Abstract:
An device and method for a pre-sampling processing is disclosed. The pre-sampling device includes a single amplifier having a virtual ground node, and a feed back circuit connected from an output of the amplifier to the virtual ground node. The feed back circuit includes a plurality of switches connected to the virtual ground node. The switches control a plurality of programmable gain settings. The feed back circuit also includes an adjustable current source that is adjusted according to an estimated echo signal. A current of the adjustable current source is summed at the virtual ground node. The feed back circuit also includes a low pass filter that is tuned to suppress received signal frequencies above a fraction of a sampling frequency of a sampler connected to the pre-sampling device.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to communication systems. More particularly, the invention relates to an apparatus and method for single amplifier pre-sampling signal processing. 
     BACKGROUND OF THE INVENTION 
     High-speed networks are continually evolving. The evolution includes a continuing advancement in the operational speed of the networks. The network implementation of choice that has emerged is Ethernet networks physically connected over unshielded twisted pair wiring. Ethernet in its 10BASE-T form is one of the most prevalent high speed LANs (local area network) for providing connectivity between personal computers, workstations and servers. 
     High-speed LAN technologies include 100BASE-T (Fast Ethernet) and 1000BASE-T (Gigabit Ethernet). Fast Ethernet technology has provided a smooth evolution from 10 Megabits per second (Mbps) performance of 10BASE-T to the 100 Mbps performance of 100BASE-T. Gigabit Ethernet provides 1 Gigabit per second (Gbps) bandwidth with essentially the simplicity of Ethernet. There is a desire to increase operating performance of Ethernet to even greater data rates. 
       FIG. 1  shows a block diagram of an Ethernet transceiver pair communicating over a bi-directional transmission channel, according to the prior art. The transceiver pair includes a first transceiver  100  and a second transceiver  105 . The first transceiver  100  includes a transmitter section  110  that receives digital data for transmission over a transmission channel  135 . The first transceiver  100  also includes a receiver section  120  that receives data. 
     The transceiver includes a digital to analog converter (DAC) for transmission, and an analog to digital converter (ADC) for reception. The hybrid circuit  130  is designed to reduce the level the transmit signal present in the receive signal path. The transmitter section  110  and the receiver section  120  are connected to a common twisted pair causing some of the transmission signals of the transmitter section  110  to be coupled into the receive signals of the receiver section  120 . The coupled signal can be referred to as an “echo” signal. The echo signal can include two separate primary components. The first component includes transmission signals due to the direct connection of the transmitter sections to the input of the receiver sections. The second component includes reflections of the transmit signal within the transmission channel. 
     The hybrid circuit  140  of the second transceiver  105  operates in the same manner as the hybrid circuit  130  of the first transceiver  100 . The transmitter section  150  and the receiver section  160  of the second transceiver  105  operate in the same manner as the transmitter section  110  and receiver section  120  of the first transceiver  100 . 
     An implementation of high speed Ethernet networks includes simultaneous, full bandwidth transmission, in both directions (termed full duplex), within a selected frequency band. When configured to transmit in full duplex mode, Ethernet line cards are generally required to have transmitter and receiver sections of an Ethernet transceiver connected to each other in a parallel configuration to allow both the transmitter and receiver sections to be connected to the same twisted wiring pair for each of four pairs. As a result, each of the four receivers typically suffers from echo signal interference. 
     The hybrid circuits  130 ,  140  can include echo signal cancellation of the above-described first component of the echo signal. Additionally, the received signals can be gain adjusted to compensate for variations in attenuation of the signals during transmission due, for example, because of variations in cable lengths. The received signals are sampled within the receiver sections  100 ,  120 . Sampling requires the received signals to be filtered prior to sampling to minimize aliasing. Therefore, the received signals should also be low-pass filtered with an Anti-Aliasing Filter (AAF). 
