RF antenna switch and method of operating the same
This is a continuation of application Ser. No. 07/997,275, filed Dec. 22, 1992 and now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electronic switches, and more particularly, to a radio frequency (RF) antenna switch and method of operating the same for use in diversity receivers. 
     BACKGROUND OF THE INVENTION 
     Receivers performing diversity functions are well known in the art for overcoming transmission errors. A particular type of diversity operation for receivers is known as antenna diversity. Receivers performing antenna diversity have an RF antenna switch before the demodulator to selectively receive RF signals from one or more antennas. Typically, the RF antenna switch is located at the input of the receiver. 
     A generic RF antenna switch is shown in U.S. Pat. No. 5,001,776. Several types of RF antenna switches for switching between RF signal paths are well known in the art. A reed switch used as an antenna switch is taught in U.S. Pat. No. 3,800,222. A circulator used as an antenna switch is taught in U.S. Pat. No. 4,380,822. A relay used as an antenna switch is taught in U.S. Pat. No. 4,982,442 and U.S. Pat. No. 5,060,293. A gallium arsenide field effect transistor is used as an antenna switch is taught in U.S. Pat. No. 4,920,285 and U.S. Pat. No. 4,987,382. A pin diode is also conventionally used as an RF antenna switch. 
     However, these conventional RF antenna switches have either high loss that reduces the receiver&#39;s sensitivity or high current drain. Portable receivers, such as radiotelephones, need high receiver sensitivity and low current drain. Therefore, there is a need for an RF antenna switch that has low loss and low current drain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an RF transceiver including a radio frequency (RF) antenna switch and constructed in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a schematic diagram of the antenna switch of FIG. 1. 
     FIG. 3 is a process flow diagram describing the operation of a receiver portion of the transceiver of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a block diagram of a radio frequency (RF) transceiver 100 including an RF antenna switch 136 and constructed in accordance with a preferred embodiment of the present invention. The transceiver 100 generally includes a receiver 101, a transmitter 103, a controller 105, a user interface 107, a first antenna 109, and a second antenna 111. The first antenna 109 is coupled to the receiver 101 for receiving RF signals. The second antenna 111 is coupled to the receiver 101 for receiving RF signals and coupled to the transmitter 103 for transmitting RF signals. 
     The receiver 101 includes a first path for receiving RF signals from the first antenna 109 and a second path for receiving RF signals from the second antenna 111. The first path includes in series: the first antenna 109, a bandpass filter 113, a matching network 123 and a mixer 125. The second path includes in series: the second antenna 111, a bandpass filter 114, a matching network 127 and a mixer 129. A local oscillator 117, coupled to a reference oscillator 115, provides a reference signal to bandpass filters 119 and 121. The bandpass filters 119 and 127 in turn are coupled to matching networks 123 and 127, respectively. The first path terminating with mixer 125 and the second path terminating with mixer 129 are combined into a common third path at the first intermediate frequency (IF) stage 131. The third path includes in series: the first IF stage 131, a second IF stage 133 and a demodulator 135. The demodulator 135 is coupled to the controller 105 for processing the received RF signal and for providing communication with the user interface 107. A DC bias network 140 provides a DC reference voltage for the mixers 125 and 129 and the first IF stage 131. 
     The receiver 101 includes an RF antenna switch 136 to perform antenna diversity. The RF antenna switch 136 includes the matching networks 123 and 127, the mixers 125 and 129, an inverter 138, the first IF stage 131 and the DC bias network 140. The controller 105 generates an antenna select signal on line 141 to alternatively enable the mixer 125 and mixer 129 via the inverter 138. For example, if the signal on line 141 is high, the mixer 125 is enabled and the mixer 129 is disabled. If the signal on line 141 is low, the mixer 125 is disabled and the mixer 129 is enabled. Enabling the mixer 125 allows the first path for receiving RF signals from the first antenna 109 to be coupled to the first IF stage 131. Enabling the mixer 129 allows the second path for receiving RF signals from the second antenna 111 to be coupled to the first IF stage 131. 
     The antenna switch of the present invention is advantageous over the antenna switches of the prior art. While the prior art antenna switches have either high loss or high current drain, the antenna switch of the present invention has both low loss and low current drain. 
     Switching between antenna paths after a gain stage (i.e. after mixers 125 and 129) reduces the switch&#39;s loss contribution to the system noise figure of the receiver 101. A traditional switch could have been at this position in the receiver. However, it would be expensive to add the complexity of matching each mixer 125 and 129 to the input terminals of the traditional switch, passing the signal through the traditional switch to its output terminal, and matching this output terminal to the input terminal of the first IF stage 131. The loss of the traditional switch would would be substantially higher than the loss contribution of the present invention. 
