inverting the I21signal or a selectable inverter92H disposed for inverting or non-inverting the I11signal.

In yet another variation, the B equivalent selectable inverter92B for inverting or non-inverting the Q21signal is embodied by a combination of a change in control logic of the selectable inverter92C and one of a selectable inverter92J disposed for inverting or non-inverting the Q22signal or a selectable inverter92K disposed for inverting or non-inverting the Q12signal. In this variation the selectable inverter92C may be eliminated when the new logic is always invert or always non-invert. In another variation, the C equivalent selectable inverter92C disposed for inverting or non-inverting the Q2Ssignal is embodied by a combination of a change in the control logic of the selectable inverter92B and one of the selectable inverter92J or the selectable inverter92K. In this variation the selectable inverter92B may be eliminated when the new logic is always invert or always non-invert.

FIG. 5is a table of selection alternatives101–104. A one of the alternatives101–104is selected by programming in the microprocessor system22(FIG. 1) in order to avoid certain undesired incoming signal frequencies received with the desired RX signal30D that result in unwanted image signal frequencies in the first IF signal that would otherwise be mixed into the second IF signal.

In the selection alternative101, the second local signal generator86is set low to a second LO frequency below the first IF frequency, the image filters82I and82Q are set high pass in order to suppress frequencies below the first IF frequency, the inverter92A is set to non-invert, the inverter92B is set to invert, and the inverter92C is set to non-invert. In the selection alternative102, the second local signal generator86is set high to a second LO frequency above the first IF frequency, the image filters82I and82Q are set low to suppress frequencies above the first IF frequency, the inverter92A is set to non-invert, the inverter92B is set to invert, and the inverter92C is set to invert.

In the selection alternative103, the second local signal generator86is set high to a second LO frequency above the first IF frequency, the image filters82I and82Q are set low to suppress frequencies above the first IF frequency, the inverter92A is set to invert, the inverter92B is set to non-invert, and the inverter92C is set to non-invert. In the selection alternative104, the second local signal generator86is set low to a second LO frequency below the first IF frequency, the image filters82I and82Q are set high to suppress frequencies below the first IF frequency, the inverter92A is set to invert, the inverter92B is set to non-invert, and the inverter92C is set to invert. It should be noted that selection alternatives101and103reverse the spectrum of the incoming RX signal30. Preferably, this spectrum reversal is compensated by a reversal in the second IF receiver98. It should also be noted that the polarities all of the selectable inverters92A–C can be reversed when appropriate fixed signal inversions are disposed elsewhere in the IF receiver64X.

Returning toFIG. 4, the image rejection in the IF receiver64X is most easily understood with numerical examples. In the first case, assume the TX center frequency38(and the LO frequency from the local signal generator72) is 2603 MHz and the frequency of the desired RX signal30is 2510 MHz. The IF signal66I and66Q is 93 MHz and the undesired image frequency that is suppressed by the combination of the complex frequency conversion in the frequency immunization converter74X and complex frequency conversions and inversions in the IF receiver64X is 2696 MHz. In the second case, assume the TX center frequency38(and the LO frequency from the local signal generator72) is 2510 MHz and the frequency of the desired RX signal30is 2603 MHz. The IF signal661and66Q is 93 MHz and the undesired image frequency that is suppressed by the combination of the complex frequency conversion in the frequency immunization converter74X and complex frequency conversions and inversions in the IF receiver64X is 2417 MHz.

Continuing the numerical example, assume the predetermined frequency of the second IF signal is 20 MHz. The frequency of the local signal generator86is set to either 73 MHz or 113 MHz for downconverting the first IF frequency of 93 MHz to the second IF frequency of 20 MHz; and the undesired image frequencies are either 53 or 133 MHz, respectively. In the first case, the first LO signal frequency of 2603 MHz in the frequency immunization converter60X downconverts signal frequencies of 2550 and 2656 MHz to the undesired image frequency of 53 MHz and downconverts signal frequencies of 2470 and 2736 MHz to the undesired image frequency of 133 MHz. The IF receiver64X is programmed to the selection alternative101or102to set the second LO frequency to 73 or 113 MHz, respectively, in order to avoid the undesired signal frequency pair 2550 and 2656 MHz or 2470 and 2736 MHz, respectively, that is determined to have the largest signal. The undesired signal frequencies in the other pair are suppressed by the combination of the filters, 90 degree phase shifts, and inversions the frequency immunization converter74X and in the IF receiver64X.

