Line driver for asymmetric digital subscriber line system

A line driver couples a data transceiver to a transmission line having a load impedance Z via a transformer with a turns ratio of 1:n, the data transceiver transmitting signals in a first frequency range and receiving signals in a second frequency range different from the first frequency range. The line driver includes an input port for receiving an input signal voltage, an output port for supplying an output signal voltage to the transformer, and a differential amplifier having a low pass filter for amplifying the input signal voltage and outputting an amplified signal voltage. The line driver further includes termination resistors having a resistance Rt, where and a positive feedback path for coupling the output signal voltage from the output port to an appropriate node of the differential amplifier so that a synthesized output impedance substantially matches the load impedance Z over the second frequency range.

FIELD OF THE INVENTION

The present invention relates to digital subscriber line (DSL) technologies. More particularly, the present invention relates to a line driver for an asymmetrical digital subscriber line (ADSL) system.

BACKGROUND OF THE INVENTION

DSL and ADSL systems use a technique called discrete multitone (DMT) for transmitting data. With DMT, a frequency band up to 1.2 MHz is split up into 256 tones (also referred to as subcarriers or subchannels) each spaced 4.3125 kHz apart. In a DSL/ADSL application, the tones are allocated for use depending on the direction of communication between a central office (CO) and a remote terminal (RT) or customer premises equipment (CPE).

Communication from a CO to a RT/CPE (such as an end user's PC modem) is referred to as “downstream.” The direction of communication from a RT/CPE to the CO is called “upstream.” A higher and wider frequency range, for example, 176 kHz to 1.1 MHz, is allocated to the downstream communication, and a lower frequency range, for example, 10 kHz to 138 kHz, is allocated to the upstream communication.

FIG. 1schematically illustrates a conventional ADSL transceiver system. A transmit signal (TX) is typically coming from a digital signal processing (DSP) processor1through a digital to analog converter (DAC)2to an analog front end (AFE)3. The AFE is a circuit block that provides the interface between the ADSL transceiver and the DSP processor, and typically includes a filter. In order to comply with strict ADSL transmission mask specifications, sufficient filtering must be provided in the transmit direction. The transmit signal is then supplied with sufficient voltage and current by the line driver5, and coupled via a transformer7to a transmission line9, such as a telephone line or twisted-pair loop. The transmission line9has a certain line impedance Z (typically 100 Ω).

As shown inFIG. 1, back termination resistors6are inserted between outputs of the line driver5and the primary of the transformer7in order to properly terminate a signal received from the transmission line9(receive signal:RX). That is, the back-termination resistors6have a specific resistance RBTso as to match the output impedance Zoutof the transceiver and the transmission line impedance Z. When the transformer7has a turns ratio of 1:n, the standard value of the back-termination resistors RBTisZ2⁢n2
for a differential line driver as shown inFIG. 1.

Although the back-termination resistors are necessary to prevent undesirable reflection of the receive signal, they waste one-half of the power provided by the line driver amplifiers. Thus, in DSL systems one conventional way of reducing system power is to reduce the value of the back-termination resistors from its standard value. The reduced termination resistance reduces the drop across the resistors and thus increases the proportion of the transmit signal that reaches to the transmission line, allowing the use of a lower supply voltage for the line driver. However, simply reducing a termination resistance causes mistermination of the receive signal as well as reducing the amount of the receive signal developed across these resistors to be sensed by a receive circuit.

An approach termed “active termination” provides a positive feedback from the line driver outputs so as to boost the reduced value of termination resistor and make the effective (or synthesized) output impedance match the line impedance.FIG. 2Aillustrates a conventional line driver10having a differential amplifier12with an active termination architecture. An input signal voltage Vinis input through input resistors R1, and amplified by the differential amplifier12to an amplified voltageVc=RfR1·(k+1)k+1-RfRF⁢Vin,
where Rfis a feedback resistance of the differential amplifier12, and RFis a resistance of the positive feedback for the active termination.

As shown inFIG. 2A, the termination resistance Rthas a value reduced from its standard value by factor k, i.e.,Z2⁢n2⁢k,
where k<1. When the line impedance has a typical value of 100 Ω, the back-termination resistance Rtis50n2⁢k.
It should be noted that for a differential structure, the total termination resistanceZn2⁢k
is divided into a pair of termination resistors. Each amplifier output is coupled via a feedback resistance RFto the opposite amplifier input so as to make a positive feedback.