     For example, in the prior art a traditional pin diode switch can be biased for losses as low as 0.4 dB with a significant bias current dissipation of 50 mA. Gallium arsenide switches have higher losses of 1.2 dB with reduced currents. Both the pin diode and the gallium arsenide antenna switch implementations are expensive. Configuring a conventional switch before a gain stage in a receiver will result in a loss in receiver sensitivity equal to the switch network&#39;s loss. Configuring the switch after a gain stage in a receiver will result in less than 2 dB loss in receiver sensitivity. Losses for a traditional switch network including a matching network, the antenna switch and a matching network are as high as 2 dB, (0.5+1+0.5 dB). This puts a design burden on the first gain stage to have a large gain (&gt;10 dB) to take over the noise and reduce its loss contribution to the system noise figure to an acceptable level. Each configuration has cost and design penalties. The present invention has a system noise figure degradation of as low as 0.1 dB, has minimal current penalty for the inverter and low parts count for occupying precious printed circuit board space. These improvements represent a significant improvement over existing antenna switch art. 
     The present invention advantageously symmetrical matches the outputs 126 and 130 of mixers 125 and 129 and the input of the first IF stage 131. The enabled mixer will present an &#34;enabled impedance&#34; to the input of the first IF stage 131. The disabled mixer will present a &#34;disabled impedance&#34; to the input of the first IF stage 131. By alternatively enabling mixers 125 and 129 the &#34;enabled impedance&#34; of one mixer and the &#34;disabled impedance&#34; of the other mixer are see at the input of the first IF stage 131. Symmetrical matching outputs of the mixers 125 and 129 maintains a desirable constant impedance to the input of the first IF stage 131. 
     In the preferred embodiment of the present invention, the antenna switch alternatively enables mixers 125 and 129. The mixers 125 and 129 represent any active gain stage, including traditional amplifiers, to be selectively enabled. Furthermore, the active stages may be located at any position in the receiver line up and not limited to location of the first mixers. 
     The first antenna 109 receives a first signal 108 in a first predetermined frequency band. The second antenna 109 receives a second signal 110 in a second predetermined frequency band. In the preferred embodiment of the present invention, the first 103 and the second 110 signals are included in substantially the same frequency band and transmitted from the same source. In this case both the first antenna 109 and the second antenna 111 and their associated first path and the second path for receiving a signals share a common design which is dependent on the predetermined frequency band of the signals to be received. The antenna switch 136 is used to select one of the first antenna 109 and the second antenna 111 for receiving the signal depending on the quality of the signal at each antenna. Alternatively, the predetermined frequency bands of the first and the second signals may be different. In this case the design of the first antenna 109 and its associated first path for receiving a signal in the first frequency band is different from the design of the second antenna 111 and its associated second path for receiving a signal in the second frequency band. The differences are dependent on the predetermined frequency bands of the first and the second signals to be received. Thus, the antenna switch 136 may also be used for a dual mode receiver (i.e. a receiver designed to received signals from two different frequency bands). If needed, the local oscillator 117 and the demodulator 135 can also be controlled in the dual mode receiver responsive to the antenna select signal at line 141 (dotted line). 
     The transmitter 103 couples the controller 103 to the second antenna 111. The transmitter includes a transmitting unit 139 coupled to a bandpass filter 137. The transmitter 103 in combination with the receiver 101 produce the transceiver 100 that receives and transmits RF signals. The frequency characteristics of the bandpass filters 114 and 137 are different to allow the transceiver 100 to send and receive RF signals via the second antenna 111 simultaneously. The transceiver 100 represents a portable or mobile unit, such as a cellular radiotelephone, in the preferred embodiment but may alternatively represent a fixed base station unit, such as that used in cellular radiotelephone systems. 
     Referring now to FIG. 2 there is shown a schematic diagram of the antenna switch 136 of FIG. 1. Portions of the schematic diagram are outlined to correspond to the matching networks 123 and 127, the mixers 125 and 129, the inverter 138, the first IF stage 131 and the DC bias network 140 of FIG. 1. Line 120 is coupled to matching network 123 which includes a capacitor C201, a capacitor C202 and an inductor L201. 
     Beginning with the matching network 123, the capacitor C201 couples the line 120 to one terminal of the inductor L201 and the capacitor C202. The second terminal of the capacitor C202 is grounded. The second terminal of the inductor L201 is coupled to the mixer 125. The matching network 123 matches the impedance at line 120 to a the impedance at the mixer 125. 
     The mixer 125 includes a transistor Q202, a resistor R204, a capacitor C206, a resister R205, an inductor L203, and a transistor Q203. The mixer 125 has inputs for receiving signals from the matching network 123, the antenna select line 141 and from the D.C. bias network 140. The antenna select signal at line 141 is coupled to the base terminal of the transistor Q202. The emitter terminal of the transistor Q202 is grounded and the collector terminal is coupled to a first terminal of the resistor R204, the capacitor C206, the inductor L203 and the resistor R205. The second terminal of the resistor R204 and the capacitor C206 are grounded. The second terminal of inductor L203 is coupled to the base terminal of the transistor Q203. The second terminal of the resistor R205 is coupled to the DC bias network 140 and the mixer 129. The emitter terminal of the transistor Q203 is grounded and the collector terminal 126 provides the output of the mixer 125. The resistor R204 and the resistor R205 in combination with the DC bias network 140 provide a voltage bias for the transistor Q203. The capacitor C206 and the inductor L203 provide a tank circuit that is series resonant at the first intermediate frequency. 
