High accuracy receiver forward and reflected path test injection circuit

There is disclosed an injection circuit for measuring radio frequency (RF) signals in an RF receiver for use in measuring the impedance match of a receive antenna and for use in calibrating receiver gain, wherein an advantageous embodiment of the injection circuit comprises: 1) a circulator coupled to the receive antenna; 2) a directional coupler coupled to the circulator; 3) an injection source coupled to the circulator and to the directional coupler, wherein the injection source is capable of injecting a test RF signal into either the circulator or the directional coupler; and 4) a terminating switch for selectively enabling or disabling the transfer of a test RF signal from the injection source to either the circulator or the directional coupler. The circulator has a reverse isolation of at least 20 dB that significantly increases the accuracy of the measurements of the RF signals compared with the accuracy that may be achieved by prior art methods. The present invention obtains the received signal strength indicator (RSSI) measurements at any instantaneous temperature and operating channel and determines voltage standing wave ratio (VSWR) measurements.

The present invention is related to that disclosed in U.S. patent application Ser. No. 09/475,604, filed Dec. 30, 1999, entitled “COMBINED SYSTEM FOR CALIBRATING RECEIVER GAIN AND MEASURING ANTENNA IMPEDANCE MATCH AND METHOD OF OPERATION.” Application Ser. No. 09/475,604 is commonly assigned to the assignee of the present invention. The disclosure of the related patent application is hereby incorporated by reference in the present application as if fully set forth herein.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to wireless communications systems and, more specifically, to radio frequency (RF) test injection circuits for measuring the antenna impedance match of a receive antenna and measuring receiver gain in a base station in a wireless network.

BACKGROUND OF THE INVENTION

In order to increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers continually implement any improvements that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, and the number of subscribers that the cellular system can service.

In many receivers characterization of forward path gain and calibration of the received signal strength indicator (RSSI) signal are presently accomplished with temperature compensation circuitry. The temperature compensation circuitry adapts to variations in gain, attenuation, and detector slopes over a range of temperatures and frequencies. Because the characteristics of devices and components used in the temperature compensation circuitry change with variations in temperature and frequency, the receivers must be calibrated and characterized at the time of manufacture. However, the characteristics of the devices and components vary within different manufacturing lots. This means that the operating characteristics of the receivers must be continuously monitored during the manufacturing process to detect changes that occur as the manufacturing process progresses.

Therefore, the receiver circuitry must be characterized by analyzing numerous individual receiver units during the manufacturing process in order to develop an accurate profile for the temperature compensation circuitry. After the receiver circuitry has been characterized, the characterization information must be stored in the memory of each of the individual receiver units. Because the manufacturing process produces component changes over a period of time, the receiver characterization process must be re-performed and the information in the memory of each of the individual receiver units must be updated.

There is therefore a need in the art for a receiver design that does not require continual re-characterization of forward path gain and continual recalibration of Received Signal Strength Indicator (RSSI) during the manufacturing process.

After a base transceiver station (BTS) has been manufactured, wireless service providers use a variety of test equipment to monitor the performance of the RF receiver and the RF transmitter in the BTS during operation. The test equipment may monitor a variety of signal parameters in the RF transmitter, including adjacent channel power ratio (ACPR), spectral purity (including in-band and out-of-band spurious components), occupied bandwidth, RHO, frequency error, and code domain power. The test equipment may also perform a variety of test functions in the RF receiver, including testing and measuring the receive antenna return loss and calibrating the receiver. Preferably, the signal parameters are remotely monitored from a central location, so that a wireless service provider can avoid the expense of sending maintenance crews into the field to test each BTS individually. Additionally, a remote monitoring system can detect the failure of an RF transmitter or an RF receiver nearly instantaneously.

