Abstract:
The terminating impedance of a networked device in a wired communication channel is controlled to avoid an impedance discontinuity when power is applied and removed from the node or other event occurs that would change the impedance of the signal interface. When the node transmits or receives signals using the communication channel, the transmit or receive device presents a matched termination to the channel. When power is removed or the device is reset, the transmit and receive circuitry is not operational and the matched impedance is therefore maintained by a separate device. The impedance may be varied slowly from a match to a high impedance to allow other devices in the network to adapt to the change in multipath environment that results from the impedance change. Alternatively, the signal interface can be switched to a passive static impedance that is maintained while power is off or the disrupting event occurs.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/870,847 filed Dec. 20, 2006, hereby incorporated by reference herein in its entirety. 

   BACKGROUND 
   1. Field 
   The invention relates to wired communication systems and specifically to controlling the impedance of an electrical signal interface of a network. 
   2. Background 
   Consider several communication nodes that are part of the same coaxial cable network or other wired network. Each node connecting to the network presents an impedance at the point of connection. Mismatches between the impedance of the node and the impedance looking into the network cause reflections. Such reflections cause a multipath signal environment that impairs passage of the signal over the medium. The input and output of the nodes include active circuitry that changes its impedance when power is applied and removed. The impedance can also change when other events occur. For example, a reset of a node can temporarily deactivate circuitry. If the nature of the reflections is known, compensation can be provided. However, as the reflections change, the compensation must change also. Adaptation to such changes takes time to complete. Sub-optimal compensation of the changed environment can cause degradation in the performance of the network, including a reduction in link margins and an increase in the data error rate. 
   Accordingly, it is desirable to ensure that the communication channel and other operational nodes of the network are not disturbed when power is applied to, or removed from one or more nodes or when another event changes the impedance that the node presents to the network. 
   SUMMARY OF THE INVENTION 
   The presently disclosed method and apparatus controls the impedance that a node presents to a network to minimize or avoid disruptions to signals communicated over the network. More specifically, the presently disclosed method and apparatus controls the impedance presented to a network by a node when power is applied to, or removed from the node or when the node experiences a reset or other event that changes the impedance. 
   In one embodiment, an Impedance Control Device is used in conjunction with a “Transitioning Node”. A Transitioning Node is any node on the network for which the impedance that the node presents to the network is changing. The Impedance Control Device will typically be placed between the Transitioning Node and the network to alter the impedance presented by the Transitioning Node to the network. Changes to the impedance of the Transitioning Node may be due to power being applied, removed or due to any other condition that will change the impedance presented to the network, such as a reset. 
   The impedance of the Impedance Control Device causes the impedance presented to the network to slowly transition so the other nodes in the network (some of which may also be transitioning) will have time to adjust to the effect that the change in the impedance of the Transitioning Node has on the network. The other nodes in the network can adapt their modulation type, signal equalization, bit loading, or they can use any other compensation mechanism used to compensate for reflections caused by mismatched impedances in the network. Examples of such compensation include using various well-known techniques suitable for the modulation type used. The rate of the slow transition is dependent on the rate of adaptation. In another embodiment, the impedance presented to the network is held constant or nearly constant during an event by switching an interface between the Transitioning Node and the network away from the Transitioning Node and to a circuit within the Impedance Control Device which has a similar impedance which is held stable. 
   Power Transition Impedance Control 
   For a power down transition 1) the Impedance Control Device detects that the main power source has been removed. 2) The Impedance Control Device immediately switches to cause a circuit having a variable impedance to be presented to the network. The initial state of the variable impedance device is an impedance that is matched to the impedance presented by the Transitioning Node when the Transitioning Node has had power applied for a long enough time for its impedance to be stable. 3) Using some energy that was stored when the device was last powered on, slowly increase the impedance of the variable impedance circuit until the stored energy is depleted and the variable impedance circuit is at maximum impedance. 
