Patent Description:
In an optical line termination, an optical transceiver receives an optical signal modulated with a data stream from an optical network unit and converts the optical signal to an electrical signal. The electrical signal from the optical transceiver is provided to a processing module for further processing as may be desired. Frequently, the optical transceiver and the processing module are provided in separate circuits (possibly in separate cards or separate integrated circuits) having different DC (direct current) offset voltage level or common mode voltage level requirements. For example, the optical transceiver can output a signal having a DC offset voltage or common mode voltage (e.g., <NUM> V) that is significantly greater than the DC offset voltage or common mode voltage (e.g., <NUM> V) for the signal that can be received by the processing module. Thus, the electrical signal from the optical transceiver cannot be provided directly to the processing module because the processing module is not equipped to handle the signal with the higher DC offset voltage or common mode voltage.

For compatibility between the optical transceiver and the processing module, the DC offset voltage or common mode voltage for the signal from the optical transceiver has to be level shifted so that the signal can be received by the processing module. One way to couple the optical transceiver to the processing module to obtain the desired level shift is with a resistive divider. However, a drawback of the resistive divider is that the resistive divider discards a significant amount of the signal being communicated between the optical transceiver and the processing module. Another way to couple the optical transceiver to the processing module to obtain the desired level shift is with capacitive coupling. One drawback to capacitive coupling is that the coupling capacitors do not provide an appropriate DC response if there are long idle times or long strings of "<NUM>" or "<NUM>" in the data stream. Still another way to couple the optical transceiver to the processing module to obtain the desired level shift is with bus transceivers. A drawback to the use of the bus transceivers is that they undesirably introduce jitter into the signal. <CIT> discloses a septup for achieving a better active lowpass filter by feeding an input signal to an active lowpass filter; highpass filtering thez resulting signal and adding the output to a lowpass filtering of the input signal.

The present application generally pertains to a coupling module in a communication device, such as an optical line termination (OLT) or optical network unit (ONU), that communicates high speed communication signals, i.e., signals transmitted at <NUM> Gbps (Gigabit per second) or greater, between a transceiver and a processing module. The coupling module can adjust the DC offset voltage level of the signal output by the transceiver to the DC offset voltage level desired for the processing module. The coupling module splits the output signal from the transceiver and passes the signal to both a high pass filter and a low pass filter that are connected in parallel. The outputs of the high pass filter and the low pass filter are then combined and provided to the processing module. The high pass filter and the low pass filter can be configured such that all or one or more predetermined ranges of frequencies of the signal from the transceiver are provided to the processing module without any significant phase shift. In addition, the coupling module can include a level shifter that is incorporated with the low pass filter. The level shifter adjusts the DC offset voltage from the transceiver to the DC offset voltage required by the processing module. The level shifter can be a shunt regulator or an operational amplifier. Both the shunt regulator and the operational amplifier can be configured to provide the appropriate level shift of the DC offset voltage such that the processing module receives the proper DC offset voltage regardless of the DC offset voltage provided by the transceiver.

One advantage of the present application is the jitter-free communication of high speed signals between an optical transceiver and a processing module in an optical communication device.

Another advantage of the present application is that the coupling module can simultaneously provide DC coupling, signal integrity, and wide (GHz to multi-GHz) bandwidth while maintaining signal swing.

Other features and advantages of the present application will be apparent from the following more detailed description of the identified embodiments, taken in conjunction with the accompanying drawings which show, by way of example, the principles of the application.

A coupling module according to the state of the art is known from <CIT>.

Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts.

