Patent Publication Number: US-2005130603-A1

Title: Low noise transceiver

Description:
TECHNICAL FIELD  
      The present invention relates generally to communications devices and more particularly to a low noise transceiver.  
     BACKGROUND OF THE INVENTION  
      Wireless devices typically transmit and receive data through the air on high frequency electromagnetic waveforms. Encoding the data to be transmitted begins data transmission. This encoded data typically has a data rate of 100 kHz to 100 MHz and is used to modulate a high frequency carrier signal. The carrier signal is often in the 800 MHz to 10 GHz range. The modulated carrier signal is then applied to an antenna for broadcasting. Reception involves receiving a radio frequency (RF) signal on an antenna, and filtering undesired spectral components. The signal is demodulated, filtered again, and decoded.  
      Various types of communications devices exist for transmitting and receiving communications signals. Transceivers are a type of device that enables both transmission and reception at a single device, often times employing the same antenna. One class of transceivers operates in a half duplex mode in which the transceiver operates in one of a transmit mode or a receive mode. Other devices, usually more complex ones, can enable concurrent transmission and reception of signals. For many devices that operate in the half-duplex mode, a switch is utilized to facilitate the desired function, either transmission or reception of signals. Accordingly, transient signals can occur during transitions between the transmit mode and the receive mode. The transients can corrupt one or both of the transmitted signals or the received signals near the time of the transition.  
     SUMMARY OF THE INVENTION  
      The present invention relates generally to a low noise transceiver. The transceiver establishes substantially identical common mode voltages at a pair of first and second nodes. A receiver also is coupled to the first node for detecting a received signal. A switch between the first and second nodes operates for connecting the nodes during a first operating mode (e.g., a transmit mode) and for disconnecting the nodes during a second operating mode (e.g., a receive mode). During the first operating mode, the first and second nodes form part of a low impedance path for diverting current away from the receiver. Additionally, since the common mode voltages exist at the first and second nodes, transients at the first node and at the receiver are mitigated as the transceiver transitions between the first and second operating modes.  
      Another aspect of the present invention provides a method for operating a transceiver. The method includes establishing a common mode voltage at a first node to which a receiver is coupled. The common mode voltage is also established at a second node. During a transmit operating mode, the first and second nodes are connected to define a low impedance path for propagating a transmission signal away from the first node. During a receive operating mode, the first and second nodes are disconnected to enable a signal provided at the first node to be detected by the receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other aspects of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings.  
       FIG. 1  depicts a block diagram of a transceiver in accordance with an aspect of the present invention.  
       FIG. 2  depicts a block diagram of a transceiver in accordance with another aspect of the present invention.  
       FIG. 3  depicts a circuit diagram of a transceiver in accordance with an aspect of the present invention.  
       FIG. 4  is a flow diagram illustrating a methodology in accordance with an aspect of the present invention.  
    
    
     DETAILED DESCRIPTION  
      The present invention relates generally to a communications device, such as a low noise transceiver. The transceiver establishes substantially identical common mode voltages at first and second nodes. A receiver also is coupled to the first node for receiving a signal. A switch interconnects the first and second nodes for connecting the nodes during a first operating mode (e.g., a transmit mode) and for disconnecting the nodes during a second operating mode (e.g., a receive mode). During the first operating mode, the first and second nodes form part of a low impedance path for diverting current away from the receiver. Additionally, since the common mode voltages exist at the first and second nodes, transients at the first node and at the receiver are mitigated as the transceiver transitions between the first and second operating modes.  
       FIG. 1  depicts an example of a system  10  that can be implemented in accordance with an aspect of the present invention. The system  10  includes a transmitter  12  coupled to an antenna  14  for transmitting desired data as electromagnetic waveforms modulated on a carrier. The antenna  14 , for example, is configured to form a resonant circuit at the carrier frequency. The transmitter  12  receives an input signal indicative of the data to be modulated and transmitted via the antenna. For example, the input data can be transmitted as an amplitude shift keying (ASK) modulated signal, a frequency shift keying (FSK) modulated signal as well as other forms of modulation.  
