Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
2001-02-21
2003-01-21
Chang, Daniel D. (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C326S082000
Reexamination Certificate
active
06509755
ABSTRACT:
BACKGROUND
The invention relates generally to transmission lines and, more particularly, to adjusting the terminating and driving impedance of a transmission line to match the characteristic impedance of the transmission line.
It is well known to skilled practitioners in the electrical arts that if a source impedance is matched to a complex conjugate of a load impedance, maximum power transfer between the source and the load is achieved. However, it is difficult to match the imaginary part of the complex impedance and half of the power is lost in the matched source impedance when using passive components for impedance matching. Although this is a characteristic of many electrical circuits, it may take on greater significance where transmission lines are considered. With transmission lines, the primary objective is to avoid reflections in the transmission line, so the characteristic impedance is assumed to be resistive.
Transmission lines, where the transmission line length is large with respect to the wavelength of the lowest transmission frequency, are commonly used for transmission of data between two or more locations. It is well known in the art of transmission lines, and particularly transmission lines for transmitting information at high data rates, that in order to maximize the efficiency of information transfer with minimum loss and dispersion effects, the terminating impedance of a receiver and the driving impedance of a transmitter must match the characteristic impedance Z
0
of the transmission line over the frequency range of interest. That is, it is desirable to maintain a uniform characteristic impedance Z
0
along the length of the signal carrying line. Any mismatch in the characteristic impedance across interconnect interfaces will cause reflection of the signal at the interface, resulting in losses and distortion of the signal in the form of attenuation, echo and cross-talk. Furthermore, multiple reflections from multiple interfaces only compound the deleterious affect on the information-carrying signal. The classical solution to the impedance matching problem involves attempting to match the distributed-parameter impedance of the transmission line with lumped-parameter impedances of resistor, capacitor and inductor circuit elements.
Wide band communication channels, like ADSL modulation over telephone conventional lines or other wideband modulation schemes, require matching of line impedances that are complex, where amplitude and phase are dependent on frequency. Telephone subscriber loops with bridged taps present impedance variations at the receiver end that are difficult to match using simple circuits. Furthermore, the impedances variations may change from loop to loop, making it impossible to design a matching circuit using generic discrete circuit components. The use of full-duplex techniques, where bi-directional transmission is conducted concurrently only further complicates the difficulty of matching interface impedances to the characteristic impedance of the transmission line.
There have been a number of different approaches to solving the characteristic impedance matching problem. In the most simple and rudimentary form, fixed resistor elements are connected across the transmission line interfaces to match the interface impedance with the characteristic impedance of the transmission line. More complex impedance matching circuits using combinations of resistor and capacitor elements are often found connected to transmission lines. Impedance matching circuits using passive components may dissipate half of the available power at the transmitter, oftentimes reducing its dynamic range by half. Although power is seldom a major consideration on a standard data transmission line, loss in dynamic range can result in excessive signal clipping with high peak to average ratios that are typical of Quadrature Amplitude Modulated signals and Discrete Multi-Tone signals, used in many modern data transmission systems.
One of the oldest and widely used approaches to match a transmitter-receiver to a transmission line is a hybrid circuit that makes use of two transformers and a balance impedance network Z
L
that, when matched to the characteristic impedance Z
0
of the transmission line, results in very high isolation between transmitter and receiver circuits. This circuit provides a line termination that matches the characteristic impedance of the line and results in no reduction in dynamic range. However, only half the power delivered by the transmitter is sent to the transmission line, the other half being wasted on the balancing impedance network Z
L
. In addition to loss of transmitted power, the balancing impedance network Z
L
cannot perfectly match a line with bridged taps or multiple interfaces. It is impractical to add switching circuits to adapt the impedance to different lines, where each line has a different configuration of taps or interfaces along the length of the line. Furthermore, this hybrid circuit makes use of multiple magnetic circuits that have inherent non-linear characteristics that produce distortion, which adversely affects signals with high peak to average ratios. These transformers also exhibit parasitic capacitance and leakage inductance that may impair circuit operation and reduce useful bandwidth.
Another approach that has received increased interest is the use of a differential driver circuit having two outputs, where each output is connected through an impedance matching resistor to each of the two terminals, respectively, of the primary winding of a transformer. The secondary winding of the transformer is connected to the transmission line. However, not only is half of the transmitter power dissipated in the two impedance matching resistors, but half of the signal amplitude is also dropped across these resistors. This results in reducing the dynamic range of the signal at the transmitter by one-half and reducing the maximum power available to drive the transmission line by one-fourth. The transformer provides for scaling the line impedance to compensate for this reduction and for generating enough peak voltage without excessive clipping. Two amplifiers, each connected across a terminating resistor receive the signal on the transmission line. This circuit may only perform better than the hybrid circuit described above in the high frequency range, where the line impedance will be mostly resistive in nature. Although more complex networks may replace these terminating resistors, the resultant configuration would also suffer from the same limitations as the hybrid circuit described above, namely low power efficiency and reduced dynamic range.
All of these solutions assume that the characteristic impedance of the transmission line is fixed and known, and therefore terminated accordingly. These solutions result in reduced power available to the transmission line, reduced dynamic range of the signal, and losses and distortion in the signal. Although more pronounced with transmission lines, these problems apply to many electrical circuits.
For the foregoing reasons, it is desirable to have a method and device for driving and receiving signals on a transmission line that does not exhibit loss of the available transmitter power to drive the line, does not suffer from a reduction in dynamic signal range, and dynamically matches the driving and terminating impedance at the interfaces to the characteristic impedance of the transmission line.
SUMMARY
The present invention is directed to a method and device for driving a load with active impedance matching that satisfies these needs. The present invention is particularly suitable for providing a method and device for driving and receiving signals on a transmission line that does not exhibit loss of the available transmitter power to drive the line, does not suffer from a reduction in dynamic signal range, and dynamically matches the transmission line interface driving and terminating impedance to the characteristic impedance of the transmission line.
In a voltage driver version of the present inv
Batzer William B.
Chang Daniel D.
Jannson Pehr B.
Ryberg John J.
Schlumberger Technology Corporation
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