The present application generally pertains to a coupling module connecting an optical transceiver and a processing module in an optical communication device, such as an optical line termination (OLT) or optical network unit (ONU). The coupling module can include a coupling network that connects a driver circuit of the optical transceiver to a receiver circuit of the processing module. In one embodiment, the coupling network is a purely passive high pass filter in parallel with an op-amp (operational amplifier) based, unity gain, low pass filter. The high pass filter can include a capacitor to provide jitter free communications. The low pass filter can be coupled to the high pass filter on the receiver side of the connection with a linking circuit. The low pass filter can include a carefully selected inductor that has acceptable low parasitic capacitance connected to the output of the op-amp. The inductor operates to protect the high speed signals from the high pass filter from being severely attenuated by the low impedance of the output of the op-amp. Alternatively, the low pass filter can include a resistor and a capacitor connected in parallel at the output of the op-amp. The high pass filter and the low pass filter are tuned to preserve the signals in the transition region between the two filters. However, in other embodiments, there can be gap in the transition region between the high pass filter and the low pass filter. The gap in the transition region between the filters can be predefined by tuning both the high pass filter and the low pass filter such that the parameters, e.g., width, of the gap are known. In a further embodiment, one or both of the high pass filter and the low pass filter may not be tuned and may result in a gap in the transition region. In still another embodiment, the coupling network can include a shunt regulator powered by signals from the driver circuit to achieve a precise, temperature independent, level shift. The high pass filter is produced by bypassing the regulator with a capacitor, or capacitor-resistor combination connected in parallel.

<FIG> depicts an embodiment of a passive optical network (PON) <NUM> for communicating data with customer premises equipment (CPE) <NUM>. Examples of PONs and telecommunication systems that can be used with the present application is described in commonly-assigned <CIT>,.

As shown by <FIG>, the PON <NUM> includes an optical line termination (OLT) <NUM>. In one embodiment, the OLT <NUM> resides on a line card of a network access device (NAD) <NUM>, which may include other OLTs of other PONs, as is described by <CIT>. The NAD <NUM> can be used to facilitate communications, both upstream and downstream, between the CPEs <NUM> and a telecommunication network (not shown). As an example, the network access device <NUM> may reside at a central office of a telecommunication network or an intermediate point between a central office and the CPEs <NUM>.

The OLT <NUM> can be coupled to an optical splitter <NUM> by an optical fiber <NUM>, and the optical splitter <NUM> is configured to split signals from the OLT <NUM> across multiple optical fibers <NUM> that are respectively coupled to ONUs <NUM> as shown. Each ONU <NUM> can receive at least one packet flow from the OLT <NUM> and convert the received packet flow(s) from the optical domain to the electrical domain. The OLT <NUM> and the optical components coupled to it, including the optical splitter <NUM>, ONUs <NUM>, and optical fibers <NUM>, <NUM> form the PON <NUM>. In one embodiment, the PON <NUM> is a gigabit passive optical network (GPON).

<FIG> shows an embodiment of an OLT <NUM> with the components used for processing upstream communications from the ONU <NUM>. It is to be understood that the OLT <NUM> shown in <FIG> may include additional equipment and/or components to perform additional functions and operations that are not shown in <FIG>, e.g., the processing of downstream communications. The OLT <NUM> can have an optical transceiver <NUM> that receives an upstream optical signal from an ONU <NUM> via optical fiber <NUM>. The signal carries a data stream transmitted by the ONU <NUM>. In one embodiment, the signal from the ONU <NUM> can be a high speed signal carrying the data stream at a data rate of between about <NUM> Gbps and about <NUM> Gbps or greater. The optical transceiver <NUM> converts the received optical signal to an electrical signal and provides the electrical signal to an input connection <NUM> of the coupling module <NUM>. The coupling module <NUM> adjusts the level of the DC offset voltage or common mode voltage of the electrical signal and provides the signal with the adjusted DC offset or common mode voltage to a processing module <NUM> via output connection <NUM>. Note that in one embodiment all of the components of the OLT <NUM> can reside on a printed circuit board (PCB), referred to as a "line card. " In other embodiments, other configurations of the OLT <NUM> are possible.

The optical transceiver <NUM> can include a photo detector or avalanche photo diode to convert the optical signal to an electrical signal. The optical transceiver <NUM> can also include an amplifier circuit such as a trans-impedance amplifier and a driver circuit to provide the electrical signal to the input connection <NUM> of the coupling module <NUM>. In addition, the optical transceiver <NUM> can be configured to be either DC coupled or AC (alternating current) coupled. The AC coupled optical transceiver <NUM> includes a capacitor connected between the driver circuit and an output connection coupled to the input connection <NUM> of the coupling module <NUM>. The DC offset voltage or the common mode voltage of the electrical signal from the optical transceiver <NUM> can range between about <NUM> V and about <NUM> V, although other voltage ranges are possible in other embodiments.