      A receiver  16  is coupled to receive a signal at a node  20 . The receiver  16  also is configured to provide a common mode voltage at the node  20 . For example, the receiver  16  includes an amplifier configured to provide a desired common mode voltage at the node  20  based on a reference voltage signal (V REF ).  
      The antenna  14  is also coupled to the node  20  for receiving a modulated signal broadcast from an external source (not shown). The modulated signal received at the antenna  14  can be modulated to encode data using one of a number of possible formats. For example, the receiver  16  can be configured to receive a FSK modulated signal, an ASK modulated signal as well as other forms of modulation. The receiver  16  provides an output signal based on the modulated signal received by the antenna  14 , such as during a receive mode of the system  10 .  
      The system  10  also includes a switch  18  coupled between the first node  20  and a second node  22 . An amplifier  24  provides a common mode voltage at the node  22 , such as based on the reference voltage (V REF ). The common mode voltages at the node  20  and  22  can be substantially the same. The switch  18  operates to selectively connect and disconnect the respective nodes  20  and  22  based on a mode selection (MODE SEL) signal. The mode selection signal can be provided from a control system or other circuitry (not shown) to indicate an operating mode of the system  10 .  
      A low impedance path, indicated at  26 , is connected between the node  22  and electrical ground (or other low potential). For example, the low impedance path includes one or more components (e.g., including a capacitor) having a lower impedance relative to the impedance of the receiver  16 . During the transmit mode, when the switch  18  is closed, the nodes  20  and  22  are coupled together for diverting transmission current from the node  20  and through low impedance path  26 . The amplifier  24  can mitigate small DC bias currents by maintaining the desired common mode voltage at the node  22 .  
      By way of further example, during the transmit mode, the mode selection signal operates the switch  18  to electrically couple the nodes  20  and  22 . Thus, the transmission signal can travel from the transmitter  12  through the antenna  14  through the switch  18  through the low impedance path  26  and to ground. During a receive mode, the mode selection signal operates to decouple the nodes  20  and  22 . As a result, a signal received by the antenna  14  is provided at the input of the receiver  16 . The receiver  16  in turn provides a corresponding output signal indicative of the signal received by the antenna  14 .  
      It will be appreciated that during transitions between the transmit mode and the receive mode, the DC voltage at the node  20  will remain substantially fixed due to the common mode voltage established at the nodes  20  and  22  by the receiver  16  and the amplifier  24 , respectively. Because such common mode voltages are provided at the respective nodes  20  and  22  transient voltages or glitches at the node  20  are mitigated. This enables improved data reception by the receiver  16 .  
      According to an aspect of the present invention, the transmitter  12 , receiver  16 , switch  18  and amplifier  24  can be implemented as part of a common integrated circuit  28 . Additionally, while the low impedance path  26  is depicted as being external to the integrated circuit  28 , it will be understood and appreciated that such path alternatively could be implemented within the IC  28 .  
       FIG. 2  depicts an example of a transceiver system  50  implemented in accordance with an aspect of the present invention. The system  50  includes a control block  52  that is operative to control operation of the system  50 . The control block is coupled to a transmit power amplifier (PA)  54  that is operative to provide a modulated output signal to an associated antenna  56 . The transmit PA  54  is configured to provide an output signal to the antenna  56  modulated at a desired carrier frequency. The antenna  56 , for example, is a loop antenna configured to resonate at the carrier frequency provided by the transmit PA  54 . The antenna  56  may have a variable resonant frequency, which can be set by a program signal (PROG) to configure one or more components to a desired impedance value.  
      The system  50  also includes a receiver  58  coupled to the antenna  56 . That is, the antenna is connected between the transmit PA  54  and the receiver  58 . The receiver  58  includes an operational amplifier (op-amp)  60  that receives a reference voltage (V REF ) signal at a non-inverting input. An inverting input of the op-amp  60  is coupled to an input node  62  of the receiver  58 , which is coupled to the antenna  56 . An output of the op-amp  60  is coupled to the control block  52  for providing an indication of a signal detected by the receiver  58 .  