FIG. 2Bshows a single-ended structure10′ corresponding to the line driver10, for simplicity. As is understood fromFIG. 2B, when the value of the feedback resistance RFis chosen to satisfyRF=Rf1-k,
a synthesized impedance Z′ seen looking into the circuit at the output node is100n2,
matching the effective output impedance Zoutto the line impedance Z=100.

There is a conventional technique to build a second order low pass filter around an amplifier, by adding a relatively small number of extra components. For example, a Rauch configuration is typically chosen because of its robustness against components variations.FIG. 3illustrates a conventional line driver14including a differential amplifier16with a Rauch filter configuration. The Rauch filter/amplifier includes an operational amplifier16, first and second input resistance R1and R2, a feedback resistance R3, and capacitances ½C1and C2, as shown inFIG. 3.

As is well understood by those of ordinary skill in the art, the transfer function of the Rauch configuration shown inFIG. 3is given as follows, which represents the second order filter characteristic:VcVin⁢(s)=R3R1·1R1⁢R2⁢C1⁢C2s2+s·GpC1+1R1⁢R2⁢C1⁢C2
whereGp≡1R1+1R2+1R3,
and s is the Laplace variable.

When applied to an ADSL line driver, depending on the sampling rate used in the ADSL system, the Rauch filter can be the only one present or part of a higher order filter. In ADSL applications this filter can be designed to have a cut-off frequency of 138 KHz for the CPE side transceiver, and 1.1 MHz for the CO side transceiver.

However, as shown inFIG. 3, the conventional line driver14with the Rauch configuration does not employ an active termination architecture. Also, the conventional impedance synthesis is used only in the line drivers configured as a pure gain stage without any filter characteristics. Because the Rauch filter is inherently frequency dependent, it is unknown to those of ordinary skill in the art how any additional components affect the required filter characteristic, or whether such additional components operate as intended.

As described above, implementing an active termination or impedance synthesis is desirable to reduce the required power of the line driver. It is also desirable to build a low pass filter around a line driver because it can eliminate extra filtering either on-chip or off-chip, so as to reduce the system cost. In addition, it is easier and less expensive to build a low pass filter around the line driver than implementing one in the AFE portion. Furthermore, providing the low-pass filter at the last stage of the transmit signal (i.e., at the line driver amplifier) is more effective in cutting off higher frequency noises. Accordingly, it would be desirable to provide both low-pass filtering and active impedance synthesis in ADSL line drivers to satisfy the transmit mask requirement.

BRIEF DESCRIPTION OF THE INVENTION

A line driver couples a data transceiver to a transmission line having a load impedance Z via a transformer with a turns ratio of 1:n, the data transceiver transmitting signals in a first frequency range and receiving signals in a second frequency range different from the first frequency range. The line driver includes an input port for receiving an input signal voltage, an output port for supplying an output signal voltage to the transformer, and a differential amplifier having a low pass filter characteristic, coupled with the input port, for amplifying the input signal voltage and outputting an amplified signal voltage. The line driver further includes termination resistors for coupling the amplified signal voltage therethrough as the output signal voltage to the output port, the termination resistors having a resistance Rt, whereRt=Z2⁢n2×k⁢⁢(0<k≤1),
and a positive feedback path for coupling the output signal voltage from the output port to an appropriate node of the differential amplifier so that a synthesized output impedance substantially matches the load impedance Z over the second frequency range.

DETAILED DESCRIPTION

FIG. 4schematically illustrates a line driver20in accordance with the present invention. The line driver20couples a data transceiver22(an AFE thereof is shown) to a transmission line24having a load impedance Z via a transformer26with a turns ratio of 1:n. The value of the load impedance Z is typically 100 Ω for telephone lines. The data transceiver22transmits signals (transmit signal: TX) in a first frequency range, and receives signals (receive signal: RX) in a second frequency range different from the first frequency range.

For example, the central office (CO) side transceiver may transmits signals in the first frequency range of 176 kHz to 1.1 MHz, which is allocated to the downstream communication to a remote terminal (RT) or customer premises equipment (CPE) side transceiver. The CO side transceiver may receive signals in the frequency range of 10 kHz to 138 kHz, which is allocated to upstream communication from a RT/CPE side transceiver to the CO side transeiver. The RT/CPE side transceiver, on the other hand, transmits signals in the frequency range of 10 kHz to 138 kHz and receives signals in the frequency range of 176 kHz to 1.1 MHz. It should be noted that these frequency ranges are examples for an illustration purpose only, and the present invention is generally applicable whenever the receive signal and the transmit signal occupy different frequency ranges.

