1. Field of the Invention
The present invention relates generally to high-speed communication, and more particularly to a hybrid circuit for use in high-speed communication devices.
2. Background Art
In high-speed communication devices, such as Cable modem and Digital Subscriber Line (DSL) devices, hybrid circuits are used in the analog front end (AFE) circuits to isolate transmitted and received signals. In particular, hybrid circuits are used to reject echo due to transmitted signals from reaching the receiver.
Conventional hybrid circuit solutions use inductive components to compensate for inductive elements present in line transformers, which generally couple the high-speed communication devices to the communication network.
However, inductive components are expensive, generate as well as pick up noise, and result in non-linearities in the performance of the hybrid circuit.
As such, there is a need to eliminate inductive components from hybrid circuits used in high-speed communication devices.
A hybrid circuit without inductors is provided herein. Embodiments can be used in high-speed communication devices, including Cable modem and Digital Subscriber Line (DSL) devices. In particular, embodiments can be used in Very High Speed DSL (VDSL) devices. Embodiments provide a cheap and compact solution to the echo rejection problem in high-speed communication devices. Furthermore, embodiments perform at least comparably to conventional solutions with respect to key performance measures.
In another aspect, a high-speed communication device that uses a hybrid circuit according to embodiments of the present invention is provided herein.
Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.
The present invention will be described with reference to the accompanying drawings. Generally, the drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.
AFE circuit 100 includes a hybrid circuit 102, a transformer 126, and a line impedance 128. AFE circuit 100 further includes resistors 120 and 122, as shown in
Typically, a line driver/transmitter 124 connects to input terminals 108 and 110 of hybrid circuit 100. Line driver/transmitter 124 provides transmit signals in the form of differential signals, which are transferred by AFE circuit 100 to a central office (CO) (not shown in
Hybrid circuit 102 is typically used to enable duplex communication in the communication device using AFE circuit 100. One function of hybrid circuit 102 includes isolating receiver 130 from transmit signals being transferred through AFE circuit 100. In other words, transmit signals are prevented from entering the receiver. This function is also known as echo rejection. Another function of hybrid circuit 102 includes providing an appropriate receiver transfer function for signals received from the CO, i.e., appropriate signal amplification/rejection at specified frequency ranges. For illustration,
Generally, hybrid circuit 102 includes a matched impedance network. For example, as shown in
Transformer 126 is coupled at output terminals of hybrid circuit 102. Transformer 126 is a line transformer that serves as an interface between circuits at the user end and circuits at the CO of the communication system. As a result, transformer 126 also serves to protect circuits at the user end from voltage surges (e.g., due to lightning strikes) that could occur over the line connecting the user and the CO. Typically, transformer 126 is designed to have a flat frequency response for the transmit/receive signal bandwidths.
AFE circuit 100 may further include a line impedance 128, as shown in
Generally, in high-speed communication systems such as DSL, for example, transmit signal bandwidths are relatively wide. Accordingly, hybrid circuit 102 needs to have correspondingly wide frequency echo rejection characteristics.
It can be shown that the transfer function from the line driver/transmitter outputs (input terminals 108 and 110) to the receiver inputs (nodes 112 and 114) is equal to:
where Vrx is the voltage at the receiver inputs due to voltage from the transmitter, Vecho. This transfer function determines the echo rejection characteristics of hybrid circuit 102.
From equation (1), it can be recognized that maximum echo rejection by hybrid circuit 102 (i.e., 100% echo rejection) can be achieved when the ratio
is equal to the ratio
which results in the transfer function described in (1) having a value of zero. In practice, however, achieving a 100% echo rejection is a challenging task, particularly with the presence of inductive components in impedance ZL, which makes ZL frequency-variable. For example, one such inductive component is due to a leakage inductance in line transformer 126, which commonly results from imperfect magnetic linking in the transformer.
The leakage inductance in line transformer 126 can be modeled as an additional inductive impedance coupled in series with the primary winding of the transformer. This causes the value of ZL and, correspondingly, the ratio
to increase with frequency.
As such, without the ratio
tracking the ratio
the echo rejection capability of hybrid circuit 102 degrades as frequency varies, with the value of the transfer function described in (1) tending away from zero.
To compensate for the variance of ZL with frequency, conventional hybrid circuits resort to using inductive elements in impedance Z1, to thereby track changes in the ratio
with equivalent changes in the ratio
small.
As such, hybrid circuit 300 can be designed to have high echo rejection capabilities. However, inductors make hybrid circuit 300 a costly solution to the echo rejection problem. Furthermore, inductors can generate as well as pick up noise and cause certain non-linearities in the hybrid circuit frequency response. The latter causes a degradation in the receiver transfer function of the hybrid circuit. Accordingly, a hybrid circuit without inductors is desirable.
As shown in
As such, hybrid circuit 400 provides a variable
ratio, by means of varying impedance Z2 instead of impedance Z1. Also, note that the placement of capacitor 402 in impedance Z2 provides that Z2 varies in the opposite direction to ZL as frequency varies. Accordingly, the ratio
continues to track the ratio
as frequency varies, thereby compensating for any inductive components in ZL.
Hybrid circuit 400 provides a cheap and compact solution to the echo rejection problem in high-speed communication devices. Furthermore, hybrid circuit 400 performs at least comparably to conventional solutions with respect to key performance measures, as will be further described below.
Conclusion
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
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Number | Date | Country | |
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20090122845 A1 | May 2009 | US |