Unfortunately, adding some types of test equipment (e.g., spectrum analyzers) to a BTS significantly increases the cost of the BTS. In some cases, the cost of the test equipment may be greater than the cost of the BTS itself. As a result, wireless service providers may not install any test equipment in the ETS. Alternatively, wireless service providers may install only a limited amount of test equipment to test only some of the functions of the BTS. The remaining functions must be monitored by maintenance crews using portable test equipment.

There is therefore a need in the art for inexpensive test equipment that may be implemented as part of the base station. In particular, there is a need for integrated test equipment that can reuse some of the existing circuitry in a base transceiver station. More particularly, there is a need for integrated test equipment that can be used to measure the impedance match of a receive antenna and that can be used to calibrate the receiver gain.

Prior art RF test injection circuits have been used to measure RF signals in an RF receiver in a base station in a wireless network for the purpose of measuring the impedance match of a receive antenna and to calibrate the receiver gain. A prior art injection circuit usually comprises a directional coupler that has an input coupled to a duplexer that is coupled to an antenna array. The output of the directional coupler is coupled to a signal amplifier. Also coupled to the directional coupler is an injection source that is capable of injecting a test RF signal into the directional coupler.

When a prior art injection circuit of this type is used to measure the impedance match of a receive antenna, the injection source injects a test RF signal into the directional coupler in the direction of the signal amplifier. Level detector circuitry that is coupled to the signal amplifier measures the RSSI level of the test RF signal to obtain a first RSSI measurement of the test RF signal.

Then the injection source injects a test RF signal into the directional coupler in the direction of the duplexer that is coupled to the antenna array. The test RF signal passes through the duplexer and hits the antenna array. RF signal energy that is not absorbed by the antenna array is reflected back through the duplexer and through the directional coupler to the signal amplifier and the level detector circuitry. The level detector circuitry coupled to the signal amplifier measures the RSSI level of the test RF signal to obtain a second RSSI measurement of the reflected test RF signal. The level detector circuitry compares the two RSSI measurements to obtain a voltage standing wave ratio (VSWR) that measures the impedance match of the antenna array.

One of the primary deficiencies of this prior art approach is the difficulty of controlling the directivity of the directional coupler. This is because directional couplers are, capable of providing only approximately 10 dB to 15 dB of reverse isolation between its input signal and its output signal. As a result, the directional coupler may transfer a signal that is 10 dB to 15 dB below its output signal back through the duplexer to the antenna array. The relatively low level of reverse isolation that is provided by the directional coupler means that a portion of the signal energy at the output of the directional coupler will be transferred back through the duplexer to the antenna array and reflected back through the duplexer to the directional coupler. The reflected energy adversely affects the RSSI measurements and causes an erroneous determination of the voltage standing wave ratio (VSWR). The same problem occurs when such a prior art injection circuit is used to calibrate the receiver gain.

There is therefore a need in the art for an improved test injection circuit for measuring radio frequency (RF) signals in an RF receiver.

SUMMARY OF THE INVENTION

To address the deficiencies of the prior art described above, it is a primary object of the present invention to provide an improved test injection circuit for measuring radio frequency (RF) signals in an RF receiver. The improved test injection circuit of the present invention may be used to obtain highly accurate RF signal measurements to determine the impedance match of a receive antenna. The improved test injection circuit of the present invention may also be used to obtain highly accurate RF signal measurements to calibrate receiver gain.

An advantageous embodiment of the improved test injection circuit of the present invention comprises: 1) a circulator coupled to an RF receive antenna; 2) a directional coupler coupled to the circulator; 3) an injection source coupled to the circulator and to the directional coupler, wherein the injection source is capable of injecting a test RF signal into either the circulator or the directional coupler; and 4) a terminating switch for selectively enabling or disabling the transfer of a test RF signal from the injection source to either the circulator or the directional coupler.

The circulator has a reverse isolation of at least 20 dB. This is significantly greater than the 10 dB to 15 dB reverse isolation of a prior art directional coupler. The use of a circulator that has at least 20 dB of reverse isolation significantly increases the accuracy of the measurements of the RF signals compared with the accuracy that may be achieved by prior art methods.

