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Texas Instruments AN-807 Reflections: Computations and Waveforms (Rev. B) Application notes
Application Report
SNLA027B – May 2004 – Revised May 2004
AN-807 Reflections: Computations and Waveforms
.....................................................................................................................................................
ABSTRACT
In this application note, the logical progression from the ideal transmission line to the real world of the long
transmission line with its attendant losses and problems is made; specifically, the methods to determine
the practicality of a certain length of line at a given data rate is discussed. Transmission line effects on
various data formats are examined as well as the effects of several types of sources (drivers) on signal
quality. A practical means is given to measure signal quality for a given transmission line using readily
available test equipment. This, in turn, leads to a chart that provides the designer a way to predict the
feasibility of a proposed data-transmission circuit when twisted-pair cable is used. This application note is
a revised reprint of section three of the Fairchild Line Driver and Receiver Handbook. This application
note, the second of a three-part series (see AN-806 and AN-808), covers the following topics:
1
2
3
4
5
6
7
8
9
10
11
12
13
Contents
Overview ..................................................................................................................... 3
Introduction .................................................................................................................. 3
The Initial Wave ............................................................................................................. 3
Cut Lines and a Matched Load ........................................................................................... 4
Kirchoff's Laws and Line-Load Boundary Conditions .................................................................. 5
Fundamental Principles .................................................................................................... 9
Tabular Method for Reflections— the Lattice Diagram ............................................................... 10
Limitations of the Lattice Diagram Method ............................................................................. 13
Reflection Effects for Voltage Source Drivers ......................................................................... 14
Reflection Effects for Matched-Source Drivers ........................................................................ 16
Reflection Effects for Current-Source Drivers ......................................................................... 17
Summary—Which are the Advantageous Combinations? ........................................................... 21
Effect of Source Rise Time on Waveforms ............................................................................ 23
List of Figures
1
Generator Driving an Infinite Transmission Line ........................................................................ 4
2
Thevenin Equivalent for Initial Wave ..................................................................................... 4
3
Voltage/Current Steps for Three Source Resistances ................................................................. 4
4
Voltages and Current on an Infinite Length Line........................................................................ 5
5
Boundary Conditions at the Line/Load Interface ........................................................................ 5
6
Waveforms for RL = R0 ..................................................................................................... 7
7
................................................................................................................................ 9
................................................................................................................................ 9
Superposition of Simple Waveforms to Form More Complex Excitations ......................................... 10
Sign Conventions for Waves ............................................................................................. 11
(a) Line Circuit to be Analyzed........................................................................................... 11
Reflection Bookkeeping with the Lattice Diagram ..................................................................... 11
Model Used for Lattice Diagram Method ............................................................................... 13
RS < R0 ...................................................................................................................... 15
RS = R0 ...................................................................................................................... 15
8
9
10
11
12
13
14
15
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16
RS > R0 ...................................................................................................................... 15
17
Source and Load Voltage Waveforms for Various R S and RL ....................................................... 16
18
Approximation of Midline Voltage with RS > R0 and RL > R0 ......................................................... 21
19
Transmission Line Model and Its Lattice Diagram .................................................................... 24
20
Waveforms for tr = 2 ≪ τ ................................................................................................. 24
21
Waveforms for tr = 2τ...................................................................................................... 25
22