      The receiver  58  also includes a resistor  64  coupled between the output of the op-amp  60  and a node  62 , although other types of components could also be utilized. The node  62  is connected with an inverting input of the op-amp  60  to form a negative feedback loop. The resistor  64  is located within the feedback loop of the receiver  58 . As a result, the output of the op-amp  60  can swing over a desired voltage range in response to signals received at the antenna  56  while the desired common mode voltage (e.g., V REF ) is maintained at the node  62 . The corresponding signal at the output of the op-amp  60  is provided to the control block  52 .  
      A transmit/receive (TX/RX) switch  66  is coupled between the node  62  and an associated node  68 . The switch  66  operates based on a mode signal provided by the control block  52 . For example, the switch  66  electrically connects the respective nodes  62  and  68  during a transmit mode and decouples the respective nodes during a receive mode. Thus, during the receive mode, a signal received at the antenna  56  is provided to the receiver  58 . In this way, the receive signal provided to the control block by the op-amp  60  corresponds to the signal received at the antenna  56 . During the transmit mode, the transmit current can be diverted through the switch  66  and away from the receiver  58 .  
      Another amplifier  70  is coupled to provide a desired common mode voltage at the node  68 . In particular, the amplifier  70  includes an op-amp  72  that receives a DC reference voltage V REF  at a non-inverting input of the op-amp. An inverting input of the op-amp  72  is coupled to the output of the op-amp such that the desired common mode voltage (e.g., V REF ) is provided at the node  68 . A low impedance path is also connected at the node  68 , which in this example is implemented as a capacitor  74 .  
      In the example of  FIG. 2 , a current sensor  76  is coupled across a current sense resistor  78  connected between the switch  66  and the node  68 . The current sensor  76  provides a current sense signal to the control block  52  indicative of current flowing through the resistor  78  during the transmit mode. The current sense resistor  78  can be implemented, for example, as a low resistance (e.g., about 1 Ω) so that it has a substantially insignificant impact on the impedance of the path between the antenna  56  and the low impedance path  74 .  
      The control block  52  can employ the current sense signal from the current sensor  76  to control the power of the transmit PA  54  during the transmit mode. For example, during the transmit mode, the control block  52  provides a mode selection signal to operate the switch  66  to a closed condition. The transmission signal provided by the transmit PA  54  is provided to the antenna  56 , encoding desired data at the resonant frequency. The antenna  56  broadcasts the transmission signal to free space as a corresponding wireless signal. The transmission signal (e.g., electrical current) also propagates through the switch  66 , through the current sense resistor  78  and to the low impedance path  74 . The amount of current flowing through the current sense resistor  78  can be utilized by the control block  52  as feedback to adjust the transmit power. During the transmit mode, an insignificant amount of the transmission current may be provided to the receiver  58  since most current is diverted through the switch  66  and to the low impedance path  74 .  
      When the control block  52  provides the mode selection signal to open the switch  66  for entering the receive mode, a signal received at the antenna  56  is provided to the input of the receiver  58 . Since the op-amp  60  maintains the input node  62  at the desired common mode voltage V REF , the output of the op-amp  60  will vary as a function of the signal received by the antenna  56 .  
      It will be appreciated that when the switch  66  is opened (e.g., a transition from the transmit mode to the receive mode), transients (or glitches) at the input node  62  are mitigated since the nodes  62  and  68  are both maintained at the desired common mode voltage V REF . Similarly, when changing from the receive mode to the transmit mode, glitches are also mitigated due to the common mode voltage at the nodes  62  and  68 .  