As shown inFIG. 4, the line driver20includes a differential amplifier30having a low pass filter characteristic, an input port28for receiving an input signal voltage Vinfrom the transceiver22, an output port29for supplying an output signal voltage Voutto the transformer26. The amplifier30amplifies the input signal voltage Vinand outputs an amplified signal voltage Vc. The low pass filter characteristic of the amplifier30may be provided implementing a Rauch configuration.

Termination resistors32are coupled between outputs of the amplifier30and the output port29. The termination resistors32have a resistance Rt, where Rt, whereRt=Z2⁢n2×k⁢⁢(0<k≤1).
That is, the value of the resistance Rtis a reduced by factor k from the “standard” valueZ2⁢n2
for a complete termination. As shown inFIG. 4, the line driver20also includes a positive feedback path34for coupling the output signal voltage from the output port29to an appropriate node of the differential amplifier30so that a synthesized output impedance Zoutsubstantially matches the load impedance Z over the second frequency range, i.e., the frequency range of the receive signal (RX).

When the line driver20is implemented in a CO side transceiver, the RX frequency range is lower than the TX frequency range, and thus the positive feedback path34includes a resistive coupling. When the line driver20is implemented in a RT/CPE side transceiver, the RX frequency range is higher than the TX frequency range, and thus the positive feedback path34includes a capacitive coupling.

FIG. 5illustrates a line driver40for a CO side transceiver in accordance with a specific embodiment of the present invention. The line driver40couples transmit signals from an AFE (not shown) to a transmission line36having a load impedance Z (typically 100 Ω) through a transformer38with a turns ratio of 1:n. The line driver40includes an amplifier50having a low pass filter characteristic, first and second input signal terminals42and44for receiving an input signal voltage Vin, and first and second output signal terminals46and48, for supplying an output signal voltage Voutto the transformer38.

As shown inFIG. 5, the amplifier50includes an operational amplifier51having first and second inputs52and54and first and second outputs56and58, for amplifying the input signal voltage Vinand outputs an amplified voltage Vcbetween the first and second outputs56and58. The operational amplifier51has a differential structure as indicated by input and output polarities.

The amplifier50also includes a first input resistor62(having a resistance R1) coupled to the first input signal terminal42, a second input resistor64(having a resistance R1) coupled to the second input signal terminal44, a third input resistor66(having a resistance R2) coupled to the first input52, and a fourth input resistor68(having a resistance R2) coupled to the second input54. A first node72connects the first input resistor62and the third input resistor66, and a second node74connects the second input resistor64and the fourth input resistor68.

The amplifier50further includes a first feedback resistor76(having a resistance R3) coupled between the first output56and the first node72, a second feedback resistor78(having a resistance R3) coupled between the second output58and the second node74, a first capacitor80(having a capacitance ½C1) coupled between the first node72and the second node74, a second capacitor82(having a capacitance C2) coupled between the first output56and the first input52, and a third capacitor84(having a capacitance C2) coupled between the second output58and the second input54.

As shown inFIG. 5, the lined driver40also includes a first termination resistor86coupled between the first output56and the first output signal terminal46, and a second termination resistor88coupled between the second output58and the second output signal terminal48. The termination resistors86and88have a resistance Rt, whereRt=Z2⁢n2×k⁢⁢(0<k≤1).
That is, the resistance Rthas a value reduced by factor k from the standard value ofZ2⁢n2.

The line driver40further includes a positive feedback path resistively coupled around the amplifier50. As shown inFIG. 5, a third feedback resistor92having a resistance R4is coupled between the second output signal terminal48and the first node72, and a fourth feedback resistor94having the resistance R4is coupled between the first output signal terminal46and the second node74. In order to match the synthesized output impedance Zoutwith the line impedance Z, the value of the resistance R4is given asR4=R3(1-k).