The processing module <NUM> can include a receiver circuit to receive the signal from an output connection <NUM> of the coupling module <NUM>. The processing module <NUM> can also include a field programmable gate array (FPGA) and/or other electrical components to further process the received signal. In one embodiment, the DC offset voltage or the common mode voltage of the electrical signal provided on the output connection <NUM> from the coupling module <NUM> can be less than about <NUM> V to correspond to the desired DC offset voltage of the processing module <NUM>. In one embodiment, the required DC offset voltage of the processing module <NUM> may be known. However, in other embodiments, the required DC offset voltage of the processing module <NUM> may have to be discovered or learned.

The coupling module <NUM> splits the electrical signal from the input connection <NUM> into two (<NUM>) signals and provides one signal to a high pass module <NUM> and the other signal to a low pass module <NUM>. The high pass module <NUM> filters the low frequency signals from the electrical signal and permits the high frequency signals to pass to the output connection <NUM>. The low pass module <NUM> filters the high frequency signals from the electrical signal and permits the low frequency signals to pass to the output connection <NUM>. In addition, the low pass module <NUM> also shifts or adjusts the level of the DC offset voltage of the electrical signal such that the DC offset voltage level is acceptable for the processing module <NUM>. The coupling module <NUM> combines the output of the high pass module <NUM> and the low pass module <NUM> at output connection <NUM>.

<FIG> show related embodiments of the coupling module <NUM> that can be used in the present application. In the embodiment of <FIG>, the high pass module <NUM> includes a capacitor <NUM> to filter the low frequency signals in the electrical signal from the optical transceiver <NUM> and permit the high frequency signals to pass to the output connection <NUM>. In one embodiment, the capacitor <NUM> can have a capacitance of about <NUM>µF. In another embodiment, the capacitor <NUM> can have a capacitance of about <NUM>µF. However, capacitor <NUM> may use different capacitances in still other embodiments. The low pass module <NUM> can include a voltage regulator such as a shunt regulator <NUM> connected in series with input connection <NUM> and in parallel to capacitor <NUM> to block the high frequency signals above a predetermined frequency in the electrical signal and permit the low frequency signals below the predetermined frequency in the electrical signal to pass to output connection <NUM>. The high frequency signals can be provided to the output connection <NUM> via capacitor <NUM>. In addition, the shunt regulator <NUM> can provide a fixed voltage drop for the DC offset voltage in the electrical signal. In one embodiment, the shunt regulator <NUM> selected for the low pass module <NUM> can have a voltage drop that directly corresponds to the required level shift in the DC offset voltage in the electrical signal from the optical transceiver <NUM> to enable the signal at the output connection <NUM> to be received by the processing module <NUM>.

The embodiment of <FIG> is similar to the embodiment of <FIG> except that the shunt regulator <NUM> is connected in parallel to a resistor network <NUM> to adjust the voltage drop to the DC offset voltage in the electrical signal provided by the shunt regulator <NUM>. In one embodiment as shown in <FIG>, the voltage drop provided by the shunt regulator <NUM> (VDR) can be based on the gate to anode voltage (VGA) of the shunt regulator <NUM> as determined by equation <NUM>.

<FIG> shows another embodiment of the coupling module <NUM> that can be used in the present application. In the embodiment of <FIG>, the high pass module <NUM> includes capacitor <NUM> to filter out the low frequency signals in the electrical signal from the optical transceiver <NUM> and permit the high frequency signals in the electrical signal from the optical transceiver <NUM> to pass to the output connection <NUM>. The low pass module <NUM> can include an op-amp <NUM> connected in series with an inductor <NUM> to filter out the high frequency signals in the electrical signal from the optical transceiver <NUM> and permit the low frequency signals in the electrical signal from the optical transceiver <NUM> to pass to the output connection <NUM>. The op-amp <NUM> and the inductor <NUM> are connected in parallel to capacitor <NUM>. In one embodiment, the inductor <NUM> can be a VISHAY IFSC0806AZER220M1 inductor having an inductance of about <NUM>µH and a low parasitic capacitance, e.g., less than <NUM> pF, to prevent the high frequency signals passed by capacitor <NUM> from reaching the low impedance at the output of the op-amp <NUM>. Other inductors having different inductances and different parasitic capacitances may be used in other embodiments.