       FIG. 3  depicts an example of a transceiver system  100  that can be implemented in accordance with an aspect of the present invention. In this example, the transceiver system  100  is illustrated as an integrated circuit. The transceiver system  100  includes a transmitter portion  102  that is coupled to an antenna  104  through an antenna pin (ANT). The antenna  104 , for example, is a loop antenna that defines a resonant circuit. The transceiver system  100  also includes a receiver portion  108  that is coupled to the antenna  104  through a low frequency (LF) pin of the IC incorporating the transceiver system. The LF pin thus corresponds to an input node  110  of the receiver portion  108  for receiving a signal received by the antenna. The input node  110  is coupled to a DECOUPLE pin through a switch device  112 . The switch device  112 , for example, is a transistor (e.g., metal oxide field effect transistor) that operates to selectively couple the LF pin with the DECOUPLE pin based on a mode selection signal provided by control circuitry (not shown).  
      The receiver portion  108  is configured to maintain a desired common mode voltage at the node  110  and the LF pin (the LF pin is essentially the same as the node  110 ). The transceiver system  100  also includes another amplifier  114  coupled to the DECOUPLE pin to maintain the desired common mode voltage at the DECOUPLE pin. Circuitry  116  is connected to the DECOUPLE pin. The circuitry  116  provides a low impedance path for sinking transmission current when the switch device  112  is closed.  
      Turning to the contents of the transmitter portion  102 , a power amplifier  120  (e.g., a class D amplifier) is coupled to provide corresponding output signal at a desired carrier frequency to encode desired data. The amplifier  120  provides its output at a node  122 , which corresponds to the ANT pin. The transmitter portion  102  is configured to provide a large output voltage (e.g., about 20 V to about 50 V peak-peak) for encoding data to be transmitted over the antenna  104 . In the example of  FIG. 3 , the amplifier  120  includes a first transistor  124  connected between the output node  122  and a peak voltage V 3 . The voltage V 3 , for example, can be fixed to a voltage in the range from about 20 V to about 50 V. The voltage V 3  thus establishes a maximum peak-to-peak voltage (V p-p ) for the amplifier  120 . Those skilled in the art will appreciate that the particular voltage V 3  can depend, among other things, on the desired transmission range and the antenna configuration.  
      A desired oscillating or pulse-width-modulated voltage signal V 1  is provided at the gate of the transistor  124 . The signal V 1  is provide by control circuitry (not shown). A second transistor  126  is connected between the output node  122  and ground. The transistor  126  is controlled by an oscillating or pulse-width-modulated voltage signal V 2 . Thus, the signals V 1  and V 2  control the respective transistors  124  and  126  so that a corresponding square wave is provided at  122  and the ANT pin.  
      Each of the signals V 1  and V 2  can be provided by control circuitry (not shown) for encoding output data at the desired carrier frequency. The antenna  104  can be tuned to resonate at the carrier. As a result, the antenna  104  operates as a resonant circuit that converts the output square wave at  122  to a corresponding sine wave for broadcasting as electromagnetic waveforms by the antenna. For example, the transmitter portion  102  can provide ASK modulated data by selectively controlling the V 1  and V 2  at the gates of the respective transistors  124  and  126 . The antenna  104 , for example, transmits an electromagnetic waveform for receipt by one or more associated receivers, transceivers or transponders.  
      The transmitter portion  102  also includes a comparator  130  that is coupled to compare the respective voltages provided by the transistors  124  and  126 . One input of the comparator  130  is coupled to a voltage divider formed of resistors  132  and  134  coupled in series between a drain of the transistor  124  and ground. Another input of the comparator  130  is coupled to a voltage divider formed of resistors  136  and  138 . This voltage divider  136 ,  138  is coupled between ground and the output node  122 , which corresponds to the voltage across the other transistor  126 . The comparator  130  in turn provides a corresponding output signal corresponding to the comparison of the sensed voltages. The comparator output signal can be utilized (e.g., by control circuitry) to control the transmitter portion  102 .  
      The receiver portion  108  includes a resistor  142  connected between an output of an op-amp  144  and the node  110  at the input of the receiver portion. The inverting input of the op-amp  144  is also connected to the input  110  to provide desired negative feedback. The node  110  is maintained at the desired common mode voltage based on a reference voltage V REF  provided at the non-inverting input. Since the resistor  142  is within the feedback loop of the amplifier, the output node  110  will remain at the common mode voltage while the output of the op-amp  144  varies as a function of the signal received by the antenna  104 . For example, the signal received at the antenna  104  can be a FSK key modulated signal provided by an external transmitter or transponder.  