Waveforms for tr = 2τ...................................................................................................... 26
23
Waveforms for tr = 2τ...................................................................................................... 26
24
Waveforms for tr = 3τ...................................................................................................... 26
25
Waveforms for tr = 3τ...................................................................................................... 27
26
Waveforms for tr = 4τ...................................................................................................... 27
27
Waveforms for tr = 4τ...................................................................................................... 28
28
Waveforms for tr = 6τ...................................................................................................... 28
29
Waveforms for tr = 6τ...................................................................................................... 28
30
Waveforms for tr = 8τ...................................................................................................... 29
31
Waveforms for tr = 8τ...................................................................................................... 29
List of Tables
2
1
(RS = 2000Ω, R0 = 100Ω, RL = 4000Ω) .................................................................................. 18
2
(RS = 500Ω, R0 = 75Ω, RL = 10 kΩ) ..................................................................................... 19
3
BASIC Program Listing ................................................................................................... 20
4
Summary of Effects ....................................................................................................... 22
AN-807 Reflections: Computations and Waveforms
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Overview
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1
Overview
In this application note, the logical progression from the ideal transmission line to the real world of the long
transmission line with its attendant losses and problems is made; specifically, the methods to determine
the practicality of a certain length of line at a given data rate is discussed. Transmission line effects on
various data formats are examined as well as the effects of several types of sources (drivers) on signal
quality. A practical means is given to measure signal quality for a given transmission line using readily
available test equipment. This, in turn, leads to a chart that provides the designer a way to predict the
feasibility of a proposed data-transmission circuit when twisted-pair cable is used. This application note is
a revised reprint of section three of the Fairchild Line Driver and Receiver Handbook. This application
note, the second of a three-part series (see AN-806 and AN-808), covers the following topics:
• The Initial Wave
• Cut Lines and a Matched Load
• Kirchoff's Laws and Line-Load Boundary Conditions
• Fundamental Principles
• Tabular Method for Reflections—The Lattice Diagram
• Limitations of the Lattice Diagram Method
• Reflection Effects for Voltage-Source Drivers
• Reflection Effects for Matched-Source Drivers
• Reflection Effects for Current-Source Drivers
• Summary—Which are the Advantageous Combinations?
• Effect of Source Rise Time on Waveforms
2
Introduction
In AN-806 it was determined that transmission lines have two important properties: one, a characteristic
impedance relating instantaneous voltages and currents of waves traveling along the line and, two, a
wave propagation velocity or time delay per unit length. In this chapter, both Z0and δ are used to compute
the line voltages and currents at any point along the line and at any time after the line signal is applied.
Also, concepts of reflections and reflection coefficients are explored along with calculating methods for
voltages and currents.
3
The Initial Wave
Application Note AN-806 also showed that for most practical purposes, where fast rise and fall time
signals are concerned, the characteristic impedance of the line actually behaves as a pure resistance
.
Figure 1 shows a generator comprised of a voltage source (magnitude V), a source resistance of RS
ohms, and a switch closing at time t = 0 connected to a lossless, infinite length transmission line having a
characteristic resistance, R0. Because the relationship of VIN to IIN is known as VIN = R0 IIN, the lossless
transmission line can be replaced with a resistor as shown in Figure 2. The loop equation is.
IIN (RS + R0) = V
(1)
Substituting VIN/R0 for IINand collecting terms shows
(2)
This shows that both source and characteristic resistances act as voltage dividers for the source voltage
V. Figure 3 shows voltage and current steps for the various source resistances. Source resistances of less
than R0 produce initial voltage steps on the line which are greater than half the compliance of the source
voltage, V. A matched source (RS = R0) produces voltage steps exactly half of V and source resistances
greater than R0 produce an initial voltage step less than one half V in magnitude. Generators can be
classified into three categories:
• Voltage source types where RS < R0
• Matched source types where RS = R0
• Current source types where RS > R0
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Cut Lines and a Matched Load
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Waveforms of these types will be discussed more fully in AN-808 on long line effects. Suffice to say that
initial voltage wave amplitude depends greatly on source resistance. Voltage source type drivers produce
higher amplitude initial voltage waves in the line than either matched source or current source type
drivers.