   For a power up transition 1) a variable impedance device within the Impedance Control Device is placed in series with the Transitioning Node. 2) When the Impedance Control Device detects that the main power source has been applied, the Impedance Control Device sets the variable impedance device to the maximum impedance. Preferably, the Transitioning Node has been powered off long enough to cause its impedance to be very high. However, if the device is powered on or off before a given transition is fully complete, the new transition direction can increase (or decrease) the impedance from the intermediate impedance the Impedance Control Device last had. 4) Based on a timing circuit within the Impedance Control Device, the variable impedance circuit changes to a minimum impedance allowing time for the Transitioning Node in series with the variable impedance circuit to reach its operating impedance 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of an Impedance Control Device. 
       FIG. 2  shows an embodiment with a matching impedance connected during power off. 
       FIG. 3  shows a block diagram of a circuit to control the switches connected in series with circuit elements used to achieve impedance control at the node interface. 
       FIG. 4  illustrates another embodiment of an impedance control system. 
       FIG. 5  shows a power up/power down (PU/PD) control circuit. 
       FIG. 6  shows a transmit/receive (T/R) switch. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a block diagram of an Impedance Control Device  100  having a switch  120 , a impedance matching circuit  150  and a controller  160 . Also shown is a node  115  which includes a transmitter  130 , a receiver  140  and a voltage reference  155 . A network is connected through a signal interface  110  and the switch  120  to a node  115 . With the switch  120  set to connect the transmitter  130  to the signal interface  110  (the switch  120  in the position shown in  FIG. 1 ), the impedance at the signal interface  110  looking back into the switch  120  is preferably matched to the impedance looking in the opposite direction (i.e., away from the switch  120  and into the network). This impedance can be 50 Ohms, 75 Ohms, or any other impedance value, typically depending upon the impedance of the cable or wire connected to the network. The position of the switch  120  is selected by a controller  160 . The controller  160  determines whether the node  115  is in transmit and receive mode and selects the position of the switch  120  accordingly. The controller  160  also detects changes in the power applied to the node  115  or detects other events that may cause the transmitter or receiver circuits to deactivate, such as a node reset event. When an event is detected, the controller  160  selects the position of the switch  120  corresponding to a matching circuit  150  appropriate to the detected event. 
     FIG. 2  shows an Impedance Control Device  400  coupled to a device interface  410 , such as a connector. The Impedance Control Device  400  includes the multi-position switch  120 , controller  160 , and matching circuit  150  of  FIG. 1 . The Impedance Control Device  400  is coupled to the transmitter  130  and the receiver  140 . The device  400  provides a predictable impedance to a node (not shown) coupled to the device interface  410  during power off. When a loss of power is detected, a switch  430  within the switch  120  connects an impedance  420  to the device interface  410  to provide a predictable impedance as the power is removed from the transmitter  130  and/or receiver  140 . The impedance  420  may be implemented as either a resistance or a complex impedance. The switch  430  maintains a connection to the impedance  420  as long as the power remains off. 
   A transmitter amplifier  450  having a predetermined output impedance, for example, 75 Ohms in the case of a coaxial cable channel, is connected to the device interface  410  by a switch  435  within the switch  120  when the node is transmitting. A receiver amplifier  440  having a predetermined input impedance, also 75 Ohms, is connected to the device interface  410  by a switch  445  when the device is receiving. The impedance presented during transmission and reception is a combination of the intrinsic impedance of the amplifiers  440 ,  450  and a series resistance  460  or a shunt resistance  470  to produce the desired total impedance. The impedance during transmission and reception takes into account the non-zero resistance of the switches  435 ,  445 . Thus, in general, the active circuit impedance will be less than the desired 75 Ohms by an amount equal to the resistance of one of the switches  435 ,  445 . The switches  435 ,  445  preferably have equal resistance. Generally, the receiver amplifier  450  is connected to the device interface  410  except when the node connected to the device interface  410  is transmitting. When power is removed from the node, the switch  430  makes a connection to an matching impedance  420 , also 75 Ohms. 