Assuming that the resistance R4is considerably greater thanZn2,VoutVc≈1k+1,
and from the specific configuration described above, the low pass filter characteristic of the line driver40is expressed asVoutVin⁢(s)=R3R1×12⁢k×2⁢k/(k+1)C1⁢C2⁢R2⁢R3s2+s⁢⁢GpC1+2⁢k/(k+1)C1⁢C2⁢R2⁢R3(1)
where s is the Laplace variable andGp≡1R1+1R2+1R3+1R4.⁢
As is well understood by those of ordinary skill in the art, Equation (1) shows a transfer function of a second order low pass filter.

The synthesized output impedance Zoutis expressed asZout⁡(s)=Z×k1-(1-k)×1C1⁢C2⁢R2⁢R3s2+s⁢⁢GpC1+1C1⁢C2⁢R2⁢R3.(2)
Since Zout(0)=Z, and Zout(∞)=Z×k, the synthesized impedance Zouthas a low-pass characteristic.

Note that the assumptionR4>>Zn2
by no means indicates a loss of generality for the implementation of the present invention. In general, in order to minimize power loss, R4is chosen in the order of several kilo ohms (1 kΩ=1000 Ω). Since the turns ratio n is generally larger than 1 and the line impedance Z is typically 100 Ω, the assumption is easily met in actual and practical implementations. Even without such considerations, the conditionR4>>Zn2
can always be met: it is well known to those of ordinary skill in the art that for a filter implemented using capacitors, operational amplifiers, and resistors, regardless of the particular architecture used, all the resistors can be scaled up by an arbitrary constant and all the capacitors can be scaled down by the same constant while the filter frequency response remains the same. In other words one can always scale values of R1, R2, R3, R4,½C1, and C2shown inFIG. 5until R4is large enough to satisfy the conditionR4>>Zn2.

Equation (1) can be expressed asVoutVin⁢(s)=G·Wn2s2+s⁢⁢WnQ+Wn2(3)
withWn2=2⁢kk+1C1⁢C2⁢R2⁢R3,WnQ=GpC1,and⁢⁢G≡R3R1·12⁢k.
Here, Wnis the natural frequency of the filter, Q is the quality factor (Q factor) of the filter, and G is the DC gain of the filter. The filter characteristic is specified by the parameters Wn, Q, and G.

Accordingly, by properly selecting component values R1, R2, R3, C1and C2(R4is a function of R3and k), a desirable filter characteristic (specified by Wn, Q, and G) around the line driver40and the active termination (factor k) can be achieved simultaneously.

FIG. 6Aillustrates an example of low-pass filter characteristic of a CO side line driver for n=1.4 and k=0.275.FIG. 6Billustrates the synthesized output impedance of the same CO side line driver. The components used in this example have values of R1=2000 Ω, R2=374 Ω, R3=5760 Ω, R4=7940 Ω, C1=250×10−12F, and C2=10×10−12F. These component values are all standard and readily available. It should be noted that the conditionR4>>Zn2
is clearly met with these standard values (R4=7940 is considerably greater thanZn2=1001.4×1.4≅51).
However, it should also be noted that these component values are examples for illustrative purpose only, and the present invention is not limited to specific component values.

As shown inFIGS. 6A and 6B, the filter characteristic meets the transmission mask specification (cut-off frequency about 1.1 MHz), and the synthesized output impedance substantially equals 100 Ω so as to achieve a proper termination of the receive signal. It should be noted that the frequency range of interest for a CO side termination is 104to 105Hz, or more specifically, for example, 10 kHz to 138 kHz. It should also be noted that these frequency ranges are specified for an illustration purpose only, and the present invention is generally applicable whenever the receive signal and the transmit signal occupy different frequency ranges.

FIG. 7illustrates an example of synthesized output impedance of the CO side line driver for different k values. Here, the same low-pass filter whose frequency response is depicted inFIG. 6Ais implemented with different component values. As shown inFIG. 7, the synthesized output impedance is substantially 100 Ω over the frequency range of interest for various k values. It should be noted thatFIGS. 6A,6B, and7are shown for illustrating an example, and different filter characteristics and synthesized impedance may be achieved using different component values.

FIG. 8illustrates a line driver100for a RT side transceiver in accordance with a specific embodiment of the present invention. The line driver100couples transmit signals from an AFE (not shown) to a transmission line136having a load impedance Z (typically 100 Ω) through a transformer138with a turns ratio of 1:n. The line driver100includes an amplifier110having a low pass filter characteristic, first and second input signal terminals142and144for receiving an input signal voltage Vin, and first and second output signal terminals146and148for supplying an output signal voltage Voutto the transformer138.