Figure 1. Generator Driving an Infinite Transmission Line
Figure 2. Thevenin Equivalent
for Initial Wave
Figure 3. Voltage/Current Steps for Three Source Resistances
4
Cut Lines and a Matched Load
In examining an infinite, lossless line (Figure 4), it is already known that the ratio of line voltage to current
is equal to the characteristic resistance of that line. The line is lossless, and the same voltages and
currents should appear at point x down the line after a time delay of xδ. If the line at point x is cut, and a
resistor of value R0 is inserted, there would not be a difference between the cut, terminated finite line and
the infinite line. The vx and ix waves see the same impedance (R0) they were launched into at time t = 0,
and indeed, the waves are absorbed into RL (= R0) after experiencing a time delay of τ = xδ. So, from an
external viewpoint, an infinite-length lossless line behaves as a finite-length lossless line terminated in its
characteristic resistance.
4
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Kirchoff's Laws and Line-Load Boundary Conditions
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Figure 4. Voltages and Current
on an Infinite Length Line
5
Kirchoff's Laws and Line-Load Boundary Conditions
The principle of energy conservation, widely known and accepted in the sciences, applies as well to
transmission line theory; therefore, energy (as power) must be conserved at boundaries between line and
load. This is expressed in an English language equation as follows.
(3)
Figure 5 shows power available at the line end is derived by the following formula. (This is assuming inphase current and voltage.)
(4)
The power absorbed by the load will be
(5)
while power not absorbed by the load is represented by
(6)
Here, the r subscript stands for reflected (not absorbed) power, voltage or current, respectively.
Applying Kirchoff's laws to point x in Figure 5, the current to the load is
iL = ix − ir
(7)
and voltage across the load is
vL = iL RL = vx + vr
(8)
Figure 5. Boundary Conditions
at the Line/Load Interface
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Kirchoff's Laws and Line-Load Boundary Conditions
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To find the ratio of vr to vx so that it can be ascertained how much power is absorbed by the load, and how
much is not absorbed (therefore, reflected), substitute vx/R0for ix and vr/R0 for ir into Equation 7.
(9)
Rearranging Equation 8 and substituting for iL in Equation 9 yields
(10)
The minus sign associated with vr/R0means, in this case, that the reflected voltage wave vr travels in the
−x direction toward the generator.
Collecting like terms of Equation 10 yields
(11)
So,
(12)
and the desired relation for vr/vx is
(13)
This ratio is defined as the voltage reflection coefficient of the load ρVL
(14)
A similar derivation for currents shows
(15)
For the remainder of this application note and AN-808, the v or i subscript on the reflection coefficient is
dropped, and ρL is assumed to be the voltage reflection coefficient of the load. Similarly, applying
Kirchoff's laws to the source-line interface, the voltage reflection coefficient of the source is
(16)
The current reflection coefficient of the source has the same magnitude as ρS, but is opposite in algebraic
sign.
When a traveling wave vx, ix meets a boundary such as the line load interface, a reflected wave is
instantaneously generated so that Kirchoff's laws are satisfied at the boundary conditions. This is the
direct result of the conservation of energy principle. Referring again to Figure 5, the effects of three
different termination resistance RL values are shown.
Case 1, RL = R0
In this case, RL is equal to the characteristic resistance of the line. Using Equation 14, the voltage
reflection coefficient of the load ρL is
(17)
Since vr/vx = ρL, then vr = ρL vx = 0 and no reflection is generated. This agrees with the discussion of cut
lines and matched load where a line terminated in its characteristic impedance behaves the same as an
infinite line. All power delivered by the line is absorbed into the load. The waveforms appear as shown in
Figure 6. The wave starting at the source at time t = 0 is reproduced at point x down the line after a time
delay of t = xδ = τ.
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Kirchoff's Laws and Line-Load Boundary Conditions
Figure 6. Waveforms for RL = R0
Case 2, RL > R0
To simplify this case, assume that RS = R0. This means that the initial voltage is
(18)
Also assume RL = 3 R0, then the load voltage reflection coefficient is
(19)
The voltage wave arriving at point x at time t = xδ generates a reflected voltage wave of magnitude
(20)
and the load voltage is
(21)
The reflected voltage wave vr generated at t = xδ = τ travels back down the line toward the source arriving
at the source at time t = 2xδ = 2τ. This wave will be absorbed without generating another reflection
because RS was picked to equal R0, making ρS equal to zero. The source voltage is now
(22)
and equilibrium is achieved.