     It is desirable to minimize the effects of echo signals. Additionally, it is desirable to provide programmable gain of received signals and filter the signals. It is desirable to provide these features with circuitry that is low cost, low noise and includes high linearity. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention includes a pre-sampling device. The pre-sampling device includes a single amplifier having a virtual ground node, and a feed back circuit connected from an output of the amplifier to the virtual ground node. The feed back circuit includes a plurality of switches connected to the virtual ground node. The switches control a plurality of programmable gain settings. The feed back circuit also includes an adjustable current source that is adjusted according to an estimated echo signal. A current of the adjustable current source is summed at the virtual ground node. The feed back circuit also includes a low pass filter that is tuned to suppress received signal frequencies above a fraction of a sampling frequency of a sampler connected to the pre-sampling device. 
     Another embodiment includes a method of processing a signal before sampling. The method includes supplying a received signal and echo signal to a preprocessing circuit, the preprocessing circuit comprising an amplifier, summing an echo signal cancellation current to a virtual ground node of the amplifier, setting a programmable gain setting of switches connected to the virtual ground node, the adjustable current source being adjusted according to an estimated echo signal, and anti-alias filtering the received signal as determined by a virtual ground node capacitance and an amplifier output capacitance. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art pair of Ethernet transceivers. 
         FIG. 2  shows an Ethernet transceiver that includes an exemplary preprocessing circuit. 
         FIG. 3  shows an exemplary preprocessing circuit. 
         FIG. 4  shows another exemplary preprocessing circuit. 
         FIG. 5  shows another exemplary preprocessing circuit. 
         FIG. 6  shows an embodiment of an echo current cancellation current source. 
         FIG. 7  is a flow chart of an exemplary method of preprocessing. 
     
    
    
     DETAILED DESCRIPTION 
     The invention includes an apparatus and method of preprocessing received analog signals before sampling. The preprocessing includes adjustable gain, filtering and echo current cancellation. The echo current cancellation is adjustable, and can be adaptive. The preprocessing can be implemented with a single amplifier, and therefore, is less expensive. 
       FIG. 2  shows an Ethernet transceiver that includes an exemplary preprocessing circuit. The preprocessing circuit  210  includes echo signal cancellation, programmable gain adjustments and anti-aliasing filtering. The echo signal cancellation can be adaptive. A sampling circuit  220  receives the preprocessed signal and generates samples. An analog to digital converter (ADC)  230  generates digital samples. The digital samples are digitally processed by digital processing circuitry  230 . 
     As previously described, the received signals include an echo signal which needs to be minimized or cancelled. Additionally, the received signals should be gain adjusted and filtered before sampling by the sampling circuit  220 . 
       FIG. 3  shows an exemplary preprocessing circuit. A signal input to the preprocessing circuit includes a received signal, and an echo signal. The echo signal, as previously described, is an undesired signal that is coupled into the received signal because the transceiver that includes the preprocessing circuit is transmitting signals simultaneous with the transceiver receiving other signals. The preprocessing circuit minimizes the echo signal. The preprocessing circuit provides a programmable gain of the received signal, and also provides anti-aliasing filtering for subsequent sampling of the received signal. This preprocessing circuit is cost effective because it only includes a single amplifier. 
     The preprocessing circuit provides a programmable gain by selectively closing switches SW 1 , SW 2 , SW 3 , SW 4 . As shown, the switches switch in a corresponding set of input resistors R 1 , R 2 , R 3 , R 4 . The inputs resistors R 1 , R 2 , R 3 , R 4  along with the feedback resistor RF set the gain of the preprocessing circuit. 
     A current source  330  provides a programmable current for canceling the echo signal. The current of the current source  330  negates current at the virtual node induced by the echo signal present at the input of the preprocessing circuit. The result is that the echo current does not flow through the feedback resistor RF, thereby subtracting the echo signal from the output (processed signal). The current source  330  can just as easily sinks current rather than sourcing current as shown. 
     The programmable current source can be controlled by estimating the echo signal. The estimate can be based upon transmit signals of the transceiver that includes the preprocessing circuit. An exemplary current source  330  includes an echo cancellation DAC (as will be described later) or a replica of a transmit signal (also described later). An input to an echo cancellation DAC can be the replica of a transmit signal, or the input can be from a digital signal processing (DSP) engine that is adaptive, and can account for reflections of transmit signal as well. 