The present invention is capable of obtaining received signal-strength indicator (RSSI) measurements at any instantaneous temperature and operating channel. The present invention is capable of using the RSSI measurements to obtain voltage standing wave ratio (VSWR) measurements for any instantaneous temperature and operating channel.

It is an object of the present invention to provide for use in an RF receiver unit a test injection circuit for accurately measuring RF signals within the RF receiver unit.

It is another object of the present invention to provide for use in an RF receiver unit a test injection circuit for accurately measuring RF signals that comprises a circulator coupled to an antenna of the RF receiver unit and an injection source coupled to the circulator that is capable of injecting a test RF signal into the circulator.

It is also an object of the present invention to provide a circulator within the RF receive path of an RF receiver unit that has a reverse isolation of at least 20 dB to increase the accuracy with which RF signals may be measured in the RF receiver unit.

It is another object of the present invention to provide level detector circuitry within an RF receive unit to obtain highly accurate measurements of the received signal strength indicator of an RF signal within the RF receive unit.

It is still another object of the present invention to provide a highly accurate method for calibrating the receiver gain of an RF receive antenna.

It is also another object of the present invention to provide a highly accurate method for measuring the impedance match of an RF receive antenna.

DETAILED DESCRIPTION

FIG. 1illustrates an exemplary wireless network100according to one embodiment of the present invention. The wireless telephone network100comprises a plurality of cell sites121-123, each containing one of the base stations, BS101, BS102, or BS103. Base stations101-103are operable to communicate with a plurality of mobile stations (MS)111-114. Mobile stations111-114may be any suitable cellular devices, including conventional cellular telephones, PCS handset devices, portable computers, metering devices, and the like.

Dotted lines show the approximate boundaries of the cells sites121-123in which base stations101-103are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions.

In one embodiment of the present invention, BS101, BS102, and BS103may comprise a base station controller (BSC) and a base transceiver station (BTS). Base station controllers and base transceiver stations are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver station, for specified cells within a wireless communications network. A base transceiver station comprises the RF transceiver unit, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces, and RF transmitters and RF receivers, as well as call processing circuitry. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver station in each of cells121,122, and123and the base station controller associated with each base transceiver station are collectively represented by BS101, BS102and BS103, respectively.

BS101, BS102and BS103transfer voice and data signals between each other and the public telephone system (not shown) via communications line131and mobile switching center (MSC)140. Mobile switching center140is well known to those skilled in the art. Mobile switching center140is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the public telephone system. Communications line131may be any suitable connection means, including a T1 line, a T3 line, a fiber optic link, a network backbone connection, and the like. In some embodiments of the present invention, communications line131may be several different data links, where each data link couples one of BS101, BS102, or BS103to MSC140.

In the exemplary wireless network100, MS111is located in cell site121and is in communication with BS101; MS113is located in cell site122and is in communication with BS102; and MS114is located in cell site123and is in communication with BS103. The MS112is also located in cell site121, close to the edge of cell site123. The direction arrow proximate MS112indicates the movement of MS112towards cell site123. At some point, as MS112moves into cell site123and out of cell site121, a “handoff” will occur.

As is well known, the handoff procedure transfers control of a call from a first cell to a second cell. For example, if MS112is in communication with BS101and senses that the signal from BS101is becoming unacceptably weak, MS112may then switch to a BS that has a stronger signal, such as the signal transmitted by BS103. MS112and BS103establish a new communication link and a signal is sent to BS101and the public telephone network to transfer the on-going voice, data, or control signals through BS103. The call is thereby seamlessly transferred from BS101to BS103. An “idle” handoff is a handoff between cells of a mobile device that is communicating in the control or paging channel, rather than transmitting voice and/or data signals in the regular traffic channels.