The op-amp <NUM> is used to control the level of the DC offset voltage or the common mode voltage provided at output connection <NUM> based on an input voltage (or offset control voltage) from an offset module <NUM>. In one embodiment, the op-amp <NUM> can be a Texas Instruments OPA2830 providing sufficient bandwidth and a low noise figure. However, other op-amps having different bandwidths and noise figures may be used in other embodiments.

The offset module <NUM> is connected to a feedback circuit <NUM> for the op-amp <NUM> at connection <NUM>. The feedback circuit <NUM> can also be connected to the output of the op-amp <NUM> and the inverting input for the op-amp <NUM>. In the embodiment shown in <FIG>, the feedback circuit <NUM> includes resistors R3 and R4. In one embodiment, R3 can have a resistance between about <NUM> kΩ and about <NUM> kΩ and R4 can have a resistance between about <NUM> kΩ and about <NUM> kΩ. However, other resistances may be used for resistors R3 and R4 in other embodiments.

In addition, the op-amp <NUM> can include an input circuit <NUM> connected between the input connection <NUM> and the non-inverting input to the op-amp <NUM>. The input circuit <NUM> can include a low pass filter to prevent the op-amp <NUM> from receiving (and having to process) higher frequency signals. In the embodiment shown in <FIG>, the input circuit <NUM> can include resistors R5 and R6 and a capacitor C1. In one embodiment, the low pass filter of input circuit <NUM> (which includes resistor R6 and capacitor C1) can be part of a positive feedback circuit for op-amp <NUM> during a transition between the low pass filter function of the low pass module <NUM> and the high pass filter function of the high pass module <NUM>. During the transition between the low pass filter and the high pass filter, the positive feedback circuit for op-amp <NUM> (which includes the low pass filter of input circuit <NUM>) can operate similar to a band pass filter to provide an output signal at the output connection <NUM> for any frequencies of the electrical signal from the optical transceiver <NUM> that may be between the low pass response of the low pass module <NUM> and the high pass response of the high pass module <NUM>. In one embodiment, the capacitor C1 can have a capacitance of about <NUM> pF and resistors R5 and R6 can each have a resistance of about <NUM> kΩ. However, in other embodiments, other capacitances may be used for capacitor C1, and other resistances may be used for resistors R5 and R6. The op-amp <NUM> can also be powered by a voltage source <NUM>. In one embodiment, the voltage source <NUM> can be <NUM> V, but different voltages can be used in other embodiments.

The low pass module <NUM> can also include a resistor <NUM> connected in series with the inductor <NUM> to form an output circuit for the op-amp <NUM>. The resistor <NUM> can be used in conjunction with inductor <NUM> to tune the low frequency response of the low pass module <NUM>, i.e., the signal frequency from which the low pass module <NUM> will no longer pass the electrical signal to the output connection <NUM>. The low frequency response can be determined based on the selection of the inductance for inductor <NUM> and the resistance for resistor <NUM>. In one embodiment, resistor <NUM> can have a resistance of about <NUM>Ω. However, other resistances may be used for resistor <NUM> in other embodiments.

The processing module <NUM> can include a resistor <NUM> connected to the capacitor <NUM>. Similarly, the optical transceiver <NUM> can include a resistor <NUM> connected to the capacitor <NUM>. The resistor <NUM> and the resistor <NUM> can be used in conjunction with capacitor <NUM> to set the high frequency response of the high pass module <NUM>, i.e., the signal frequency below which the high pass module <NUM> will no longer pass the electrical signal to the output connection <NUM>. The high frequency response can be determined based on the selection of the capacitance for capacitor <NUM> after accounting for the resistances of resistor <NUM> and resistor <NUM> as established in their respective devices.

In one embodiment, the capacitor <NUM>, resistor <NUM>, resistor <NUM>, inductor <NUM> and resistor <NUM> can be configured as a Boucherot cell or a Zobel network. The capacitor <NUM> can be configured to provide a predetermined high frequency response based on the known resistances for resistor <NUM> and resistor <NUM>, and the inductor <NUM> and resistor <NUM> can be configured to provide a predetermined low frequency response such that when combined with the predetermined high frequency response, the predetermined high frequency response overlaps with the predetermined low frequency response for a minimal frequency range to provide the output connection <NUM> with all the frequencies of the electrical signal without any substantial phase shift in the signals.