   The switches  430 ,  435 ,  445  can be constructed using transistors. For example, the switch  435  can be a field effect transistor (FET) connected with a first lead  490  connected to a common terminal  480 , a second lead  492  connected to the series resistance  460 . The gate  494  on the FET is used to turn the transistor on (i.e., reduce the resistance between the first lead  490  and the second lead  492 ). In one embodiment, the transmitter amplifier  450  and receiver amplifier  440  are connected through transistors  435 ,  445  that require positive bias on the gate to turn on the transistor  435 ,  445 . The matching impedance  420  connects through a transistor  430  that uses a negative bias to turn off the transistor  430 . When power is removed, the negative bias is not generated and the gate voltage returns to zero volts and the transistor  430  conducts, thereby connecting the matching impedance  420 . In one embodiment, this transistor  430  can be implemented with a depletion mode MOSFET or other semiconductor device that conducts when no gate bias is present. 
   In another embodiment, when the controller  160  detects an event during which power remains, but the active circuitry is deactivated, such as during a reset event, the switch  120  selects the matched impedance  150  for the duration of the event. At the end of the event, the switch  120  returns to the state in which either the receiver  140  or transmitter  150  is selected, as appropriate. 
   In yet another embodiment, when an event is detected, the switch  120  selects the matching circuit  150 . In this embodiment, the matching circuit  150  has an impedance that is variable starting from an impedance that is equal to or similar to the impedance of the node when it is “on” and slowly changing to an impedance that is equal to or similar to the impedance of the node when it is “off”, or changing from “off” to “on”. 
   The rate of the slowly varying signals is dependent on the speed of adaptation of the network. The total transition time for changing the impedance can range from microseconds to minutes. The transition time can be different for the power up, power down, and event conditions. For example, the power down transition can take many seconds, while the power up transition can take microseconds. 
   As noted with regard to  FIG. 2 , the switch  120  can be implemented with field effect transistors (FETs) where one terminal of each FET is connected to a common point and the other terminal is connected in series with each of the devices and the FETs are controlled to enable one FET at a time. 
     FIG. 3  shows a block diagram of a circuit to control the switches connected in series with circuit elements used to achieve impedance control at a node interface. Power on/off detection and signal control  260  detect the application or removal of power to the node by monitoring, for example, the 3.3-volt power supply of the node. When power is applied, signal control  260  asserts the power up signal. When power is removed, signal control  260  asserts the power down signal. Additionally, the signal control  260  is responsive to an event signal, such as device reset, and can initiate the power up or power down signals in response. 
   The power up signal passes through delay  270  to control RX_MUX, the signal used to select the receiver switch driving signal. The delay interval defines the period of time that RX_MUX selects the RXU_Cntl signal (varying voltage) to drive the switch. After the delay interval expires, RX_MUX selects the RX_Cntl (digital level) signal to drive the switch in the normal transmit/receive mode. 
   Signal transition control  280  creates a slow varying signal in response to the power up signal to produce RXU_Cntl, which drives the receiver switch to slowly turn on the switch and vary the resistance in series with the receiver circuit. 
   Signal transition control  280  can be an R-C network that creates an exponential voltage on a capacitor. Alternatively, signal transition control can be a constant current source that produces a substantially linear voltage across a capacitor. Signal transition control can be designed to discharge the capacitor quickly, and immediately change the output voltage level, when the input signal is deactivated, or to provide a slow transition in both directions of input signal transition. 
   Signal transition control  290  creates a slow varying signal in response to the power down signal to produce POR_Cntl*, which drives the matched impedance switch to slowly turn on the switch and vary the resistance in series. The power down signal can be an active low signal so that the inactive state maintains a voltage on a capacitor, thus storing energy. When the active-low power down signal is asserted, POR_Cntl* can be asserted to immediately turn on the matched impedance switch, then slowly drop the signal voltage to turn off the switch and increase the effective impedance at the node interface. 
   In one embodiment, the matched impedance is switched to the node interface upon power removal, then after transitioning to a high impedance and the other network nodes adapt to the new multipath environment, the un-powered node can be removed without a disruption in the multipath environment of the network. How the RX_MUX, RXU_MUX, and POR_Cntl* signals are used is discussed in further detail below with regard to  FIG. 6 . 