FIG. 2illustrates in greater detail exemplary base station101in accordance with one embodiment of the present invention. Base station101comprises base station controller (BSC)210and base transceiver station (BTS)220. Base station controllers and base transceiver stations were described previously in connection with FIG.1. BSC210manages the resources in cell site121, including BTS220. BTS220comprises BTS controller225, channel controller235, which contains representative channel element240, transceiver interface (IF)245, RF transceiver unit250, antenna array255and impedance measurement controller251.

BTS controller225comprises processing circuitry and memory capable of executing an operating program that controls the overall operation of BTS220and communicates with BSC210. Under normal conditions, BTS controller225directs the operation of channel controller235, which contains a number of channel elements, including channel element240, that perform bi-directional communications in the forward channel and the reverse channel. A “forward” channel refers to outbound signals from the base station to the mobile station and a “reverse” channel refers to inbound signals from the mobile station to the base station. In an advantageous embodiment of the present invention, the channel elements operate according to a code division multiple access (CDMA) protocol with the mobile stations in cell121. Transceiver IF245transfers the bi-directional channel signals between channel controller235and RF transceiver unit250.

Impedance measurement controller251, in conjunction with circuitry located in RF transceiver unit250, measures the receiver gain and the impedance match for the receive antenna portion of antenna array255. Portions of RF transceiver unit250and the operation of impedance measurement controller251are described below in greater detail in connection withFIGS. 3 and 4.

Antenna array255transmits forward channel signals from RF transceiver unit250to mobile stations in the coverage area of BS101. Antenna array255also transfers to transceiver unit250reverse channel signals received from mobile stations in the coverage area of ES101. In an advantageous embodiment of the present invention, antenna array255is multi-sector antenna, such as a three sector antenna in which each antenna sector is responsible for transmitting and receiving in a one hundred twenty degree (120°) arc of coverage area. Additionally, RF transceiver unit250may contain an antenna selection unit to select among different antennas in antenna array255during both transmit and receive operations.

FIG. 3illustrates a portion of exemplary RF transceiver unit250in accordance with an advantageous embodiment of the present invention. As will be fully described, the injection circuit of the present invention uses a circulator as a circuit element. An advantageous embodiment of the present invention comprises both a circulator and a directional coupler. RF transceiver unit250comprises a circuit branch300(shown inFIG. 3) for receiving an RF signal. Circuit branch300comprises antenna array255, duplexer310, circulator370, directional coupler320, low noise amplifier (LNA)330, receive path circuitry335, and level detector circuitry340. Circuit branch300also comprises injection source350and single-pole double-throw (SPDT) terminating switch360. Impedance measurement controller251enables and controls the measurement function of level detector circuitry340.

Duplexer310filters the signal path to and from the antenna array255as RF transceiver unit250transmits (forward channel) signals and receives (reverse channel) signals. Duplexer310isolates the receive signals in a receive signal frequency band (e.g., 1850-1910 MHz) from the transmit signals in a transmit signal frequency band (e.g., 1930-1990 MHz). Duplexer310permits the sharing of antenna array255by the RF receiver portion and the RF transmitter portion of RF transceiver unit250.

Receive (reverse channel) signals from antenna array255pass through duplexer310to circulator370. In this exemplary advantageous embodiment of the present invention, circulator370is a device that has three terminals. The three terminals are a first input terminal (coupled to duplexer310) and a second input terminal (coupled to terminating switch360) and an output terminal (coupled to directional coupler320). When circulator370receives a signal at one of its terminals, circulator370transfers the signal to an adjacent terminal. Some circulators are designed to circulated a signal in a clockwise direction. Some circulators are designed to circulate a signal in a counterclockwise direction. In the exemplary advantageous embodiment of the present invention shown inFIG. 3circulator370circulates a signal in a clockwise direction.

When circulator370receives an RF signal from duplexer310on the first input terminal of circulator370, then circulator370transfers the signal to directional coupler320. At the same time, circulator370transfers to duplexer310any signal that circulator370receives on its second input terminal from single-pole double-throw (SPDT) terminating switch360. As shown inFIG. 3, the second input terminal of circulator370is connected to one output of terminating switch360. As described more fully below, the signal from terminating switch360originates from injection source350and is used to measure a reflected signal from antenna array255.