If the circuit in Figure 5 is analyzed using simple circuit theory and neglecting the transmission line effects,
it is easily seen that
(23)
This agrees exactly with Equation 22 and will always be the case. After all reflections cease and the circuit
reaches equilibrium, the steady state voltages and currents on the line are the same as those produced
using simple dc circuit analysis. Waveforms for RL > R0 (specifically RL = 3 R0) appear in Figure 7.
In general, the case where RL > R0is viewed in the following manner. Because the line is capable of
delivering more power than can be instantaneously absorbed by the load, the excess power is returned to
the source and absorbed in the source resistor (assuming RS = R0).
An upper limit on the voltage reflection coefficient is found by allowing RL to go to infinity. In this case,
Equation 14 goes to +1.
Case 3, RL < R0
In this case, again set RS = R0and allow RL to equal R0/3. The initial wave, as before, is
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Kirchoff's Laws and Line-Load Boundary Conditions
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(24)
and the load voltage reflection coefficient is
(25)
Therefore, the reflected voltage wave vr is
(26)
which starts propagating back toward the source at time t = τ. The load voltage at time t = τ is
(27)
The (−V/4) reflected wave arrives back at the source at time t = 2τ. Because RS is set equal to R0, ρS is,
then, equal to zero and no reflected wave will be generated. The voltage at the source is now
(28)
From a dc circuit analysis, the steady state voltage is
(29)
This agrees with the result of Equation 28. The waveforms for Case 3 (RL < R0) appear in Figure 8.
An interpretation of the actions occurring when load resistance is less than the characteristic line
resistance is as follows: when power available at the line end is less than the power the load can absorb,
a signal is sent back to the source saying, in essence, “send more power”.
It has been shown that a ratio of line and load resistance (ρ) can be used to calculate the voltages and
currents in terms of a wave arriving at the boundary, possibly generating a reflected, reverse-traveling
wave to satisfy the conservation of energy principle at the line-to-load boundary. This ratio is
(30)
where RB represents the resistance into the boundary, RB is RS when considering the source-to-line
interface and RB would be RL when considering the line-to-load interface. It is obvious that if discussing
impedances, then ZSwould be substituted for RS in Equation 30, and there may be some phase angle
between the voltage and current waves.
The forward traveling wave, vx, plus the reflected wave, vr, is equal to the load voltage (VL). Since vr is ρL
vx, this can be expressed as
vx(1 + ρL) = vL
(31)
This quantity (1 + ρ) can be defined as the voltage transmission coefficient of the load and it is known that
(32)
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Fundamental Principles
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RS = R0, RL = 3R0
Figure 7.
Figure 8.
The cases with various load resistances can be summarized.
Circuit at time t = τ (one line delay time)
Condition
6
1.
RL = R0
ρL = 0
No reflection is produced—circuit reaches steady state immediately.
2.
RL > R0
ρL > 0
Positive voltage reflection—wave is sent back toward source. Voltage
at load is higher than steady stage voltage (overshoot).
3.
RL < R0
ρL < 0
Negative voltage reflection—wave is sent back toward source. Voltage
at load is lower than steady state voltage (undershoot).
Fundamental Principles
Before examining the algorithm for keeping track of reflections, there are two principles to keep in mind.
• Energy (as power) is conserved at boundary conditions (as explored previously)
• The principle of linear superposition applies. This means any arbitrary excitation function can be
broken down into step functions, or ramps. The reaction of the circuit to each part can be analyzed,
and the results can be added together when finished. This means that a positive pulse of duration t is
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Tabular Method for Reflections— the Lattice Diagram
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examined by superimposing two step functions, one positive and one negative, starting after a delay of
t (Figure 9). It also means the voltage at any point on the line is the sum of initial voltage plus the sum
of all voltage waves that have arrived at or passed through the point up to and including the time of
examination. Also, the current on the line is, at any point, the sum of initial current plus any forward or
reverse traveling currents passing the point up to and including the time the current is examined.
