Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2001-09-06
2002-12-31
Chin, Stephen (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C333S018000, C330S304000
Reexamination Certificate
active
06501792
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the fields of communications, digital data communications, receivers, digital data receivers, equalizers, cable equalizers, automatic gain control (AGC) systems, DC restorers, and quantized feedback (QFB) DC restorers.
BACKGROUND OF THE INVENTION
FIG. 1
shows the concept of a basic communication system
8
including a transmitter
12
, a transmission medium
14
(such as a cable or wire) which is corrupted by noise
16
, and a receiver
18
. In serial digital data communications, the input signal
10
consists of an input pulse train or sequence. The input signal
10
is attenuated and distorted by the medium
14
, through which it is transmitted, before a received signal
17
arrives at the receiver
18
which after processing the signal
17
provides the output signal
20
. Distortion is caused by variable delay (dispersion) and variable attenuation of high frequency components. This distortion results in pulse spreading and consequential interference between neighbouring pulses known as ISI (intersymbol interference).
As shown in
FIG. 1A
, receiver
18
may typically include an automatic or adaptive equalizer
60
to offset the undesirable frequency effects of the cable (or other transmission medium), a DC (direct current) restorer
62
to restore or regenerate the DC component of the transmitted input, and an automatic gain control circuit
64
which provides the necessary gain for the equalizer
60
, as explained below. The adaptive aspect of the cable equalizer is particularly useful, for example, where one receiver is capable of receiving several different signals transmitted from different locations and over cables having different lengths.
FIG. 1B
illustrates a communications system wherein a receiver
18
receives signals from a number of different transmitters (
12
-
1
,
12
-
2
,
12
-
3
, and
12
-
4
) that respectively transmit over cables (
14
-
1
,
14
-
2
,
14
-
3
, and
14
-
4
) which are of different lengths. An automatic cable equalizer in the receiver
18
should be able to equalize signals which have been transmitted over any cable length between some minimum length (e.g. zero length) and some maximum length.
Theoretically, an equalizer should have a frequency characteristic that is the inverse of the transmission medium and which restores high frequency components and eliminates dispersion. In practice however, this also increases noise at the receiver by increasing the noise bandwidth and boosting high frequency noise components. As is well known in the art, the loss over a cable (such as a co-axial cable) of length L may be approximated in frequency domain terms by:
L
(
j
&ohgr;)=
e
−AL
(j&ohgr;)
½
, &ohgr;=2
&pgr;f
where A is a constant. As is common practice and to facilitate understanding, the analysis of equalizer functionality is carried out in the frequency domain. Note that the function L(j&ohgr;) if expanded and expressed in the form of a numerator polynomial divided by denominator polynomial has an infinite number of poles and zeros. As a result, and as is further well known in the art, in a typical implementation of an automatic cable equalizer, the inverse cable loss function is approximated as:
G
(
j
&ohgr;)=1
+Kf
(
j
&ohgr;)
where K is a control variable which varies depending on the length of the cable over which the signal was transmitted from zero at the minimum cable length to unity (or some other constant) at the maximum cable length. The equalizer function circuitry
22
is illustrated in
FIG. 2
where the circuitry for providing the variable gain K is shown at
24
, the circuitry which realizes the function f(j&ohgr;) is shown at
26
, and the summing function is shown at
28
. When the amplitude of the transmitted signal is a standard amplitude which is known, the amount by which the amplitude of the received signal (see below) has been attenuated may be used to provide an appropriate value for the gain K
25
(and correspondingly indicate the length of the cable over which the received signal was transmitted). As will be explained below, this may be obtained, via an AGC system and a DC restorer.
The poles and zeros of the function f(j&ohgr;) are chosen so that 1 +f(j&ohgr;) provides a good approximation to the inverse cable loss L(j&ohgr;) at the maximum cable length.
FIG. 2A
illustrates a possible implementation of a circuit which may achieve an f(j&ohgr;) transfer function. Note that in
FIG. 2A
the f
in
and f
out
signals, which are respectively the input and output of the f(j&ohgr;) circuit, are shown as differential signals whereas in
FIG. 2
these signals are shown as single-ended. Referring to
FIG. 2A
, transistors
74
and
76
form a differential pair whose emitter terminals are connected through an impedance network
78
(each emitter terminal is also connected to a reference through current sources
80
and
82
respectively). The impedance network typically comprises a plurality of resistor-capacitor circuits cascaded together in parallel. The values of the resistor and capacitor components define the poles and zeros of f(j&ohgr;). The collectors of transistors
74
and
76
are coupled to Vcc through resistors
70
and
72
respectively. The input to f(j&ohgr;) is applied between the base terminals of transistors
74
and
76
, and the output of f(j&ohgr;) is taken between the collector terminals of
74
and
76
.
The equalization approach illustrated in
FIG. 2
is, however, subject to several drawbacks. First, since the best approximation to the desired inverse cable loss response occurs at the extreme values of the control variable K, i.e. when K=0 (corresponding to the minimum cable length) and when K=1 (corresponding to the maximum cable length), the accuracy of the approximation deteriorates for intermediate values of K (corresponding to intermediate cable lengths). As the accuracy of the approximation worsens, the resulting errors cause increased jitter in the recovered data.
Second, the above approach is overly susceptible to noise associated with the f(j&ohgr;) function. Typically, the function f(j&ohgr;) can provide a gain of more than 40 dB at a frequency of 200 MHz. As shown in
FIG. 2
, to prevent overload of the f(j&ohgr;) function by the larger input levels associated with short cable lengths, the circuitry for the gain control function K
24
must be physically placed ahead or in front of the circuitry which realizes the f(j&ohgr;) function
26
. As a result, the noise associated with the function f(j&ohgr;) is never attenuated and is always present at the output, irrespective of the value of K. Again, this causes an increase in jitter, particularly for lower values of K.
Third, the function G(j&ohgr;) is also chosen to delay high frequency signals in an inverse manner to the dispersion characteristic of the cable. When K is varied, the delay through the equalizer is also varied. Therefore when K varies in an undesirable manner, for example due to the presence of noise on the K controlling signal
25
, the resulting delay modulation further contributes to jitter.
In addition, ideally a cable equalizer capable of multi-standards operation should be able to trade cable length for data rate as cable length is varied (for e.g., 800 Mbits/second at 100 metres, 200 Mbits/seconds at 400 metres). To minimize noise and ensure stability, the bandwidth of the function G(j&ohgr;) should also vary inversely with cable length. In practice, however, adding circuitry for realizing a variable bandwidth function to the equalizer of
FIG. 2
results in increased circuit noise and delay modulation, and therefore jitter.
The above described problems render the cable equalizer of
FIG. 2
overly susceptible to producing jitter. This prior art cable equalizer is also unsuitable for multi-standards use since standards with higher data rates, and consequentially shorter critical or maximum cable lengths, fall into the non-optimal intermediate operating region and because of the increased jitter levels
Bereskin & Parr
Gennum Corporation
Kim Kevin
LandOfFree
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