The signals that circulator370transfers to directional coupler320pass through directional coupler320. Directional coupler320combines the signals from circulator370with the signals, if any, from the other input of directional coupler320(i.e., the input of directional coupler320that is coupled to terminating switch360and injection source350). The resulting signals then pass to low noise amplifier (LNA)330. Low noise amplifier330amplifies the signals from directional coupler320and transfers the amplified signals to receive path circuitry335. The output of receive path circuitry335is coupled to transceiver IF245. Level detector circuitry340is coupled to receive path circuitry335. Impedance measurement controller251is coupled to level detector circuitry340and is capable of enabling and controlling the measurement function of level detector circuitry340.

In this advantageous embodiment of the present invention, the injection circuitry comprises injection source350that is coupled to circulator370and to directional coupler320by a single-pole double-throw (SPDT) terminating switch360. The output of injection source350is provided as an input to terminating switch360. Terminating switch360may transfer the injection signal from the injection source350either to the second input terminal of circulator370or to the coupled-input of directional coupler320.

Impedance measurement controller251is capable of controlling terminating switch360. Impedance measurement controller251selectively enables and disables the output from injection source350,by controlling the position of terminating switch360depending upon the type of RF signal measurement to be performed. When impedance measurement controller251causes terminating switch360to close toward the right side of terminating switch360(as shown in FIG.3), injection source350provides an injection signal to directional coupler320. When impedance measurement controller251causes terminating switch360to close toward the left side of terminating switch360(as shown in FIG.3), injection source350provides an injection signal to circulator370. The injection signal is an RF signal with a frequency that is preferably in the central portion of the frequency operating range of the RF receiver portion of RF transceiver unit250.

As previously mentioned, impedance measurement controller251enables and controls the measurement function of level detector circuitry340. The level detector circuitry340provides AGC and RSSI level detection for use by impedance measurement controller251. In an advantageous embodiment of the present invention, level detector circuitry340uses AGC detectors in transreceiver IF245for detecting AGC levels, rather than providing a separate AGC detector for measurement purposes. If measurements are not being performed, impedance measurement controller251disables the output of injection source350. When injection source350is disabled, no signal is being injected into RF transceiver unit250for measurement purposes.

The injection signal from injection source350may be used to measure the return loss of antenna array255, as well as measure parameters associated with the performance of the RF receiver portion of RF transceiver unit250. When injection source350is connected to directional coupler320through terminating switch360, directional coupler320provides an output for level detector circuitry340that is a combination of the injection signal from injection source350and the RF receive signal, if any. The output signal from directional coupler320is used for measuring the RSSI level of signals in the RF receive path for use in determining the return loss for antenna array255or for calibrating the RF receiver of RF transceiver unit250.

In this advantageous embodiment of the invention, the return loss for antenna array255may be measured. In the first step of the measurement process, impedance measurement controller251causes terminating switch360to close toward the right side of terminating switch360(as shown inFIG. 3) to cause a first injection signal to be transferred to directional coupler320. First injection signal is coupled in the direction of low noise amplifier (LNA)330, receive path circuitry335, and level detector circuitry340. For convenience, this direction will be referred to as the “receiver forward path.” The first injection signal is treated as a normal input signal which goes through receive path circuitry335to automatic gain control (AGC) circuit (not shown). The AGC circuit controls the gain of the first injection signal in accordance with well known AGC principles. Level detector circuitry340then determines the RSSI level of the first injection signal and records the RSSI level of the first injection signal in impedance measurement controller251.

