Wave transmission lines and networks – Automatically controlled systems – With control of equalizer and/or delay network
Reexamination Certificate
2000-03-16
2001-05-29
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Automatically controlled systems
With control of equalizer and/or delay network
C375S230000
Reexamination Certificate
active
06239667
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to attenuation equalizing of transmission lines.
Transmission lines generally represent conductive connections between system elements carrying signal power. However, due to non-ideal physical properties of any transmission line, the transmission lines more or less attenuate the signals to be transmitted.
FIG. 1
a
shows a schematic example of a transmission line
10
connected between a signal generator
20
at a node
25
and a termination impedance
30
at a node
35
. The transmission line
10
shall also comprise a serial impedance
40
. Both impedances
30
and
40
are normally designed to match the characteristic impedance of the transmission line
10
. The signal generator
20
in
FIG. 1
a
is exemplarily depicted as a pulse generator for generating rectangular pulses, e.g., in a digital system.
FIG. 1
b
shows the transmission line effect for an example of a typical transmission line
10
with a frequency dependent attenuation in the circuit according to
FIG. 1
a
, whereby the x-axis shows the time in seconds and the y-axis shows the ratio of the input signal at node
25
divided by two times and the output signal at node
35
. In this example, the transmission line
10
shall provide an impedance of 50 &OHgr; with a propagation delay of 3.2 ns, and the impedances
40
and
30
shall also be 50 &OHgr;. The rise time (defined as the time interval of a leading edge between the instants at which the instantaneous value first reaches specific lower and upper limits of 10% and 90% of the signal amplitude) of a stimulus signal
50
from the signal generator
20
is assumed to be 0.8 ns. The stimulus signal
50
appears at the impedance
30
as an attenuated signal
60
, attenuated by almost 10% and it takes almost 10 ns until the output has achieved its final value.
In case that the signal generator
20
applies pulses at the node
25
, and in particular rectangle pulses, the falling edge of a transmitted pulse at node
35
might already ‘start’ before the rising edge has reached its maximum amplitude, due to the transmission line effect as shown in
FIG. 1
b
. This leads not only to a degradation in shape of the original pulse, but also to a timing error as a change in the propagation delay for the negative slope with varying pulse widths.
FIG. 1
c
shows the influence of the transmission line effect for pulses with decreasing pulse widths, whereby the x-axis shows the time in seconds and the y-axis shows the output signal at node
35
.
FIG. 1
d
depicts the dependency of the timing-error on the pulse width, whereby the x-axis shows the pulse width in seconds and the y-axis shows the timing-error in seconds.
FIGS. 1
c
and
1
d
are both based on the values of the example of
FIG. 1
b.
It is apparent that an increasing timing-error occurs with a decreased pulse width. In particular in testing applications, e.g. in digital IC testers, with a required timing accuracy of 300 ps or less, an error of 65 ps at 1 ns pulse width (compare
FIG. 1
d
) represents a significant portion. Slower transition times increase the error, whereas faster transition times decrease it.
Attenuation equalizers as corrective networks are commonly used in order to compensate the attenuation characteristics of transmission lines. The attenuation equalizers are generally designed to make an absolute value of a transfer impedance, with respect to two chosen pairs of terminals, substantially constant for a certain frequency range.
FIG. 2
a
depicts a common concept to avoid transmission line effects, as indicated by the
FIG. 1
, by providing an attenuation equalizer
100
for amplifying the higher frequencies more than the lower frequencies, or for attenuating the lower frequencies. The attenuation equalizer
100
is coupled between the node
25
and a node
105
before the transmission line
10
.
An example of the attenuation equalizer
100
in
FIG. 2
a
, as an R-C network for an ordinary high pass filter well known in the art, is given in
FIG. 2
b
. The attenuation equalizer
100
comprises a parallel connection of a resistor
110
and a capacitor
120
, coupled with one connection to the node
25
and with the other connection to a node
130
. A second resistor
140
is shunted between the node
130
and ground, and a buffer
150
might be connected in series between the nodes
130
and
105
.
FIG. 2
c
shows the stimulus signal
50
from the signal generator
20
and the corresponding attenuated signal
60
for a resistor-ratio to be chosen as 10.25:1 and a time-constant of 1.8 ns of the RC network in
FIG. 2
b
. The rising edge of the attenuated signal
60
has been improved though the amplitude has been decreased. The thus improved error-curve is depicted in
FIG. 2
d.
A more detailed investigation still shows some errors since the simple approach cannot compensate the effect totally. More complex circuits including two or more time constants are able to reduce these even more. However, if simple R-C networks are applied as the attenuation equalizer
100
, certain drawbacks have to be encountered:
The compensation ratio is fixed.
The required time constant can hardly be achieved in an on-chip application. E.g., a 100 &OHgr; resistor requires a 18 pF capacitor which requires a large amount of silicon space.
The resulting time constant varies with large on-chip resistor tolerances (e.g., ±20%).
Bringing the signal off-chip for compensation and on-chip for further buffering introduces additional capacitances and inductances in a possibly highly sensitive high-frequency signal path.
Other attenuation equalizers are known in the art, such as EP 0 607 702 A2 disclosing an attenuation equalizer designed for long transmission lines (about 100 m) and frequencies in the range of 125 MHz. JP 7007375 discloses a further attenuation equalizer, wherein an element constant is controlled by a filter constant control circuit. Data given to the circuit connects a switch to a pulse waveform generator side, connects another switch to a transmission line side, and connects a further switch to a waveform measuring instrument side.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved attenuation equalizer. The object is solved by providing a frequency filtering in combination with a current compensation.
According to the invention, an attenuation equalizer for compensating an attenuation characteristics of a transmission line comprises a first node, a second node, and a first frequency filter unit coupled to the first node and/or to the second node for frequency filtering an applied signal. A first current source is coupled to the first node in order to control a current provided by the first current source to the second node.
The invention allows to greatly reduce timing errors which occur when high frequency digital signals are fed through transmission lines which do not have the highest quality concerning attenuation. This permits to employ, e.g., digital signals well in the GHz range, transmission lines with small dimensions (increasing functionality per space/volume), and/or smaller cables (which are more flexible, giving more degrees of freedom for an optimum mechanical design). The costs for extremely good cables which usually rises exponentially with quality can be kept down, smaller and flexible cables can be used in conjunction with smaller and cheaper interconnections, and/or varying electrical length which occur quite often in a test-setup (on load boards or when probing on the wafer) can be compensated (even automatically). Other high frequency sensitive components (e.g., relays, connectors) also can be selected for small space and low price. The solution according to the invention is also suitable for both signal directions in a bidirectional signal application.
According to a first aspect of the invention, the attenuation equalizer comprises a signal path with the first node and the second node. A correction path for correcting the frequency behavior of the signal applied to the first node co
Hewlett--Packard Company
Jones Stephen E.
Lee Benny
LandOfFree
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