In one embodiment, the optical transceiver <NUM> can communicate with the processing module <NUM> using a differential signal transmitted over two connections, e.g., a positive connection and a negative connection. Thus, the embodiments of <FIG>, <FIG>, <FIG> (as described below), <NUM> (as described below) and <NUM> (as described below), if used with a differential signal, would be connected between each of the input connections <NUM> and the output connections <NUM> for the differential signal as shown in <FIG>.

In another embodiment, a comparator circuit can be connected between the optical transceiver <NUM> and the input connection <NUM> to determine whether the optical transceiver <NUM> is AC coupled or DC coupled. The determination of whether the optical transceiver <NUM> is AC coupled or DC coupled is used to change the voltage provided by the offset module <NUM> to the op-amp <NUM> to adjust the DC offset voltage or the common mode voltage provided by the op-amp <NUM> to accommodate the coupling configuration of the optical transceiver <NUM>. In addition, if the optical transceiver <NUM> is AC coupled, the comparator circuit can also be used to apply a reference bias to the input circuit <NUM> for op-amp <NUM>. In one embodiment, the comparator circuit can include a comparator output that goes high when the optical transceiver <NUM> is DC coupled.

<FIG> shows an embodiment of the offset module <NUM> used to provide the input voltage to the op-amp <NUM> to control the level shift of the DC offset voltage or the common mode voltage provided at output connection <NUM>.

A digital to analog converter (DAC) <NUM> provides the input voltage for op-amp <NUM> at connection <NUM>. The DAC <NUM> receives a value from a processing element <NUM>, such as an FPGA, that corresponds to the desired output from the DAC <NUM>, i.e., the input voltage at connection <NUM>. The processing element <NUM> receives DC offset level information and then uses the DC offset level information to generate the input value for the DAC <NUM>. In one embodiment, the processing element <NUM> communicates with the comparator circuit or the optical transceiver <NUM> to obtain information on the DC offset level output by the optical transceiver <NUM> based on the type of equipment in optical transceiver <NUM>. In another embodiment, the processing element <NUM> may receive information directly from the optical transceiver <NUM> or the processing module <NUM> via a communication bus that informs the processing element <NUM> whether the optical transceiver <NUM> is AC coupled or DC coupled.

In an alternate embodiment, the processing element <NUM> can communicate with a DC offset level circuit <NUM> (see <FIG>) coupled to the processing module <NUM> to obtain information regarding the DC offset voltage level to be received by the processing module <NUM>. The processing element <NUM> can also store (or obtain) information relating to the DC offset level required by the processing module <NUM>. The processing element <NUM> can then use the DC offset level information from the processing module <NUM> to determine the level shift required from the low pass module <NUM>. The processing element <NUM> can then generate the signal or value for the DAC <NUM> to obtain the proper input voltage for op-amp <NUM> so the op-amp <NUM> can provide the proper DC offset voltage or the common mode voltage for the processing module <NUM>.

<FIG> shows an embodiment of a coupling module <NUM> similar to the embodiment of the coupling module <NUM> shown in <FIG>. However, the coupling module <NUM> of <FIG> also includes a DC offset level circuit <NUM> connected between the processing module <NUM> and the offset module <NUM> of the coupling module <NUM>. The DC offset level circuit <NUM> can receive information or feedback from the processing module <NUM> and then use the information from the processing module <NUM> to control the offset module <NUM> to provide a voltage to the op-amp <NUM> such that the op-amp <NUM> provides a desired level in the DC offset voltage.