      It will be understood and appreciated that the receiver portion  108  allows for large transmission signals at the ANT pin (e.g., about 20-50 V p-p ) while maintaining the signals at the LF and DECOUPLE pins comparatively small (e.g., about less than about 5 V p-p ). Additionally, it will further be appreciated that the configuration depicted herein enables the receiver portion  108  to have a large dynamic range. That is, while the input node  110  is maintained at the desired common mode voltage (e.g., corresponding to V REF  2.5 volts), the output of the op-amp  144  can swing between 0 and about two times the V REF  (e.g., about 5 volts) based on the signal received by the antenna  104 .  
      The antenna  104  is depicted as including a resistor  156  in series with an inductor  158 . A capacitor  160  is coupled between the inductor  158  and electrical ground. Another capacitor  162  is coupled between the LF pin and a node between the inductor  158  and capacitor  160 . The capacitor  160 , for example, corresponds to a trim capacitor that can be set to a desired capacitance so that the antenna  104  defines a resonant circuit that can resonate at a desired frequency, namely, at the carrier provided by the transmitter portion  102 .  
      By way of example, the resistor  156  can have a resistance of about 49 Ω and the inductor  158  can have an inductance of about 466 μH±10%. The capacitor  162 , for example, can be at about 3 nF and the trim capacitor  160  can be set to about 3.37 nF to achieve resonance at approximately 127 KHz. For such an antenna configuration, the capacitance of the circuitry  116  that defines the low impedance path can be set to about 1 μF, which contributes about 0.3% of the series capacitance with the capacitor  162  during the transmit mode. Those skilled in the art will appreciate that other values can be utilized to achieve resonance at other frequencies.  
      During the transmit mode, the transmitter portion  102  provides transmission current to the antenna  104 . Since the switch device  112  is closed in this operating mode, the transmission current is diverted away from the receiver portion  108  to the circuitry  116  that defines the low impedance path. The transmission current can also be sensed through a current sense resistor  166  (e.g., about 1 Ω). For instance, a comparator  168  is coupled across the current sense resistor  166  to provide a corresponding output signal indicative of the transmission current. The output of the comparator  168  can be utilized to further control the transmitter portion  102  during the transmit mode to maintain a desired power level for the transmission. For example, associated control circuitry (not shown) can be utilized to control the pulse-width-modulated input signals at V 1  and V 2  based on the output signal provided by the comparator  168 . In this example, the output of the comparator  168  is provided through a buffer  170 .  
      As mentioned above, the transmission current through the current sense resistor  166  is also provided to the circuitry  116  that defines a low impedance path. In this example, the circuitry  116  includes a capacitor  174  (e.g., about 1 μF) that has an associated series resistance  176  (e.g., about 40-100 mΩ). By selecting the capacitor  174  to have a substantially greater capacitance than the series capacitance provided by the capacitor  162  (e.g., about 3 nF), while in the transmit mode, the capacitor  162  dominates. As a result, the circuitry  116  provides a desired low impedance path for the transmission current provided at the resonant frequency of the antenna  104 . Additionally during the transmit mode, the amplifier  114  can mitigate DC bias currents by maintaining the desired common mode voltage at the DECOUPLE pin.  
      It is to be appreciated that when the transistor  112  is activated to disconnect the LF and DECOUPLE pins (corresponding to a transition from the transmit mode to the receive mode), transients or glitches at the LF pin (and node  110 ) are mitigated. This is because the receiver portion  108  and the amplifier  114  maintain desired common mode voltages at the respective LF and DECOUPLE pins. Transients are also mitigated for transitions from the receive mode to the transmit mode when the LF and DECOUPLE pins are connected through activation of the switch device  112 . Since the configuration in  FIG. 3  substantially prevents spurious signals from being injected into the receiver portion, reception can be improved relative to other transceiver designs.  