It has also been established that the steady state solution for voltages and currents on the line can be
found by simple dc circuit analysis.
In examining reflection effects for the remainder of this application note, the following conventions are
used.
A voltage or current wave traveling toward the point of interest will have the subscript “i” for incident wave,
A voltage or current wave traveling away from the point of interest will have the subscript “r” for reflected
wave,
The subscript “S” means the parameter applied to the source (vSfor the voltage at the source, etc.), and
The subscript “L” means the parameter applied to the load (vL for the voltage at the load, etc.)
Sign conventions for voltage waves and their associated currents are shown in Figure 10.
Figure 9. Superposition of Simple Waveforms to Form More Complex Excitations
7
Tabular Method for Reflections— the Lattice Diagram
The waves going up and down the line can be monitored by drawing a time scale, as a vertical line with
time increasing in the down direction, to represent the location on the line under examination. Because
voltages at the source and load ends of the transmission line are normally of primary interest, two time
scales are necessary. Drawing arrows from one time scale to the other as in Figure 10 shows the direction
of travel of the waves during a specific time interval. Since the main concern is only with the waveforms at
the line ends, time scales are ruled off in multiples of the time delay of the line τ. If a unit-step type wave is
launched from the source at time t = 0+, it is known that the magnitude of the wave will persist unchanged
until a wave arrives back from the load after a round trip delay time of two line delays. The source time
scale then is incremented in multiples of 2 mτ where m = 0, 1, 2, 3,… Likewise, the first wave arrives at
the load after a single time delay, so the first increment ruling on the load time scale is τ, or one time delay
of the line. Because the subsequent waves arrive back at the load in increments of 2τ, the load time scale
is ruled off in multiples of (2m + 1)τ where m = 0, 1, 2, 3,… The operation of the lattice diagram is
discussed using the example in Figure 12 which is the lattice diagram for the associated circuit.
time t = 0− (just before the switch closes)
The voltages at the source and load are equal with a magnitude of vinitial. Assume that no initial voltage is
present. So, in this case, the voltage at the source and load equals zero.
Vinitial = vS(0−) = VL(0−) = 0
(33)
time t = 0+ (just after the switch has closed)
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Tabular Method for Reflections— the Lattice Diagram
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Figure 10. Sign Conventions for Waves
Figure 11. (a) Line Circuit to be Analyzed
(b) Lattice Diagram
Figure 12. Reflection Bookkeeping with the Lattice Diagram
The first wave vi(1) is launched at the source and begins to travel toward the load end of the line. As
previously mentioned, a voltage divider action between RS and R0 is used to derive the magnitude of the
initial voltage wave.
(34)
At this time, the voltage at the source is the sum of the initial voltage plus the voltage wave vi(1) just
generated.
(35)
Because the switch closure represents a step function, the source voltage remains at this level until a
wave returns after reflecting from the load at time t = 2τ.
time t = τ
The incident voltage wave vi(1) now arrives at the load and generates a reflected voltage wave
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Tabular Method for Reflections— the Lattice Diagram
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(36)
where ρL is the voltage reflection coefficient of the load. The reflected voltage wave vr(1) immediately
starts traveling back toward the source becoming the incident voltage wave v i(2) which arrives back at the
source at t = 2τ. The voltage at the load is now the sum of the initial voltage plus the incident voltage wave
vi(1) that just arrived plus the reflected voltage wave that is just departing.
(37)
Again, because of the step function excitation, the load voltage remains unchanged until the new wave
arrives at time t = 3τ.
time t = 2τ
vi(2) now arrives at the source and generates a reflected voltage wave vr(2) of magnitude
(38)
where ρS is the source voltage reflection coefficient.
