Pulse or digital communications – Cable systems and components – Transformer coupling
Reexamination Certificate
1998-03-04
2002-04-02
Chin, Stephen (Department: 2634)
Pulse or digital communications
Cable systems and components
Transformer coupling
C375S295000
Reexamination Certificate
active
06366618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to data communications and, more specifically, the present invention relates to low voltage integrated circuits transmitting data over higher voltage communications lines.
2. Background Information
As the speeds of electronic circuits increase, there is a continuing trend to reduce the operating voltage of the integrated circuits from the traditional 5 volt DC power supply. For instance, present day integrated circuits commonly operate at approximately 3.3 volts and 2.2 volts. Lower voltage integrated circuits will soon be desired to accommodate higher integrated circuit speeds in the near future. However, many existing communications systems require 5 volt peak-to-peak signals that cannot be easily produced by these lower voltage integrated circuits.
At the present time, there are some known methods of producing higher voltage data communications signals, such as for example 5 volt peak-to-peak signals, with lower voltage integrated circuits, such as for example approximately 3.3 volts or lower. However, most known methods suffer from a variety of problems including undue amounts of noise and interference, and/or are impractical to implement.
FIG. 1A
is a schematic of one known method of producing higher voltage level data communications signals with lower voltage integrated circuits. A data communications circuit is illustrated with an isolation transformer
101
including a center-tap
103
coupling primary winding
105
to Vcc. Primary load resistors
109
and
111
are coupled between center-tap
103
and the respective ends of primary winding
105
of transistor
101
. A data communications line
121
is coupled to the ends of secondary winding
107
. As shown in
FIG. 1B
, the characteristic impedance Z
0
of data communications line
121
may be alternatively represented as a secondary load resistor
115
, which would be coupled in parallel across the ends of secondary winding
107
. Two current sinking drivers
117
and
119
are connected to the ends of primary winding
105
.
One problem with the data communications circuit illustrated in
FIG. 1A
is that center tap
103
results in inherent second harmonic distortion, which manifests as common mode signal currents and attendant electromagnetic interference (EMI) problems.
With the dual current sinking drivers
117
and
119
at the transformer ends of primary winding
105
, and with Vcc applied to center-tap
103
, the circuit of
FIG. 1A
resembles a push-pull audio power amplifier in topology and current sinking drivers
117
and
119
only drive half of the primary winding
105
at a time. Since the remaining non-driven half of the primary winding
105
is rather tightly magnetically coupled, autotransformer action occurs from the non-driven winding. Consequently, all of the parasitic reactances and IR drops associated with the non-driven winding, as well as the reflected non-linear B-H magnetizing characteristic from the transistor core appear as additional loads to the driving side of the circuit. Since there is no negative feedback in the circuit, as would be used in an analogous audio power amplifier to reduce distortion, the circuit of
FIG. 1A
suffers from quite high harmonic distortion due to non-linear loading. At frequencies of operation occurring in networking systems, this translates into excessive EMI and loss of signal quality.
FIG. 2
is a schematic of another known center-tapped primary data communications circuit similar to the circuit shown in FIG.
1
A. In particular,
FIG. 2
shows an isolation transformer
201
with a center-tap
203
on primary winding
205
. Data communications line
221
is coupled across the ends of secondary winding
207
. A characteristic impedance of data communications line
221
is represented in
FIG. 2
as a secondary load resistor
215
, coupled in parallel across the ends of secondary winding
207
. Two current sinking drivers
217
and
219
are connected to the transformer ends of primary winding
205
. Each end of primary winding
205
is coupled to center-tap
203
through primary load resistors
209
and
211
, respectively.
In order to achieve acceptable power consumption, transformer
201
of
FIG. 2
is also driven in a push-pull fashion, similar to the circuit discussed above in FIG.
1
A. Therefore, since the non-driven half of the primary winding
205
is rather tightly magnetically coupled, autotransformer action occurs in the non-driven half of primary winding
205
, and all of its parasitic reactances and IR drops, as well as its reflected non-linear B-H magnetizing characteristic from the transformer core, appear as additional loads to the driving side of the circuit resulting in high harmonic distortion, excessive EMI and loss of signal quality.
FIG. 3
is a schematic of a known data communications circuit utilizing a step-up transformer
301
. Principal problems associated with step-up transformer
301
include the increased circuit sensitivity due to step-up transformer
301
parameters and the very low impedances that result on the primary winding side of step-up transformer
301
. In particular, primary winding
305
of transformer
301
is driven end-to-end by current generator
317
. Primary load resistor
309
is coupled in parallel across the ends of primary winding
305
. Data communications line
321
is coupled end-to-end across secondary winding
307
. The characteristic impedance of data communications line
321
is represented in
FIG. 3
as a secondary load resistor
315
, coupled in parallel across the ends of across secondary winding
307
.
As shown in
FIG. 3
, transformer
301
is a step-up type transformer, which enables the required higher level voltage signals to be achieved on data communications line
321
from a lower level voltage integrated circuit. Since transformer
301
is a step-up type, primary load resistor
309
must be a low impedance load in order for the impedance to be matched across the system. For instance, assuming step-up transformer
301
has a turns ratio of 1:1.41 and that the characteristic impedance
315
of the transformer line
321
is 100 ohms, current generator
317
must operate into an impedance of 50 ohms reflected through a 1:2 impedance transformation. This results in primary load resistor
309
being only 25 ohms in this example. This low impedance is very difficult to implement on a matched impedance circuit board layout.
A 1:1.41 ratio is chosen in this illustration because it is the lowest transformation ratio that is practical to use with 3.3 volt driver circuits. If an integral ratio, such as for example 1:2, were selected for step-up transformer
301
, which is relatively easy to wind, current generator
317
would have to operate into an even lower load impedance. Specifically, if step-up transformer
301
has a turns ratio of 1:2, primary load resistor
309
would be 12.5 ohms in the case of a 100 ohm secondary load resistor
315
.
Non-integral transformation ratios are difficult to achieve accurately with the low number of turns present on high frequency transformers. This is exacerbated by the fact that the output signal level, or launch level, of network drivers must be tightly controlled to allow proper operation of receive-end adaptive line equalizers. Thus, the resulting step-up transformer of
FIG. 3
is difficult and expensive to manufacture and may have to be custom-matched with a lower voltage physical layer of a an integrated circuit. As integrated circuit voltages continued to decrease, such as for example to 2.2 volts or lower, correspondingly even higher step-up ratios and even lower drive impedances will need to be adopted if the configuration of the schematic shown in
FIG. 3
is utilized.
Thus, what is desired is an method and an apparatus providing higher voltage output signals in communications lines from lower level voltage integrated circuits using transformers that do not suffer from the problems discussed above.
SUMMARY OF THE INVENTION
A data communications circuit
Blakely , Sokoloff, Taylor & Zafman LLP
Chin Stephen
Fan Chieh M.
Nortel Networks Limited
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