In one embodiment, the DC offset level circuit <NUM> can use information from the processing module <NUM> to control the offset module <NUM> to set the level shift of the DC offset voltage in op-amp <NUM> to account for the optical transceiver <NUM> being either AC coupled or DC coupled. In addition, the DC offset level circuit <NUM> can use information from the processing module <NUM> to control the offset module <NUM> to make minor adjustments to the DC offset voltage from op-amp <NUM>, e.g., increase the voltage or decrease the voltage, to obtain an optimal DC offset voltage for the processing module <NUM>. Further, after establishing the optimal DC offset voltage for the processing module <NUM>, the DC offset level circuit <NUM> can ensure that the optimal DC offset voltage is still being received by the processing module <NUM>. If the optimal DC offset voltage is not being received by the processing module <NUM>, the DC offset level circuit <NUM> can control the shift module <NUM> to make minor adjustments to the DC offset voltage from op-amp <NUM> to re-establish the optimal DC offset voltage for the processing module <NUM>. As shown in <FIG>, the offset module <NUM> provides the input voltage to the inverting (-) input of the op-amp <NUM>. However, in another embodiment, the offset module <NUM> can provide a second input voltage to the non-inverting (+) input of the op-amp <NUM> to further control the DC offset voltage provided by the op-amp <NUM>.

<FIG> shows another embodiment of the coupling module <NUM>. The embodiment of the coupling module <NUM> shown in <FIG> includes a linking circuit <NUM> connecting the high pass module <NUM> and the low pass module to the output connection <NUM>. Similar to the embodiment of the coupling module <NUM> shown in <FIG>, the coupling module <NUM> splits the electrical signal from the input connection <NUM> into two (<NUM>) signals and provides one signal to a high pass module <NUM> and the other signal to a low pass module <NUM>. The high pass module <NUM> filters the low frequency signals from the electrical signal and permits the high frequency signals to pass to the output connection <NUM>. The low pass module <NUM> filters the high frequency signals from the electrical signal and permits the low frequency signals to pass to the output connection <NUM>. In addition, the low pass module <NUM> also shifts or adjusts the level of the DC offset voltage of the electrical signal. The linking circuit <NUM> combines the output of the high pass module <NUM> and the low pass module <NUM> at output connection <NUM>.

<FIG> shows an embodiment of a coupling module <NUM> similar to the embodiment of the coupling module <NUM> shown in <FIG>. However, the low pass module <NUM> of <FIG> does not include inductor <NUM> and resistor <NUM> as the output circuit, but instead includes a capacitor <NUM> and resistor <NUM> connected to the output of the op-amp <NUM> as the output circuit. The capacitor <NUM> and resistor <NUM> can be configured to filter out the high frequency signals in the electrical signal from the optical transceiver <NUM> and permit the low frequency signals in the electrical signal from the optical transceiver <NUM> to pass to the output connection <NUM>. The resistor <NUM> can be connected in series with the output of the op-amp <NUM> and the capacitor <NUM> can be connected in parallel with the resistor <NUM>. The capacitor <NUM> and resistor <NUM> can be configured to provide a predetermined low frequency response such that when combined with the predetermined high frequency response, the predetermined high frequency response and the predetermined low frequency response are tuned to provide the output connection <NUM> with all the frequencies of the electrical signal without any substantial phase shift in the signals. In one embodiment, the capacitor <NUM> can have a capacitance of about <NUM>µF and the resistor <NUM> can have a resistance of about <NUM>Ω, but the capacitor <NUM> and resistor <NUM> may have different values in other embodiments.

In addition, <FIG> also shows the linking circuit <NUM> connecting the high pass module <NUM> and the low pass module <NUM> to the output connection <NUM>. The linking circuit <NUM> can be an attenuator circuit that can be used to match the gain of the high pass path through the high pass module <NUM> to the gain of the low pass path through the low pass module <NUM> in order to obtain a flat frequency response. In one embodiment, the linking circuit <NUM> can include a resistor <NUM> connected in series with the output of the high pass module <NUM> (capacitor <NUM>), a resistor <NUM> connected in series between resistor <NUM> and output connection <NUM>, and a resistor <NUM> connected in parallel between resistor <NUM> and resistor <NUM>. In one embodiment, resistor <NUM> and resistor <NUM> can each have a resistance of about <NUM>Ω and resistor <NUM> can have a resistance of about <NUM>Ω. However, other resistances may be used for resistors <NUM>, <NUM> and <NUM> in other embodiments. In another embodiment, the linking circuit <NUM> can also operate as a <NUM>Ω constant impedance, <NUM> dB attenuator to attenuate any reflections that may occur between the optical transceiver <NUM> and the processing module <NUM>. While the linking circuit <NUM> (attenuator) has been shown in a "T" configuration in <FIG>, the linking circuit <NUM> may have other configurations in other embodiments.