     FIG. 4  illustrates another embodiment of an impedance control system having a power supply  180 , a voltage divider  190 , a power up/power down (PU/PD) control circuit  200 , a controller  165 , a transmit/receive (T/R) switch  310 , a transmit power amplifier (TX PA), a receive low noise amplifier (RX LNA) and a matching network  350 . In accordance with the embodiment shown in  FIG. 4 , when power is applied, a first voltage of 3.3 volts and a second voltage of 2.5 volts are presented to the PU/PD control circuit  200 . The 2.5 volt power source is the voltage divider  190 . The PU/PD control circuit  200  outputs three signals, RXU_Cntl, POR_Cntl, and RX_MUX to the T/R switch  310 . These signals provide control to the switch as will be explained in greater detail below. The matching circuit  350  is placed in series between the network and the T/R switch  350 . A second RF path  195  from the network is provided directly to the T/R switch  310 . The second RF path  195  is connected through the T/R switch  310  to either the TX PA  330  or the RX LNA  340 , as will be explained in greater detail below. The determination as to whether the network is connected to the TX PA  330 , the RX LNA  340  or through the matching network  350  is controlled by the controller  165 . 
   Referring now to  FIG. 5 , an example of the details of the PU/PD control circuit  200  is shown.  FIG. 6  illustrates the details of one embodiment of the T/R switch  310  that is controlled by the PU/PD control circuit  200  of  FIG. 5 . A Power On Reset Block  210  (POR) shown in  FIG. 5  detects the transient of the power supply coming up from 0 to full operating power (vdd) or down from vdd to 0. Alternatively, the POR  210  can detect a transient on the power supply that is indicative of a power transition. 
   The POR  210  generates a signal entitled “POR_Cntl”. The POR_Cntl signal is initially in the logical “0” state (inactive) during a power up transition (i.e., when power is initially applied). The POR_Cntl signal is coupled to the input of an inverter  235  to create a signal entitled “POR_Cntl*”. POR_Cntl* is the logical inverse of POR_Cntl, however, when power is first applied, the inverter  235  does not have power and the output of the inverter  235  will remain low for some period of time. Power is supplied to the inverter  235  by the signal RXU_Cntl. As will be seen further below, the RXU_Cntl is connected indirectly to the 3.3 volt power supply line provided by the power supply  180  and rises more slowly then the 3.3 volt line directly from the power supply  180 . 
   POR_Cntl* is coupled to a switch  355  shown in  FIG. 6 . When the voltage of POR_Cntl* is low, the switch  355  will be in a high impedance state (the switch is turned off). Thus, the matching network  350  shown in  FIGS. 3 and 1B  is not connected to the network. As shown in  FIG. 5 , the power supply  180  provides 3.3 volts to the circuit  200 , but it will be understood by those skilled in the art that other voltages would also be appropriate. As noted above, RXU_Cntl is coupled to the 3.3 volt power supply line as well, but not directly. Rather, RXU_Cntl is coupled to the 3.3V power supply line through either an FET  220  or an FET  230 . In addition, a capacitor  250  is provided on the RXU_Cntl line. The capacitor  250  will cause the voltage on RXU_Cntl to rise relatively slowly compared to the voltage on the 3.3 volt power supply line. The signal RXU_Cntl is also coupled to the positive input to a comparator  240 . Power to the comparator  240  is applied directly from the 3.3 volt power supply  180 . The negative input to the comparator  240  is coupled to the 2.5 volt output of the voltage divider  190 . Once RXU_Cntl charges up to a level that is greater than the 2.5 volt line provided to the PU/PD control circuit  200  by the voltage divider  190 , the output RX_MUX transitions from a logical “0” level to a logical “1”. 
   The output RX_MUX of the comparator  240  is coupled from the comparator  240  shown in  FIG. 5  to the “select” input of a multiplexer  347  shown in  FIG. 6 . The RXU_Cntl signal is coupled from the PU/PD control circuit  200  of  FIG. 5  to the “0” input of the multiplexer  347  shown in  FIG. 6 . RXU_Cntl is increasing gradually from 0 volts to a full vdd level depending on the C_PD value and leakage current at this point. Up until that point, RX_MUX is at a logical zero. RX_MUX being at a logical “0” will cause the “0” input to be coupled to the gate of the FET  345  and a logical “1” will cause the multiplexer  347  to couple the signal coupled to the “1” input to the gate of the FET  345 . Accordingly, RXU_Cntl, which is provided from the controller  165  (shown in  FIG. 4 ) and coupled to the “0” input of the comparator  240 , is passed through to the gate of the FET  345 . The signal applied to the gate of the FET  345  will keep the FET  345  initially at a high impedance and will slowly turn the FET  345  on as RXU_Cntl rises. When RXU_Cntl gets above the 2.5 volt reference voltage the FET  345  will be fully on. In addition, as RXU_Cntl rises above the 2.5 volt level, the comparator  240  will switch and RX_MUX will rise causing the multiplexer  347  to couple RX_Cntl to the gate of the FET  345 . 