In the second case the first LO frequency of 2510 in the frequency immunization converter60X downconverts signal frequencies of 2457 and 2563 MHz to the undesired image frequency of 53 MHz and downconverts signal frequencies of 2377 and 2643 MHz to the undesired image frequency of 133 MHz. The IF receiver64X is programmed to the selection alternative103or104to set the second LO frequency to 73 or 113 MHz, respectively, in order to avoid the undesired signal frequency pair 2457 and 2563 MHz or 2377 and 2643 MHz, respectively, that is determined to have the largest signal. The undesired signal frequencies in the other pair are suppressed by the combination of the filters, 90 degree phase shifts, and inversions the frequency immunization converter74X and in the IF receiver64X.

FIG. 6is flow chart of a method in the transceiver10for immunizing the receiver20from self-jamming by the TX jamming signal34. In a step112the receiver20sets the LO frequency of the local signal generator72to the TX center frequency38. In a step114the frequency immunization converter60uses the LO frequency at the TX center frequency38for simultaneously frequency downconverting the desired RX signal30and the undesired TX jamming signal34to the desired IF signal66at an IF frequency and the residual jamming signal68at near to zero frequency, respectively. In a step116, the high pass filter62passes the desired IF signal66and effectively eliminates the residual jamming signal68. Then, in a step118the IF receiver64(or64X) selects a second LO frequency and signal inversions for avoiding the worst of the undesired image frequencies.

FIG. 7is a flow chart of a method for avoiding undesired image frequencies in the present invention. In a step122the LO signal of the local signal generator72is set to the TX center frequency38. The TX center frequency38is either greater than or less than the RX frequency of the RX signal30. When the TX center frequency38is greater than the frequency of the RX signal30, the microprocessor system22selects one of selection alternatives101and102. When the TX center frequency38is less than the RX frequency of the RX signal30, the microprocessor system22selects one of selection alternative103and104.

In a step124for TX center frequency38greater than the RX frequency, the frequency of the largest image signal for the second IF frequency that accompanies the RX signal30D is determined. The determination may be a pre-determination based upon an expected signal environment or a measurement of the signals accompanying the RX signal30D after the filtering of the splitter14and the bandpass filter52.

Continuing to use the numerical example above, one pair of image frequencies is 2550 and 2656 MHz and the other pair is 2470 and 2736 MHz. In a step126when either 2550 or 2656 MHz is determined to be the largest image signal, the selection alternative102is chosen for setting the second LO frequency above the first IF signal66I and66Q. In the present example the second LO frequency is set to 113 MHz. When either 2470 or 2736 MHz is determined to be the largest image signal, the selection alternative101is chosen for setting the second LO frequency below the first IF frequency. In the present example the second LO frequency is set to 73 MHz. Then, in a step128, the image filters82I and82Q, and the inversions of the selectable inverters92A–C are then selected for passing the desired IF frequencies and suppressing the other pair of image signal frequencies.

In a step134for TX center frequency38less than the frequency of the RX signal30, the frequency of the largest image signal for the second IF frequency that accompanies the RX signal30D is determined. The determination may be a pre-determination based upon an expected signal environment or a measurement of the signals accompanying the RX signal30D after the filtering of the splitter14and the bandpass filter52.

Continuing to use the numerical example above, one pair of image frequencies is 2457 and 2563 MHz and the other pair is 2377 and 2643 MHz. In a step136when either 2457 or 2563 MHz is determined to be the largest image signal, the selection alternative103is chosen for setting the second LO frequency above the first IF signal66I and66Q. In the present example the second LO frequency is set to 113 MHz. When either 2377 or 2643 MHz is determined to be the largest image signal, the selection alternative104is chosen for setting the second LO frequency below the first IF frequency. In the example the second LO frequency is set to 73 MHz. Then, in a step138, the image filters82I and82Q, and the inversions of the selectable inverters92A–C are then selected for passing the desired IF frequencies and suppressing the other pair of image signal frequencies.