<FIG> shows an embodiment of a coupling module <NUM> similar to the embodiment of the coupling module <NUM> shown in <FIG>. However, the offset module <NUM> of <FIG> has been replaced with a first offset module <NUM> and a second offset module <NUM> and input circuit <NUM> has been replaced with input circuit <NUM>. In addition, a differential module <NUM> is connected to the input circuit <NUM>.

First offset module <NUM> and second offset module <NUM> can be used to control the DC offset voltage that is provided by op-amp <NUM>. First offset module <NUM> can provide an input voltage for the inverting (-) input of op-amp <NUM> and can be used for fine or smaller adjustments, e.g., <NUM> V adjustments, of the DC offset voltage from op-amp <NUM> to optimize the performance of the processing module <NUM>. The first offset module <NUM> can receive information from processing module <NUM> regarding the desired DC offset voltage for the processing module <NUM>. The first offset module <NUM> can use the information from the processing module <NUM> to determine the appropriate input voltage to provide to connection <NUM> to obtain the desired DC offset voltage from the op-amp <NUM>. In one embodiment, the first offset module <NUM> can include a duty-cycle integrating DAC, but other configurations of the first offset module <NUM> can be used to obtain the desired voltage at connection <NUM>.

The second offset module <NUM> can provide an input voltage for the non-inverting (+) input of the op-amp <NUM> and can be used for gross or larger adjustments, e.g., <NUM> V adjustments, of the DC offset voltage to account for the optical transceiver <NUM> being AC coupled. The second offset module <NUM> can receive information from the processing module <NUM> regarding whether the optical transceiver <NUM> is AC coupled or DC coupled. In one embodiment, the processing module <NUM> has to discover whether the optical transceiver <NUM> is AC coupled or DC coupled through a trial and error process. In another embodiment, the processing element <NUM> may receive information directly from the optical transceiver <NUM> via a communication bus that informs the processing element <NUM> whether the optical transceiver is AC coupled or DC coupled. If the optical transceiver <NUM> is DC coupled, the second offset module <NUM> can be disabled or "no-loaded" in one embodiment. The second offset module <NUM> can use the information from the processing module <NUM> to determine the appropriate input voltage to provide to the input circuit <NUM> (which is connected to the non-inverting input of op-amp <NUM>) through resistor R8. In one embodiment, the second offset module <NUM> can include a PNP transistor having a pull-up voltage of <NUM> V, but other configurations of the second offset module <NUM> can be used to provide the desired voltage to input circuit <NUM>. In one embodiment, resistor R8 can have a resistance of about <NUM> kΩ, but can have other resistances in other embodiments.

The input circuit <NUM> can be connected between input connection <NUM> and the non-inverting input of the op-amp <NUM>. The input circuit can include a resistor R5 connected to the input connection <NUM> and a resistor R6 connected between resistor R5 and the non-inverting input to the op-amp <NUM>. A capacitor C2 is connected between resistor R5 and a ground connection and resistor R7 is connected between resistor R6 and the ground connection. In addition, the inputs from the second offset module <NUM> and the differential module <NUM>, after passing through resistor R8 and resistor R9, respectively, can be connected to the input circuit <NUM> between resistor R5 and capacitor C2. In one embodiment, the capacitor C2 can have a capacitance of about <NUM> pF, resistor R5 and resistor R6 can each have a resistance of <NUM> kΩ and the resistor R7 can have a resistance of about <NUM> kΩ. However, in other embodiments, other resistances may be used for resistors R5, R6 and R7 and other capacitances may be used for capacitor C2. In a further embodiment, the input circuit <NUM> can be used for low pass or band pass filtering of frequencies of the electrical signal from the optical transceiver <NUM> as previously described with respect to input circuit <NUM>.