     An undesirable feature of this embodiment is that the echo cancellation current is conducted through the switches SW 1 , SW 2 , SW 3 , SW 4 . The echo signal is typically much greater in amplitude than the desired received signal. The large amplitude of the echo signal can cause non-linear distortion within the switches SW 1 , SW 2 , SW 3 , SW 4 , thereby eroding the effects of echo cancellation. 
     The received signal is filtered by a two pole filter provided by capacitance Cvn of the virtual ground node of the amplifier  330 , and output capacitance CO. Typically, the capacitance values of capacitors Cvn and CO are estimated through computer simulation of the processing circuitry. A roll-off frequency of the two pole low-pass filter can be realized by setting the values of the capacitors Cvn and CO. 
       FIG. 4  shows another exemplary preprocessing circuit which only requires a single amplifier  440 . As with the previous circuit of  FIG. 3 , switches SW 1 , SW 2 , SW 3 , SW 4  are programmed to set the gain of the preprocessing circuit. Feedback resistors RF 1 , RF 2 , RF 3 , RF 4  are switched to control the gain of the preprocessing circuit. 
     The programmable current source  430  of  FIG. 4  sums with the received signal and current source at the virtual ground node of the amplifier  440 . Therefore, the echo current cancellation current of the current source  430  cancels the echo signal current before the switches SW 1 , SW 2 , SW 3 , SW 4 . Therefore, only the cancelled echo signal current and received signal current flows through the switches SW 1 , SW 2 , SW 3 , SW 4 . The combination of the received signal current and cancelled echo current is substantially less than the original echo signal current, and therefore, improves linearity of the switches SW 1 , SW 2 , SW 3 , SW 4 . 
     The embodiments of  FIG. 3  and  FIG. 4  are shown as single input amplifiers. Preferable embodiments typically include differential input amplifiers. Single input amplifier are depicted in these figures for clarity. 
       FIG. 5  shows another exemplary preprocessing circuit, showing the preprocessing circuit in greater detail.  FIG. 5  shows more clearly why the amplitude of the echo signal can be large relative to the received signal. The transceiver of the preprocessing circuit can simultaneously transmit signals by way of a transmitter DAC  550  through an Ethernet twisted pair, and receive signals through the Ethernet twisted pair. The transceiver is coupled to the Ethernet twisted pair through a transformer  530 . Both the transmit section and the receiver section of the transceiver are connected to the transformer  530 . One input/output of the transformer  530  is shown as grounded. Typically, the input and output include differential signals, and the two connections to the transformer  530  are both connected to an Ethernet twisted pair, rather than being grounded. 
     The echo cancellation current is generated by a MSEC (mixed signal echo cancellation) DAC  540 . A first embodiment includes the MSEC DAC  540  being controlled by the N-bit transmit input of the transmitter DAC  550 , and can additionally include a gain and phase adjustment of the N-bit input. Typically, the N-bit input is a replica (representation) of the N-bit transmit input, rather than the actual N-bit transmit input. That is, typically the N-bit transmit under goes some sort of processing before being received by the MSEC DAC  540 , as designated by the gain and phase adjust block  590 . 
     Another embodiment includes the MSEC DAC  540  being controlled by a DSP engine  560 . An exemplary DSP engine  560  receives and adaptively filters the N-bit transmit input. The adaptive filtering can include adaptively modifying filter coefficients of an IIR or FIR filter. The coefficients can be modified based upon an echo signal (reflected) received through the transformer  530 , and processed, for example, by the previously described pre-processing circuit  210 , the sampling circuit  220 , and an ADC  595 . More specifically, the DSP engine  560  receives the N-bit transmit input and an output signal from the ADC  595 . The output signal from the ADC  595  is fed to the DSP engine  560  and adaptively sets or modifies filter coefficients (for example, IIR or FIR) for filtering of the N-bit transmit input, generating a drive signal for the MSEC DAC  540 . The net result is that the echo cancellation current of the MSEC DAC  540  cancels both components of the echo signal. More specifically, echo signal due to the direct connection of the transmitter sections to the input of the receiver sections, and echo signal due to reflections of the transmit signal within the transmission channel. 
       FIG. 5  shows that switch implemented as transistors M 1 , M 2 , M 3 , M 4  which are driven by control signals d&lt; 1 &gt;, d&lt; 2 &gt;, d&lt; 3 &gt;, d&lt; 4 &gt;. The control signals set the gain of the processing circuitry. 