As shown inFIG. 8, the amplifier110includes an operational amplifier151having first and second inputs152and154and first and second outputs156and158, for amplifying the input signal voltage Vinand outputs an amplified voltage Vcbetween the first and second outputs156and158. The operational amplifier151has a differential structure as indicated by input and output polarities.

The amplifier110also includes a first input resistor162(having a resistance R1) coupled to the first input signal terminal142, a second input resistor164(having a resistance R1) coupled to the second input signal terminal144, a third input resistor166(having a resistance R2) coupled to the first input152, and a fourth input resistor168(having a resistance R2) coupled to the second input154. A first node172connects the first input resistor162and the third input resistor166, and a second node174connects the second input resistor164and the fourth input resistor168.

The amplifier110also includes a first feedback resistor176(having a resistance R3) coupled between the first output156and the first node172, a second feedback resistor178(having a resistance R3) coupled between the second output158and the second node174, a first capacitor180(having a capacitance ½C1) coupled between the first node172and the second node174, a second capacitor182(having a capacitance C2) coupled between the first output156and the first input152, and a third capacitor184(having a capacitance C2) coupled between the second output158and the second input154. It should be noted that although the same denotations R1, R2, R3, C1and C2are used for a simplicity reason, actual values of the resistance and capacitance are different from those of the line driver40.

As shown inFIG. 8, the lined driver100also includes a first termination resistor186coupled between the first output156and the first output signal terminal146, and a second termination resistor188coupled between the second output158and the second output signal terminal148. The termination resistors186and188have a resistance Rt, whereRt=Z2⁢n2×k⁢⁢(0<k≤1).
That is, the resistance Rthas a value reduced by factor k from the standard value ofZ2⁢n2.

The line driver100further includes a positive feedback path capacitively coupled around the amplifier110. As shown inFIG. 8, a first feedback capacitor192having a resistance C3is coupled between the second output signal terminal148and the first input152, and a second feedback capacitor194having the capacitance C3is coupled between the first output signal terminal146and the second input154. In order to match the synthesized output impedance Zoutwith the line impedance Z, the value of the capacitance C3is given as C3=(1−k)×C2.

When a value12⁢π⁢⁢fC3
is considerably greater thanZn2
for a frequency f ranging from 0 to fc, where fcbeing a cut-off frequency of the low pass filter characteristic of the line driver100, the transfer function is approximated asVoutVc≈11+k.
Then from the specific configuration described above, the low pass filter characteristic of the line driver100is expressed asVoutVin⁢(s)=R3R1×1(k+1)×(k+1)2⁢kC1⁢C2⁢R2⁢R3s2+s⁢⁢GpC1+(k+1)2⁢kC1⁢C2⁢R2⁢R3(5)
where s is the Laplace variable andGp≡(1R1+1R2+1R3).
As is well understood by those of ordinary skill in the art, Equation (5) shows a transfer function of a second order low pass filter.

The line driver100has a synthesized output impedance Zoutexpressed asZout⁡(s)=Z×k1-(1-k)⁢(s2+s·GpC1)s2+s⁢⁢GpC1+1C1⁢C2⁢R2⁢R3.(6)
Since Zout(0)=Z×k, and Zout(∞)=Z, the synthesized impedance Zouthas a high pass characteristic.

Note that the assumption12⁢π⁢⁢fC3>>Zn2
by no means indicates a loss of generality for the implementation of the present invention. In general, in order to minimize power loss, C3is chosen less than a couple of hundred pico farads (1 pF=1×10−12F). Since the turns ratio n is generally larger than 1 and the line impedance Z is typically 100 Ω, the assumption is easily met for a cut-off frequency fcwhich is about 138 kHz for actual and practical RT/CPE applications. Even without such considerations, the condition12⁢⁢π⁢⁢fC3>>Zn2
can always be met: it is well known to those of ordinary skill in the art that for a filter implemented using capacitors, operational amplifiers, and resistors, regardless of the particular architecture used, all the capacitors can be scaled down by an arbitrary constant, and all the resistors scaled up by the same constant while the filter response remains the same. In other words one can always scale values of R1, R2, R3, ½C1, C2, and C3shown inFIG. 8until C3is small enough to satisfy the condition12⁢⁢π⁢⁢fC3>>Zn2
for a frequency range f<fc.