The differential module <NUM> can provide an input voltage for the non-inverting input of the op-amp <NUM> to support signal maintenance when the optical transceiver <NUM> is AC coupled. The differential module <NUM> can be used to support signal maintenance during arbitrarily long periods of unchanging signal at the input connection <NUM> such as during idle times or during the transmission of long sequences of "<NUM>"s or "<NUM>"s. The input voltage from the differential module <NUM> can maintain a voltage at the non-inverting input of the op-amp <NUM> during times when the signal from an AC coupled optical transceiver <NUM> may sag or fluctuate. The differential module <NUM> can receive information from the processing module <NUM> regarding whether the optical transceiver <NUM> is AC coupled or DC coupled. If the optical transceiver <NUM> is DC coupled, the differential module <NUM> can be disabled or "no-loaded" in one embodiment. The differential module <NUM> can use the information from the processing module <NUM> to determine the appropriate input voltage to provide to the input circuit <NUM> (which is connected to the non-inverting input of op-amp <NUM>) through resistor R9. In one embodiment, the differential module <NUM> can include an Schmidt trigger inverter and a line driver, but other configurations of the differential module can be used to provide the desired voltage to input circuit <NUM>. In one embodiment, resistor R9 can have a resistance of about <NUM> kΩ, but can have other resistances in other embodiments.

<FIG> shows an embodiment of a coupling module <NUM> used with a differential signal. As shown in <FIG>, the differential signal can have a positive (+) path and a negative (-) path for both input connection <NUM> and output connection <NUM>. Each of the paths, i.e., the positive path and the negative path, can include a high pass module <NUM>, a low pass module <NUM> and a linking circuit <NUM> to connect the input connection <NUM> and the output connection <NUM> and provide the appropriate level shift between the input connection <NUM> and the output connection <NUM>. In addition, the coupling module <NUM> can use the differential module <NUM> to provide an input signal (or voltage) to the low pass modules <NUM> for both the positive path and the negative path. The differential module <NUM> can include a differential feedback circuit <NUM> to receive and process the information from the processing module <NUM>. The output of the differential feedback circuit <NUM> can be inverted by inverter <NUM> and then split in order to be provided to the low pass modules <NUM> for the positive path and the negative path. The output from the inverter <NUM> is provided directly to the low pass module <NUM> for the positive path, but the output from the inverter <NUM> is inverted by inverter <NUM> before being provided to the low pass module <NUM> of the negative path. By providing opposed signals to the low pass modules <NUM> on the positive path and the negative path, the differential module <NUM> is able to maintain signal quality during idle times or during the transmission of long sequences of "<NUM>"s or "<NUM>"s.

Although the figures herein may show a specific order of method steps, the order of the steps may differ from what is depicted. Also, two or more steps may be performed concurrently or with partial concurrence. Variations in step performance can depend on the software and hardware systems chosen and on designer choice. Software implementations could be accomplished with standard programming techniques, with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps.

The coupling module <NUM> is described in various embodiments for use within an OLT. However, it is possible to use the coupling module <NUM> in other types of communication devices, such as an ONU. As an example, an ONU may be configured according to the block diagram shown by <FIG> having a coupling module <NUM> that is coupled between and optical transceiver <NUM> and a processing module <NUM>, as described above for the OLT <NUM>.

Claim 1:
An apparatus comprising a coupling module (<NUM>) and a processing module (<NUM>) coupled to said coupling module, said coupling module comprising:
an input connection (<NUM>) connected to an input module to receive a high speed communication signal from the input module;
a high pass module (<NUM>) connected to the input connection (<NUM>) for receiving the high speed communication signal from the input connection (<NUM>), the high pass module (<NUM>) configured to filter the high speed communication signal and output a first filtered signal including signal frequencies greater than a first predetermined frequency;
a low pass module (<NUM>) connected to the input connection (<NUM>) in parallel with the high pass module (<NUM>) for receiving the high speed communication signal from the input connection (<NUM>), the low pass module (<NUM>) configured to filter the high speed communication signal and output a second filtered signal including signal frequencies less than a second predetermined frequency, the low pass module (<NUM>) configured to adjust a DC offset voltage of the high speed communication signal to provide a predetermined DC offset voltage to the second filtered signal;
an output connection (<NUM>) connected to the high pass module (<NUM>) and the low pass module (<NUM>), the output connection (<NUM>) for receiving the first filtered signal from the high pass module (<NUM>) and the second filtered signal from the low pass module (<NUM>), the output connection (<NUM>) configured to combine the first filtered signal and the second filtered signal to form a combined signal and provide the combined signal to the processing module (<NUM>) having a required DC offset voltage; and
wherein the predetermined DC offset voltage of the second filtered signal corresponds to the required DC offset voltage of the processing module.