Telephonic communications – Line equalization or impedance matching
Reexamination Certificate
1998-01-23
2001-01-30
Chan, Wing F. (Department: 2743)
Telephonic communications
Line equalization or impedance matching
C379S394000
Reexamination Certificate
active
06181792
ABSTRACT:
RELATED APPLICATION
Co-pending application entitled “A Communication Interface Having Combined Shaping Of Receive Response And Synthesized Matching Impedances For Different Frequency Bands And A Design Method Therefor”, and assigned to the same assignee as that of the present invention.
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to communication interfaces and design methods therefor. More particularly, the invention relates to a reconfigurable telephone line interface having synthesized matching impedances for different frequency bands on connecting telephone lines and a design method therefor.
2. Description of Prior Art
Communication circuits, more particularly, telephone line interface circuits, are required to present a terminating impedance to voice or data signals on connecting cable pairs. The terminating impedance prevents, or at least minimizes, noise on the cable pairs in the form of reflections or echos due to mismatches between the terminating impedance of the interface circuit and the transmission impedance of the cable pair. In most countries, the terminating impedance for cable pair must meet requirements specified by a country's telephone administration. In some countries, the required terminating impedance is different in characteristic for adjacent frequency bands served by the interface circuit. Synthesis of a terminating impedance that facilitates matching the impedance requirements of different countries would simply implementations for the different countries and lead to a single circuit that could be programmed to meet the requirements of the different countries.
Recently, active filters using transconductance amplifiers with feedback have been used to synthesize a terminating impedance. Prior art related to terminating impedances for telephone interface circuits using a transconductance amplifier with feedback for matching the impedance requirement of a connecting telephone line or cable pair includes the following:
U.S. Pat. No. 5,500,894 issued Mar. 19, 1996 discloses a telephone line interface circuit with AC and DC transconductance loops for providing selectable impedances to a cable pair. The '894 patent discloses a transconductance loop with a filter network having a transfer function designed to provide a synthesized terminating impedance which may be programmed to meet the terminating impedance requirements of different telephone systems. However, '094 does not disclose a cascade of sub-filter networks to meet the terminating impedance requirements of different frequency bands on a cable pair for different countries while having little or no effect upon the other bands.
U.S. Pat. No. 4,894,864 issued Jan. 16, 1990 discloses a transconductance loop for synthesizing the terminating impedance of a cable pair. However, there is no partitioning the loop into programmable sub-filter networks providing different terminating impedances that may be required by different countries.
U.S. Pat. No. 4,381,561 issued Apr. 26, 1983 discloses impedance matching for an interface circuit using digital impedance synthesizing by a digital filter and D/A converter in a negative feedback loop to match a full duplex analog telephone line to a digital switching system. The '561 patent does not disclose a transconductance loop for synthesizing analog signals on a cable pair into matching impedances for different frequency bands.
U.S. Pat. No. 4, 359,609 issued Nov. 16, 1982 discloses an interface circuit having a feedback loop, either current or voltage controlled by which the circuit impedance can be easily set to match a telephone circuit. The '609 patent does not disclose partitioning a filter network in the loop to provide synthesized impedances matching different frequency bands on a cable pair.
U.S. Pat. No. 4,351,060 issued Sept. 21, 1982 discloses an automatic equalizing function implemented digitally using a recursive digital filter having programmable coefficients and a transconductance amplifier to provide an output impedance matching a telephone line. The '060 patent does not disclose matching a synthesized impedance to different frequency bands on a cable pair.
U.S. Pat. No. 4,961,219 issued Oct. 2, 1990 discloses synthesing an impedance across a telephone line by detecting a voltage difference across the telephone line and using an RC circuit to develop a pair of impedance output signals matching the impedance of the telephone line circuit. The '219 patent does not disclose synthesizing matching impedances to that of different telephone line circuits.
None of the prior art discloses or suggests an easily reconfigurable terminating impedance for a communication interface using a transconductance amplifier with feedback that provides different synthesized impedances for different frequency bands and, in particular, uses a notched response to separate one frequency band from another frequency band.
SUMMARY OF THE INVENTION
An object of the invention is a communication interface and design method which provides a terminating impedance matching different frequency bands occurring on a connecting communication circuit.
Another object is a telephone line interface circuit and design method using active filters including feedback loops to provide terminating impedances matching different frequency bands occurring on a connecting telephone circuit.
Another object is a reconfigurable telephone line interface circuit and design method using a transconductance amplifier and feedback loop for providing terminating impedances matching different frequency bands occurring on different connecting telephone circuits.
Another object is a telephone line interface circuit and design method using a transconductance amplifier with feedback loops including multiple feedback networks which provide matching terminating impedances for different frequency bands occurring on different connecting telephone circuits.
Another object is a telephone line interface circuit and design method using a transconductance amplifier and feedback loop including cascaded filter networks which provide a notch response to separate different frequency bands on different connecting telephone line circuits.
Another object is a standard communication interface which may be reconfigured to provide different matching impedances for different connecting communication circuits having multiple frequency bands thereon.
These and other objects, features and advantages are achieved in a telephone line interface circuit and design method providing a synthesized terminating impedance matching different frequency characteristics for received signals in different frequency bands, as required in some countries. A transconductance amplifier and feedback loop provide the terminating impedance for signals received from the telephone line and a source impedance for signals transmitted by the interface to the telephone line. Signals with different frequencies requiring different transmission impedances will generally be received from the telephone line. These different impedance characteristics are matched using different filter networks in the feedback loop. Each network provides a matching terminating impedance to the telephone line according to the frequency band applied to the telephone line. The network shapes the synthesized impedance based upon the relation Z=1/G
m
H where Z=the terminating impedance; G
m
is the transconductance of the amplifier and H is the transfer function of the feedback network. For multiple frequencies on the telephone line requiring terminating impedances with different characteristics, multiple feedback networks are included in the feedback loop. Each feedback network is designed to match the impedance requirements of a frequency band without affecting the terminating impedance and frequency band of the other frequency bands. In some instances, a notch response is provided by the filter network required to separate the frequency bands.
REFERENCES:
patent: 4351060 (1982-09-01), Treiber
patent: 4359609 (1982-11-0
Derby Jeffrey Haskell
Thomas David Ross
Chan Wing F.
International Business Machines - Corporation
McConnell Daniel E.
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