Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2001-10-22
2003-12-16
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C330S258000
Reexamination Certificate
active
06664820
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of electrical cable driver buffers. Specifically, the present invention relates to a circuit for effectuating a universal cable driver buffer that maintains a constant output voltage for a variety of cable termination electrical characteristics.
2. Related Art
Electronic communication media have become a ubiquitous and crucially important aspect of modern technology, commerce, industry, and leisure. Much modern electronic communications includes transmission of digital data. Some electronic communications media include cable-based modalities. One such cable-based communications modality effectuates electronic communications via coaxial cable. One coaxial cable standard for many communications applications has an impedance of 75 Ohms (&OHgr;).
Communications via coaxial cable routinely utilize devices to place electronic signals, including digital data, onto and to pick such signals off from a coaxial cable. One such group of such devices are transmitter-receivers, also known as transceivers. In the design of components, such as a clock recovery integrated circuit (IC), needed for such transceivers, the intent is to empower the transceiver as a unitary device to drive, e.g., to pass signals effectively onto, a 75&OHgr; coaxial cable.
Drivers for coaxial cables employed in many communication applications are designed to comply with communications standards to assure interconnectivity among and between a plethora of communications networks and systems worldwide. One such standard with widespread adoption is ITU-G703, which is incorporated herein by reference. This standard is promulgated by the International Telecommunications Union (ITU) of Geneva, Switzerland. Standard ITU-G703 demands that compliant cable drivers impress a signal to be transmitted via a 75&OHgr; coaxial cable with an amplitude of one Volt (1 V).
P-channel 13.5 mA constant current sources are a conventional cable driver mainstay. These devices employ operational amplifiers (Op Amps) to achieve the signal gain required to effectively drive a 1 V signal on a 75&OHgr; coaxial cable. Referring to Conventional Art
FIG. 1
, one such op amp based cable driver is depicted. Such conventional cable drivers employ a bipolar implementation.
Their design approach incorporates complementary symmetry amplifiers. Such conventional drivers achieve an output voltage swing of twice the signal voltage specification. This is because compliance with telecommunications standards demands that, in order to properly terminate the communications cable, a termination resistor R
OT
, equal in ohmic resistance value to a source impedance Z
OS
, is connected in series with the line; the output voltage is accordingly divided between the two equal impedances, each dropping one half of the amplifier's output signal voltage. For the steady state conditions of the present discussion, an impedance Z
OC
of the coaxial cable itself is negligible.
Such conventional cable drivers are switchable, such that their output current may be delivered to either of two loads. As depicted in Conventional Art
FIG. 1
, the output current of a cable driver may be delivered from either an inverting or a non-inverting source output terminal to respective load resistors R
OT
or R
OT
by parallel coaxial cable runs. Users of these drivers and the connected coaxial cables delivering the currents being driven by the drivers have the option of compliantly utilizing one or both of two differential signals with no penalty. In one option, a user elects to utilize one single-ended signal. In this case, to remain compliant with ITU-G703, the other available application must be terminated by an equivalent load resistance. In the present example, R
OT
and R
OT
' are compliantly equal in ohmic resistance values.
The preferable method of compliantly terminating the cables delivering the output of a conventional driver is to utilize two separate 75&OHgr; resistors. In this case, the output signal may be taken from either resistor, switched between them, or taken from both and auctioneered, according to the preference of the user for a particular application. In any case, the 13.5 mA signal dropped across the 75&OHgr; resistor develops the 1 V output signal, in compliance with ITU-G703.
However, in using a constant current source to generate a signal with a 1 V amplitude, a problem arises with respect to parasitic capacitance. Parasitic capacitances arise at the output of the driver from a number of sources. Sources of parasitic capacitance there include (1) the capacitance of printed circuit boards used in the construction of both the drivers and the load; (2) routing capacitances arising from the layout of conductors carrying the signal, within the driver, the cable capacitance of coaxial cable itself, and within the load; (3) the capacitance arising between the bonding pads to which the load resistors are soldered or otherwise electrically coupled and mechanically mounted and the dielectric material constituting the material from which the load printed circuit boards are constructed; and (4) capacitance associated with all conductive copper, aluminum and/or metallic traces.
A charge-discharge loop is associated with the parasitic capacitances. The parasitic capacitance charges and discharges cyclically in accordance with the output signal. The charge-discharge cycle of the parasitic capacitance is deleterious for a number of reasons. One detrimental effect is that, with signals on the order of the amplitude under discussion herein, a non-negligible amount of current intended to be passed in the load is diverted to supplying the charging current.
Another effect of parasitic capacitance is adverse to the signal risetime and correspondingly degrades bandwidth and data transfer rate and capacity. Absent parasitic capacitance, the risetime is wholly dependent upon the signal itself, and follows from the device generating the signal. Parasitic capacitance however, in association with the load impedance, develops a charge-discharge time constant proportional to the capacitance, which adds delay to the signal risetime. In as much as bandwidth is inversely proportional to the signal risetime, the delayed signal risetime reduces bandwidth and the rate and capacity of data signal transmission accordingly.
The degradation of signal risetime due to parasitic capacitance is illustrated by reference to Conventional Art FIG.
2
. In a first exemplary circuit with an associated parasitic capacitance of five pico Faradays (pF) has a corresponding signal risetime of just over 1 nanosecond (nS). A second exemplary circuit passes the identical 1 V amplitude signal as the first circuit. However, the second circuit has an associated parasitic capacitance of ten pF. It is seen that the signal risetime associated with the second circuit is twice that of the first circuit, between 2 and 3 nS.
In an effort to counter these detrimental effects, efforts must be taken to minimize parasitic capacitance. For instance, extreme care must be taken in the design layout of printed circuit boards used in drivers and loads and quality control of both materials selected for them and their construction. Also, extreme care is needed in the placement of load resistors and the routing and connection of cable. Further, connection of the cables, usually by BNC type connectors, adds to parasitic capacitance, each BNC connector adding a degree of capacitance and exacerbating the problem.
However, efforts at minimizing parasitic capacitance may be cumbersome and pose an undue burden on users and makers of cable drivers. Efforts such as exercise of care in printed circuit board layout and quality control are burdensome and expensive. Further, they may not effectuate all applications and in others may not suffice.
A further problem arises, which may be characterized as sensitivity to variations in load resistance inherent in conventional constant current drivers. With a constant 13.5 mA output current producing a 1 V p
Goerke Lawrence R.
National Semiconductor Corporation
Wagner , Murabito & Hao LLP
Wells Kenneth B.
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