     The matching network 127 coupling line 122 to the mixer 129 includes a capacitor C203, an inductor L202, and a capacitor C204. The match circuit 127 has the same topography and operates in the same fashion as the match circuit 123 described above. 
     The mixer 129 has inputs for receiving signals from the matching network 127, the antenna select line 141 via the inverter 138 and from the D.C. bias network 140. The mixer 129 includes a transistor Q201, a resistor R203, a capacitor C205, a resistor R206, an inductor L204 and a transistor Q204 has the same topography and operates in the same fashion as the mixer 125 described above. 
     The first IF stage 131 receives inputs at 126 and 130 from the mixer 125, the mixer 129 and the DC bias network 140 to produce an output at line 132. The first IF stage 131 includes transmission lines T201, T202 and T203, a capacitor C207, an inductor L205, a resistor R207, a capacitor C211, a capacitor C209 and a bandpass filter 200. The collector terminal 126 of the mixer 125 is coupled to a first terminal of the transmission line T201. Likewise, the collector terminal 130 of the mixer 129 is coupled to a first terminal of the transmission line T202. A second terminal of the transmission line T201 is coupled to a second terminal of the transmission line T202. The junction of the second terminals of the transmission lines T201 and T202 is coupled to a first terminal of the capacitor C207 and a first terminal of the inductor L205 and a first terminal of the resistor R207. The second terminal of capacitor C207 is grounded. The second terminal of the inductor L205 and the second terminal of the resistor R207 are coupled to a first terminal of the capacitor C211, a first terminal of terminal of the capacitor C209 and the DC bias network 140. A second terminal of the capacitor C211 is grounded. A second terminal of the capacitor C209 is coupled to a first terminal of the transmission line T203. A second terminal of the transmission line T203 is coupled to a bandpass filter 200. The output terminal 132 of the bandpass filter 200 is coupled to the second IF stage 133 of FIG. 1. The transmission line T201 and the capacitor C207 or the transmission line T202 and the capacitor C207 form a series resonant short at both the local oscillator frequency and the radio frequency signal. The inductor L205, the resistor R207, the capacitor C211, the capacitor C209 and the transmission line T203 form a matching network to match between the output of either mixer 125 or mixer 129 and the bandpass filter 200. 
     Since the mixers 125 and 129 are alternatively enabled, the impedance formed by the first IF stage 131 includes the impedance seen at the output of the enabled mixer as well as the impedance seen at the output of the disabled mixer. 
     The DC bias network 140 includes a resistor R208, a capacitor C210 and an inductor L211. The DC bias network 140 conventionally provides a voltage bias for the mixers 125 and 129 and the first IF stage 131. 
     FIG. 3 is a process flow diagram describing a method of operation of the diversity receiver 10 1 of FIG. 1. The flow starts at step 301. At step 303, a first RF signal is received via a first path including in series: a first antenna 109, a first bandpass filter 113, a first matching network 123, a first mixer 125 having a first output terminal. At step 305, a second RF signal is received via a second path including in series: a second antenna 111, a second bandpass filter 114, a second matching network 127, a second mixer 129 having a second output terminal. At step 307, the the first path and the second path are combined via a third path at the output terminals of the first and second mixers 125 and 129. At step 309, the first mixer and the second mixer are alternatively enabled for alternatively coupling the first RF signal and the second RF signal, respectively, to the third path. 
     In the preferred embodiment of the present invention transistors Q201-4 are available from Motorola Inc. under part nos. MSD1819A-RT1 (Q201 and Q202) and MRF941 (Q203 and Q204). Transmission lines T201 and T202 have a physical length of 11.43 mm and T203 has a physical length of 27.94 mm. The values for the resistors, capacitors and inductors are as follows: 
     
         ______________________________________                                    
R203, R204 and R207 = 2.7 KΩ                                        
                    L201 and L202 = 15 nH                                 
R205 and R206 = 10 KΩ                                               
                    L203 and L204 = 68 nH                                 
R208 = 330 Ω  L205 = 390 nH                                         
                    L211 = 1000 nH                                        
C201 and C202 = 27 pF                                                     
                    C209 = 10,000 pF                                      
C203 and C204 = 2.7 pF                                                    
                    C210 = 1500 pF                                        
C205 and C206 = 33 pF                                                     
                    C211 = 18 pF                                          
C207 = 6.8 pF                                                             
______________________________________