Equation (5) can be expressed asVou⁢tVin⁢(s)=G·Wn2s2+s⁢WnQ+Wn2(7)
withWn2=k+12⁢⁢k×C1⁢C2⁢R2⁢R3,WnQ=GpC1,⁢and⁢⁢G≡R3R1·1k+1.
Here, Wnis the natural frequency of the filter, Q is the quality factor (Q factor) of the filter, and G is the DC gain of the filter. The filter characteristic is determined by the parameters Wn, Q, and G.

Accordingly, by properly selecting component values R1, R2, R3, C1and C2(C3is a function of C2and k), a desirable filter characteristic (specified by Wn, Q, and G) around the line driver100and the active termination (factor k) can be achieved simultaneously.

FIG. 9Aillustrates an example of low-pass filter characteristic of a RT side line driver for n=4 and k=0.35.FIG. 9Billustrates the synthesized output impedance of the same RT side line driver. The components used in this example have values of R1=2658 Ω, R2=12365 Ω, R3=3678 Ω, C1=590×10−12F, C2=95×10−12F, and C3=61×10−12F. These values are all standard and readily available. It should be noted that the condition12⁢⁢π⁢⁢fC3>>Zn2
for the cut-off frequency fc138 kHz is easily met with these standard values(12⁢⁢π⁢⁢fC3=18906
is considerably greater thanZn2=6.25).
However, it should also be noted that these component values are examples for illustrative purpose only, and the present invention is not limited to specific component values.

As shown inFIGS. 9A and 9B, the filter characteristic meets the transmission mask specification (cut-off frequency about 138 kHz), and the synthesized output impedance substantially equals 100 Ω (100±20 Ω) to achieve a proper termination of the receive signal. It should be noted that the frequency range of interest for a RT side termination is 105to 106Hz, for example, 176 kHz to 1.1 MHz, and the synthesized impedance has an adequate value over this frequency range. It should also be noted that these frequency ranges are specified for an illustration purpose only, and the present invention is generally applicable whenever the receive signal and the transmit signal occupy different frequency ranges.

FIG. 10illustrates an example of synthesized output impedance of the RT side line driver for different k values. Here, the same low-pass filter whose frequency response is depicted inFIG. 9Ais implemented with different component values. As shown inFIG. 10, the synthesized output impedance is substantially 100 Ω over the frequency range of interest for various k values. It should be noted thatFIGS. 9A,9B, and10are shown for illustrating an example, and different filter characteristics and synthesized impedance characteristics may be achieved using different component values.

A transceiver system in accordance with the present invention includes a CO side and a RT/CPE side transceivers both having a line driver in accordance with the present invention as described above. In accordance with a specific embodiment of the present invention, the transceiver system includes the CO side transceiver having the line driver40, as shown inFIG. 5, and the RT/CPE side transceiver having the line driver100, as shown inFIG. 8.

In a line driver according to the present invention, a portion of the transmit signal is fed back to an appropriate node of the line driver including an amplifier having a low pass filter characteristic, for example, having the Rauch configuration. The polarity of the transmit signal which is fed back is chosen to establish a positive feedback path around the line driver and thus give the appropriate boost to the back termination resistors which are reduced in value. The transmit signal which is fed back can be coupled resistively or capacitively, depending on the frequency range of the receive signal.

When the positive feedback path is resistively coupled, the synthesized impedance takes the form of a low-pass, and when the feedback path is capacitively coupled, the synthesized impedance has a high-pass characteristic. In the both cases, the frequency response around the line driver shows the required low-pass filter characteristic. Since the received signal in ADSL CPE applications occupies a frequency band above the transmitted signal, a high-pass characteristic is acceptable in the synthesized impedance. This simply means that the active termination will provide the necessary termination for the high frequency down stream data while misterminating the line at lower frequencies where there is no useful signal. Thus a capacitive coupling is appropriate at the CPE side. On the other hand, for the CO side the received signal is at a frequency band lower than the transmitted signal, hence a low-pass characteristic in the synthesized impedance is acceptable, thus requiring resistive coupling. In this case positive feedback around the line driver will ensure proper termination for the lower frequency upstream data while misterminating the line for higher frequencies where there is no useful signal for reception.