Telecommunications – Receiver or analog modulated signal frequency converter – With wave collector
Reexamination Certificate
2000-09-18
2003-04-08
Chin, Vivian (Department: 2682)
Telecommunications
Receiver or analog modulated signal frequency converter
With wave collector
C455S063300, C455S025000, C455S279100, C379S028000
Reexamination Certificate
active
06546235
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electronic circuits and, more particularly, to a system for canceling distortion caused by electromagnetic coupling among nodes in such circuits.
BACKGROUND OF THE INVENTION
For millions of homes throughout the world, a convenient way to access a data network such as the Internet is via a data modem connected to a standard, band-limited telephone line. The data modem typically has a transmit amplifier, a receive amplifier and a hybrid which allows full duplex communication of transmit and receive signals across the telephone line.
With the evolution of data networks in general and the Internet in particular, users have begun to demand the speedy transmission of large amounts of data in the form of imagery, music, software downloads and so on. In order to allow data transmission to occur at sufficiently high rates with an acceptable level of accuracy, it is crucial for the transmit and receive amplifiers in the data modem to operate in a highly linear manner. That is to say, the distortion level at the output of each amplifier must be very low compared with the level of the useful signal. In addition, it is important from the customer's point of view that the data modem be inexpensive and consume little power so as to be of a reasonable size without the risk of overheating.
In an attempt to satisfy these constraints, most full duplex data transmission schemes rely on the use of different frequency bands for the transmit and receive signals such that the transmit and receive amplifiers operate in non-overlapping regions of the frequency spectrum. A “guard band” typically separates the transmit and receive frequency bands. The use of distinct frequency bands helps reduce the amount of leakage from the transmit amplifier through the hybrid into the receive amplifier, which in turn reduces the probability that a symbol output by the receive amplifier will be erroneous.
However, there are strict limitations on the width of a guard band that can be used in a practical system when contemplating the transmission of data at high speeds across a standard telephone channel initially designed for voice transmission in the 300-3500 Hz frequency range. Thus, it is often the case that only a narrow guard band separates the transmit and receive frequency bands. It therefore becomes even more important to make the transmit and receive amplifiers linear so as to reduce the likelihood of the transmit signal being distorted and spilling over into the receive band and also to reduce the likelihood of the receive signal being distorted and spilling over into the transmit band. For this reason, the linearity requirements associated with the transmit and receive amplifiers in a data modem are in some cases so severe as to require a difference of 85 dBc between the levels of signal and distortion at the amplifier output.
Unfortunately, regardless of the degree of linearity of a transmit or receive amplifier, use of the amplifier in a full duplex data modem circuit will nevertheless result in the presence of distortion at the amplifier output. Such distortion can be traced to electromagnetic coupling of signals from other areas of the circuit into the signal present at the amplifier input. The amplifier therefore amplifies both the useful signal and the distortion, resulting in the appearance of an amplified useful signal and an amplified distortion component at the output. The presence of an amplified distortion component at the output makes it appear as though the amplifier did not behave linearly whereas the problem is really rooted in the fact that the useful signal at the input to the amplifier was contaminated with electro-magnetically induced distortion to begin with.
This phenomenon is now described in greater detail with reference to
FIG. 1
, which shows a transmit amplifier
100
and a receive amplifier
150
assumed to be in proximity to one another in a data modem circuit. The transmit amplifier
100
is implemented as a class AB amplifier with two transistors
110
,
120
whose emitters are connected together and also to an input stage
130
of a hybrid. An input voltage V
IN
(t) is applied simultaneously to the base of both transistors
110
,
120
. When the input voltage V
IN
(t) is a sinusoid at a frequency fo as shown in
FIG. 2A
, the transistors
110
,
120
conduct during alternating half-cycles of the sinusoid. Assuming the transmit amplifier
100
to be highly linear from an input-output point of view, an output voltage V
OUT
(t) taken at the emitter junction and prior to the input stage
130
of the hybrid will very closely resemble the input voltage V
IN
(t) and will thus be a sinusoid at frequency f
0
.
The frequency content of the input and output voltages V
IN
(t), V
OUT
(t) is shown in FIG.
2
B. The frequency spectrum is separated into a transmit band
240
, a receive band
230
and a guard band
220
which separates the transmit and receive bands. The input voltage V
IN
(t) and the matching output voltage V
OUT
(t) are both represented in the frequency spectrum of
FIG. 2B
by a spike at a frequency f
0
in the transmit band
240
, which illustrates the sinusoidal nature of the input and output voltages.
However, as now explained, the currents supplying the two transistors
110
,
120
exhibit characteristics which are in contrast to those of the input and output voltages V
IN
(t), V
OUT
(t). Because each transistor
110
,
120
in the class AB amplifier
100
only conducts during alternating half-cycles, the respective supply currents (denoted I
S1
(t) and I
S2
(t)) will also consist of half-cycles. Those skilled in the art will appreciate that such half-cycles are replete with second- and higherorder distortion components.
By way of example and with reference to
FIG. 2C
, there is shown a trace of the supply current I
S1
(t) as a function of time along with its associated frequency content in FIG.
2
D. Shown at
210
are multiple distortion components that are basically harmonics of the frequency f
0
. Beat frequencies may also appear due to the introduction of second-, third- and higher-order distortion components by the amplifier
100
in the presence of DMT-type signals. Since the guard band
220
is relatively narrow, significant ones of the distortion components
210
appear in the receive band
230
and, through electro-magnetic induction, these distortion components will affect the current I
R
(t) and the voltage V
T
(t) at the input of the receive amplifier
150
.
Specifically, the supply current I
S1
(t) travels around a loop
160
in the data modem circuit which defines a certain surface area. Meanwhile, the input current I
R
(t) to the receive amplifier
150
, as received from an output stage
140
of the hybrid, travels around a different loop
170
in the data modem circuit which will have a surface area of its own. Due to the mutual proximity of the two loops
160
,
170
, the supply current I
S1
(t) feeding the transistor
110
in the transmit amplifier
100
will electro-magnetically couple its way into the voltage V
T
(t) and the current I
R
(t) at the input of the receive amplifier
150
and will manifest itself as a parasitic distortion component.
This parasitic distortion component depends on the electromagnetic flux induced by loop
160
onto loop
170
(denoted &phgr;
160→170
). For instance, considering the effects on the voltage V
T
(t), this can be expressed in mathematical terms as:
V
T
(
t
)=
V
R
(
t
)+
V
P
(
t
),
where
V
P
(
t
)=
d
(&phgr;
160→170
)/
dt=&agr;·dl
S1
(
t
)/
dt=&agr;·j&ohgr;·I
S1
(
t
),
and where j=(−1), &ohgr; is the frequency of operation and &agr; is a coupling factor that depends on the dimensions and configurations of the two loops
160
and
170
and on their relative orientation and proximity. The value of &agr; is generally unknown a priori and may be complex, meaning that it introduces an arbitrary change in both magnitude and phase.
Thus, it is seen that due to the effects of electromagnetic
Anderson Carl C.
Gorcea Dan V.
Chin Vivian
Lee John J
Nortel Networks Limited
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