     The capacitances Cvn and CO in association with the amplifier and the feedback resistors form a complex low pass transfer function (low pass filter). The roll-off frequency of the low pass filter can be adjusted (vary the capacitance values of Cvn and Co) to provide the appropriate anti-aliasing filtering that is needed for the subsequent sampling. The desired values of Cvn and CO are typically determined through simulations. 
     The inclusion of the filtering capacitor Cvn can lead to non-stability of the amplifier  520 . A compensation capacitor Ccomp is included to maintain stability of the amplifier with the inclusion of the filtering capacitor Cvn. 
       FIG. 6  shows an-exemplary embodiment of the MSEC DAC  540 . A similar embodiment can be used for the TX DAC  550 . The DAC architecture of  FIG. 6  is referred to as a segmented current steering (CS) DAC. This architecture includes MSB (most significant bit) current sources that are thermometer decoded, and includes LSB (least significant bit) current sources that are binary decoded. The CS DAC shown includes an M-bit digital input. Segmentation of N1 bits, can include the N1 LSBs implemented as N1 current sources having magnitudes of I, 2I, 4I . . . 2^(N1−1)*I.  FIG. 6  only shows two LSB current sources  632 ,  634  in an LSB current source block  630 . However, other LSB current sources can be included. For the top M−N1 MSBs, the current sources can be implemented as 2^(M−N1) thermometer decoded current sources, each current source sourcing a current of magnitude I*2^(N1).  FIG. 6  only shows two MSB current sources  622 ,  624  in an MSB current source block  620 . However, other current sources can be included. 
     An exemplary embodiment includes M=12, and N1=6. The corresponding current sources of the LSB current source block  630  for this embodiment would be I, 2I, . . . 32I, and the current sources of the MSB current source block  620  would be 63 current sources each sourcing a current having a magnitude of  641 . 
     Connecting the appropriate current sources to the output of the CS DAC includes latching a digital input which drives corresponding switches. In  FIG. 6 , the CS DAC includes a switch input block  610  that latches the digital input. The switch input block  610  drives switches  626 ,  628 ,  636 ,  638 , thereby setting the magnitude of the current being sourced or sunk by the CS DAC. 
     The output (current source) is designated by a differential output hyb+ and hyb−. The amplifier of  FIG. 5  is shown as a single input amplifier. However, as previously described, the preferable embodiment is probably a differential input amplifier. The differential outputs output hyb+ and hyb− are adaptable for use with a differential amplifier. 
       FIG. 7  is a flow chart showing steps included within a method of processing a transceiver signal before sampling. A first step  710  includes supplying a received signal and echo signal to a preprocessing circuit, the preprocessing circuit comprising an amplifier. A second step  720  includes summing a current of an adjustable current source to a virtual ground node of the amplifier, the adjustable current source being adjusted according to an estimated echo signal. A third step  730  includes setting a programmable gain setting of switches connected to the virtual ground node. A fourth step  740  includes anti-aliasing filtering the received signal as determined by a virtual ground node capacitance and an amplifier output capacitance. 
     The method of  FIG. 7  can be used in conjunction with the embodiments of  FIGS. 3 ,  4  and  5 . 
     A previously described, an embodiment includes an output impedance of the single amplifier being much greater than an output load impedance of the single amplifier. Another embodiment includes an output impedance of the single amplifier being much less than an output load impedance of the single amplifier. 
     An exemplary embodiment includes each of the plurality of switches being connected to resistive elements, and the programmable gain settings being dependent upon which switches are closed. Non-linearity of the switches can be minimized by only conducting received signal current and cancelled echo signal current through the plurality of switches and amplifier feedback resistance, and conducting the received signal and cancelled echo current through feedback of the amplifier. Linearity of the amplifier can be improved by the adjustable current canceling the echo signal current. 
     An embodiment of the adjustable current source is an echo cancellation DAC. The echo cancellation DAC can be driven by a replica of a transmit signal, or the echo cancellation DAC can be driven by an adaptive DSP controller which provide for cancellation of reflections of the transmit signal. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The invention is limited only by the appended claims.