      In view of the foregoing structural and functional features described above, certain methodologies that can be implemented will be better appreciated with reference to  FIG. 4 . While, for purposes of simplicity of explanation, the method of  FIG. 4  is shown and described as being implemented serially, it is to be understood and appreciated that the illustrated actions, in other embodiments, may occur in different orders and/or concurrently with other actions. Moreover, not all illustrated features may be required to implement a method according to an aspect of the present invention. It is to be further understood that the following methodology can be implemented in hardware, such as one or more integrated circuits, software, or any combination thereof.  
       FIG. 4  depicts a flow diagram of an example method that can be implemented in accordance with an aspect of the present invention. The method begins at  200  such as in connection with providing power to circuitry (e.g., an integrated circuit or circuit board) utilized to implement the method. At  210 , a common mode voltage is established at node N 1 . Node N 1 , for example corresponds to a connection to an associated antenna, such as for receiving an input signal from one or more external sources. The input signal, for example, can be an FSK modulated signal, although other types of modulation also can be utilized. At  220 , a common mode voltage is established at node N 2 . The node N 2 , for example, corresponds to a DECOUPLE node to which a low impedance path is connected. The low impedance path provides a path during a transmit mode for diverting electrical current away from a receiver coupled to the node N 1 .  
      At  230 , mode control is implemented to decide whether the method is in a transmit mode or a receive mode. For example, the mode control is implemented by a control system according to a predefined control algorithm for the circuitry implementing the method. In the transmit mode (TRANSMIT), the methodology proceeds from  230  to  240 . At  240 , a transmission signal is provided to an antenna at a desired carrier frequency. For example, the transmission signal is provided to an output pin of an integrated circuit to which an antenna is coupled. The antenna can be a loop antenna configured to broadcast electromagnetic waveforms at the carrier frequency based on the transmit signal provided at  240 . At  250 , nodes N 1  and N 2  are coupled together. This creates a connection to a low impedance path connected at node N 2  so that the transmission signal can be diverted away from N 1  as well as away from a receiver coupled at node N 1 .  
      At  260 , the transmission current can be sensed. The sensed current can then be utilized to adjust transmit power (as needed) at  270 . From  270 , the methodology returns to  230  in which the methodology can remain in the transmit mode or switch to a receive mode, such as based on a mode control signal.  
      If the mode control at  230  causes the method to enter the receive mode (RECEIVE), the method proceeds to  280 . In the receive mode, at  280 , nodes N 1  and N 2  are decoupled. This results in disconnecting the low impedance path from the circuit as well as forcing electrical current propagated by the antenna (e.g., corresponding to a signal received by the antenna) to the receiver coupled at node N 1 .  
      For example, electromagnetic waveforms can be received at an antenna and, in turn, converted to an electrical signal and provided to the receiver. Because the nodes N 1  and N 2  are decoupled during the receive mode, electrical current corresponding to the received signal is provided to an input of the receiver. At  290 , the signal received at the node N 1  is detected by the receiver, which can then be processed. The signal can be detected, for example, by adjusting an output of an associated amplifier of the receiver based on electrical current provided by the antenna so as to maintain the desired common mode voltage at node N 1 . Since the common mode voltage is maintained at the node N 1 , variations in the output of the receiver amplifier represents the received signal, which can be processed in a desired manner. The received signal, for example, can correspond to an FSK modulated signal, although other types of modulation can also be utilized.  
      From  290 , the methodology returns to  230  in which the method can either remain in the receive mode or switch back to the transmit mode. It can be appreciated that as the method switches between the receive mode and the transmit mode, glitches at node N 1  (e.g., an input of the receiver) are mitigated. This is because the common mode voltages established at  210  and  220  are maintained throughout the method. As a result, a more accurate indication of the received signal can be provided at the input of the receiver thereby improving reception of the transceiver. Those skilled in the art will understand and appreciate various circuit configurations, including analog and/or digital circuitry, that can be utilized to implement the method described above.  
      What has been described above includes examples and implementations of the present invention. Because it is not possible to describe every conceivable combination of components, circuitry or methodologies for purposes of describing the present invention, one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.