The reflected voltage wave vr(2) starts back toward the load end of the line and becomes the incident
voltage wave vi(3) arriving at the load at time t = 3τ. The voltage at the source is now the sum of the
voltage that was there plus the incident voltage wave just arrived plus the reflected voltage wave just
departed for the load.
(39)
time t = 3τ
vi(3) arrives at the load generating vr(3)
vr(3) = ρLv i(3)
(40)
vr(3) departs back toward the source becoming v i(4) to the source. The load voltage is now
vL(3) = vL(1) + vi(3) (1 + ρL)
(41)
time t = 4τ
When vi(4) arrives at the source and generates v r(4), then
vr(4) = ρSv i(4)
(42)
starts back toward the load to become vi(5) to the load. The load voltage is now
vL(4) = vL(2) + vi(4) (1 + ρL)
(43)
This process can continue ad infinitum or until no measurable changes are detected. The reflection
process at that time is considered complete and the line assumes a steady state condition. Steady state
conditions can be found by applying simple dc circuit theory to source load circuits.
Summarizing this lattice diagram method, any time t = mτ and m > 1, the following relationships exist:
If m is odd, the vi(m) wave is arriving at the load and generates a reflected wave
vr(m) = ρLv i(m)
(44)
This becomes vi(m + 1) as it starts toward the source. The voltage at the load at time t = mτ will be
vL(m) = vL(m − 2) + vi(m) (1 + ρL)
(45)
This is the sum of the voltage that was there before the wave arrived, i.e., vL(m − 2), plus the wave
arriving vi (m) and the reflected wave vr(m) departing.
If m is even, the vi(m) wave is arriving at the source and generates a reflected wave
vr(m) = ρSv i(m)
(46)
This becomes vi(m + 1) as it starts toward the load. The voltage at the source is now
vS(m) = vS(m − 2) + vi(m) (1 + ρS)
12
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(47)
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This is the sum of the voltage that was present vS(m − 2) plus the incident wave arriving vi(m) plus the
reflected wave departing vr(m).
The voltage and current at the source end of the line for a lossless line can be expressed as a summation.
(48)
(49)
where e(t) is the generator voltage as a function of time, and u(t) is the unit step function.
Likewise, the load voltage and load current for the lossless line can be expressed as a summation.
(50)
(51)
A similar expression of summation can be developed for the voltage (or current) at any point along the line
at any time.
Because the lattice diagram is tabular in method, a computer program can be written relieving the
designer of bookkeeping and repetitive calculations. A BASIC computer program for lattice diagrams
appears in >Figure 13.
8
Limitations of the Lattice Diagram Method
Before using the lattice diagram to explore reflection effects with various source and load characteristics, it
is necessary to pause at this point and examine the models used by the lattice diagram.
First, both the line driver and receiver are simulated either by a constant input or output resistance. The
source has two voltage sources and a switch representing the internal source voltage at a time less than
zero and equal to (or greater than) zero. The receiver is represented by a single resistor shunting the line
end opposite the driver site. The line itself is represented by its characteristic resistance R0 and its total
one-way time delay (τ). This is equal to length times propagation delay per unit length. This model is
shown in Figure 13.
Figure 13. Model Used for Lattice Diagram Method
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Reflection Effects for Voltage Source Drivers
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Because most data communication circuits are voltage types, that is, the receiver senses the line voltage
to decide if a logic One or logic Zero is present, the primary interest is in voltages at the source and load
as a function of time. Major exceptions include the current loops used in teletypewriters, telegraphs, and
burglar alarm systems. The majority of data communications circuits used in computers, peripherals, and
general controllers are voltage types.
The lattice diagram method cannot easily use source or receiver current/voltage relationships that are
non-linear; i.e., not purely resistive. For non-linear current/voltage characteristics such as found in diodes,
a graphic method can be used called the reflection diagram or the Bergeron method.