In the second step of the measurement process, impedance measurement controller.251causes terminating switch360to close toward the left side of terminating switch360(as shown inFIG. 3) to cause a second injection signal to be transferred to the second input terminal of circulator370. The second injection signal is identical to the first injection signal and has the same frequency and amplitude as the first injection signal. Circulator370transfers the second injection signal to duplexer310. The second injection signal travels through duplexer310and hits antenna array255. Energy that is not absorbed by antenna array255reflects back through duplexer310, around circulator370and down circuit branch300in the direction of the receiver forward path through low noise amplifier330. The reflected second injection signal is treated as a normal input signal which goes through receive path circuitry335to an automatic gain control (AGC) circuit (not shown). The AGC circuit controls the gain of the second injection signal in accordance with well known AGC principles. Level detector circuitry340then determines the RSSI level of the second injection signal and records the RSSI level of the second injection signal in impedance measurement controller251.

A software algorithm in impedance measurement controller251compares the two recorded RSSI levels and determines a voltage standing wave ratio (VSWR) measurement. Impedance measurement controller251then stores the result as the return loss measurement for antenna array255.

The injection circuit of the present invention will work even if there is no duplexer310in circuit branch300. That is, the injection circuit that comprises circulator370will work in an RF receiver that does not include a duplexer. Duplexer310is used in RF transceiver units that are capable of both transmitting and receiving RF signals. In an RF receiver without an RF transmitter duplexer310will not be present.

The use of circulator370in the advantageous embodiment of the present invention facilitates the control of the directivity of the injected signals. Specifically, circulator370is capable of providing at least approximately 20 dB of reverse isolation between its input signal and its output signal. As a result, circulator370may transfer a signal that is at least approximately 20 dB below its output signal back through duplexer310to antenna array255. The level of reverse isolation provided by circulator370exceeds the level of reverse isolation provided by a directional coupler such as directional coupler320.

The inclusion of circulator370in circuit branch300improves the accuracy in the measurements of signals from antenna array255by increasing the level of isolation between duplexer310(or antenna array255) and low noise amplifier330with respect to the level of isolation available in prior art designs. The relatively high level of reverse isolation provided by circulator370(compared to the level of reverse isolation provided by directional couplers) means that in the present invention it is less likely that a portion of the signal energy at the output of circulator370will be transferred back through duplexer310to antenna array255and reflected back through duplexer310to circulator370and into the receiver forward path direction. Any additional energy that is reflected back into the receiver forward path direction adversely affects the RSSI measurements and causes an erroneous determination of the voltage standing wave ratio (VSWR). In the present invention, the use of circulator370in conjunction with directional coupler320reduces the levels of the additional reflected energy and significantly improves the accuracy of the RF signal measurements.

Impedance measurement controller251initiates an RSSI calibration by enabling injection source350and injecting an injection signal through terminating switch360to directional coupler320. Directional coupler320combines the injection signal from terminating switch360with an RF receive signal from circulator370, if any, and transfers the resulting signal to low noise amplifier330. The resulting signal is treated as a normal input signal which goes through receive path circuitry335to an automatic gain control (AGC) circuit (not shown). The AGC circuit controls the gain of the resulting signal in accordance with well known AGC principles. Level detector circuitry340then determines the RSSI level of the resulting signal and records the RSSI level of the resulting signal in impedance measurement controller251.

Impedance measurement controller251uses the RSSI measurement to offset, an existing RSSI curve that is stored in impedance measurement controller251. The amount of offset represents the variation in RF receiver gain caused by component performance variations due to temperature and frequency. Impedance measurement controller251stores the offset and the RSSI measurement for use in calibrating the RF receiver gain.

During the RSSI calibration measurement, the presence of circulator370provides at least 20 dB of reverse isolation in circuit branch300and serves to reduce the amount of signal energy that reaches duplexer310. Any signal energy that reaches circulator370from directional coupler320is transferred toward terminating switch360and is not reflected back into the receiver forward path direction.

Prior art injection circuitry without circulator370allows signal energy to travel toward duplexer310. Depending upon the amount of antenna load, the signal energy reflects off the antenna load and back into the receiver forward path direction. The reflected signal energy that is allowed to occur in prior art injection circuitry adversely affects the signal measurement process and the accuracy of the measured signals.