   The signal POR_Cntl which is generated by the POR circuit  210  shown in  FIG. 5  is at a high voltage (3.3 volts in the present embodiment) when the POR circuit  210  detects power. Resistor R_PU  212  and C_PU  214  create a slow changing voltage at node POR_PU at the gate of the FET  220  shown in  FIG. 5 , which controls the N device  220 . When vdd rises from 0 to 3.3 volts, POR_Cntl is driven high and the N device  220  turns on. POR_Cntl* (the logical complement of POR_Cntl) is initially at low voltage level, since power is applied to the inverter  235  by RXU_Cntl. POR_Cntl* is coupled to the P device  230  which turns on and starts charging the capacitor C_PD  250 . By doing this, the signal RXU_Cntl is now at high level, ‘1’, that is, full vdd level, which can be 5V, 3.3V, 1.8V, 1.2V, or 1V or other voltage. 
   When power is removed, the POR circuit  210  drives its output, POR_Cntl, to a low voltage level. This causes the N device to turn off. POR_Cntl* is then driven to a high voltage by the inverter  235 . POR_Cntl* turns off the P device  230 , isolating the already charged tank capacitor C_PD  250 . By doing this, the signal RXU_Cntl remains at a high voltage level (full vdd level) and decreases gradually depending on the value of the capacitor C_PD  250  and the leakage current at this circuit node. RX_MUX drifts to a low voltage level no matter what is the state of RXU_Cntl is because the comparator  240  generating the RX_MUX is powered by the 3.3 volt power supply which has been powered down. The impedance of the RX LNA  340  is now high because the power to the RX LNA  340  is off. Furthermore, because POR_Cntl* is held at a high voltage, the FET  355  is held on and so the matching network  350  is coupled through the FET  355  to the network. As the voltage of POR_Cntl drops, the impedance through the FET  355  will start to increase and will change gradually from low impedance to high impedance. 
   It should be noted that the switch  310  can be implemented with individual switches in series with each of the switched components, each switch with a separate control signal. Any of the components can be connected to the common switch node using the corresponding control signal. 
   TX power amplifier  330  is connected to the network through the switch through FET  335  when the TX_Cntl signal is at a high voltage. TX_Cntl is generated by the controller  165  shown in  FIG. 4 . It should be noted that any other switching device responsive to the control signal TX_Cntl may be used rather than the FET  335  shown in  FIG. 6 . 
   During normal operation, RX_MUX will be high, and so the multiplexer  347  will couple RX_Cntl to the gate of the FET  345 . RX LNA  340 , or any other receiver component, is connected to the network through FET  345 . As noted above, FET  345  is controlled by either RX_Cntl (a digital signal) or RXU_Cntl (a varying level signal). 
   Power Shutdown Modes 
   These modes are designed for the case when the power supply is available, but it is desired to minimize device operating current. 
   Complete shutdown mode (C) is as complete a shutdown as possible while power is still being applied to the device. When shutdown mode C is enabled all active devices of the affected IC are disabled with the exception of the T/R switch, which is directed to connect the RF I/O port to the passive matching circuit. This mode is preferred for critical power conservation, such as battery backup operation for lifeline services. 
   Partial shutdown mode (P) is a partial shutdown mode. When shutdown mode P is enabled all transmit active devices are disabled. However, receive active devices remain enabled, and the T/R switch is directed to connect the RF I/O port to the receive path. This mode is recommended for all other power down requirements, such that power on and power off (sleep) states will have minimal impact to the system.