The presence of circulator370in circuit branch300reduces the amount of signal energy that is reflected back into the receiver forward path direction. In this manner, circulator370increases the accuracy of the measurements of the signals over the accuracy that may be achieved by prior art methods that use injection circuitry.

The present invention provides an improved apparatus and method for injecting a test signal (i.e., an injection signal) into an RF receiver to obtain the corresponding RSSI measurement at any instantaneous temperature and operating channel. The ability to obtain such RSSI measurements eliminates the need for the characterization and compensation circuitry employed by prior art methods that do not use injection circuitry.

In order to further increase the accuracy of signal measurements, impedance measurement controller251may make sure that no incoming RF receive signal is present from duplexer310(or antenna array255if there is no duplexer310) that would interfere with the measurement of the injection signal prior to the initiation of the measurement process. In an exemplary advantageous embodiment of the present invention, injection source350provides an 1880 MHz injection signal that falls directly in the center of the assigned forward channel frequency range (1880 MHz±660 kHz) with a power level that is high enough to be discernable above expected noise levels. The use of an injection signal in this frequency range allows BTS101to perform voltage standing wave ratio (VSWR) measurements and calibration while also handling normal communications traffic.

An advantageous embodiment of the injection circuitry of the present invention has been described that comprises circulator370and directional coupler320. It is possible, however, to use circulator370without directional coupler320. In this embodiment of the present invention an injection signal is transferred to circulator370and then transferred by circulator370to duplexer310. The injection signal passes through duplexer310and hits antenna array255. Energy that is not absorbed by antenna array255reflects back down circuit path300in the direction of the receiver forward path through low noise amplifier330. Level detector circuitry340then obtains the RSSI measurement as previously described. The presence of directional coupler320is not required to obtain RSSI measurements for the reflected injection signal. Directional coupler320is used to obtain a reference for the voltage standing wave ratio (VSWR) measurement.

FIG. 4illustrates an exemplary flow diagram400that describes the operation of the exemplary advantageous embodiment of the present invention in RF transceiver unit250for measuring the return loss of antenna array255. Initially, impedance measurement controller251enables injection source350to generate a first injection signal (process step405). The frequency of the first injection signal is preferably at the center frequency of the RF receiver operating range. Next, impedance measurement controller251enables terminating switch360to transfer the first injection signal from injection source350through terminating switch360to one input of directional coupler320(process step410).

Directional coupler320combines the first injection signal from injection source350with the RF receive signal from circulator370and transfers the combined signal to level detector circuitry340via low noise amplifier330(process step415). Level detector circuitry340in RF transceiver unit250receives the amplified signal from low noise amplifier330, uses the existing AGC detector of transceiver IF245to automatically adjust the gain of the signal from low noise amplifier330, and measures the resultant RSSI level for the first injection signal and records the resultant RSSI level for the first injection signal in impedance measurement controller251(process step420).

Next, impedance measurement controller251enables terminating switch360to send a second injection signal from injection source350to the second input of circulator370. The second injection signal has the same frequency and amplitude as the first injection signal. Circulator370then passes the second injection signal through duplexer310to antenna array255(process step425). Next, duplexer310transfers the reflected second injection signal from antenna array255to level detector circuitry340through circulator370, directional coupler320, and low noise amplifier330(process step530). Level detector circuitry340in RF transceiver unit250receives the amplified signal from low noise amplifier330, uses the existing AGC detector of transceiver IF245to automatically adjust the gain of the signal from low noise amplifier330, and measures the resultant. RSSI level for the reflected second injection signal and records the resultant RSSI level for the reflected second injection signal in impedance measurement controller251(process step435). Impedance measurement controller251uses a software algorithm to compare the two measured RSSI levels and determines a voltage standing wave ratio (VSWR) measurement (process step440).