NOTE: A French hydraulic engineer, L.J.B. Bergeron developed the method to study the
propagation of water hammer effects in hydraulics. See references, AN-806.
Signals exchanged using lattice diagrams are of the unit step variety. When ramps or more complex
waves are exchanged, the complexity of the bookkeeping increases dramatically. Additionally, the lines
are presumed to be lossless, although a constant line attenuation factor could be accommodated without
excessive bookkeeping. These limitations should be kept in mind when examining various source and load
resistance combinations and their reflection characteristics.
There are three classes of source resistance, RS < R 0, RS = R0 and RS > R0. There are also three classes
of load resistance, RL< R0, RL = R0 and RL > R0. This gives nine types of single driver, single receiver line
circuits. Each circuit will be examined in turn to determine reflection effects for these combinations with
evaluations of each combination for voltage type communications.
9
Reflection Effects for Voltage Source Drivers
Initial waves launched by a voltage source type driver (R S < R0) are greater than one-half the magnitude
of the internal voltage source. Referring to Figure 13, the initial voltage wave is derived as follows.
(52)
while the voltage at the source at t = 0+ is
(53)
If the receiver switching point is at the mean of the driver voltage swing, the initial wave always has
sufficient magnitude to indicate the correct logic state as it passes the receiver site. This maximizes the
noise margins of the receiver.
Since RS < R0, the source voltage reflection coefficient ρS is less than zero. Any voltage waves, then,
arriving back at the source are changed in sign, reduced in amplitude (assuming RS > 0Ω), and sent back
toward the load. If the load resistance equals the characteristic line resistance (RL = R0), the voltage
reflection coefficient of the load is
(54)
No reflections, therefore, are generated at the load. The voltage wave produced at the source is
reproduced at the load after a time delay of τ = ℓδ, and the line assumes a steady state condition.
Figure 17 illustrates the source and load voltage waveforms for this case.
If RL is greater than R0, ρ L is positive. Waves arriving at the load generate the same polarity reflections as
the arriving waves. ρS and ρLare of opposite signs, so a dampened oscillatory behavior of the load voltage
is expected. The oscillation period or ringing is 4τ. The overshoot of vL from t = τ to 3τ may cause
breakdown of the input circuitry of a receiver, depending on the receiver voltage rating. The undershoot at
t = 3τ or 5τ can reduce the noise immunity of a receiver or even cause a logic level misinterpretation—an
error in the data. These waveforms are shown in Figure 17.
If RL is less than R0, then ρ L is negative and a wave arriving at the load generates a reflection opposite in
polarity to the incident wave. This causes the voltage at the source to overshoot steady state voltage at t =
0. Each reflection returning from the load causes the source voltage to continually step down toward the
steady state voltage VSS. These steps last for 2τ, or one round trip delay. Load voltage starts an increasing
step-up waveform towards VSS at time t = τ, with steps again taking one round trip delay, 2τ. A line
receiver placed in the middle of the line sees an entirely different waveform—dampened oscillations much
14
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like the load voltage in Figure 17. This is caused by the negative signs of both source and load voltage
reflection coefficients. Each time an incident wave arrives at either source or load, the reflected wave
generated at that time has a sign opposite to the sign of the incident voltage wave. The voltage at a
distance half way down the line is composed of these forward and reverse traveling waves arriving at that
point commencing at time t = 0.5τ, and with each new wave passing that point after one line delay (τ).
These waveforms are shown in Figure 17.
The optimum load resistance for voltage signal communications on transmission lines driven by a low
impedance source (RS < R0) is equal to the characteristic line resistance. Large signal line voltages are
produced and there are no reflection effects complicating the waveforms Figure 15.
However, a matched load (RL = R0) is a dc load on the driver, thus it increases system power dissipation.
But, it does preserve signal fidelity and amplitude allowing use of multiple bridging receivers (Rin ≫ R0)
along the line.
The unterminated case (RL > R0) reduces dc driver loading and also reduces system power dissipation
over the matched load case. The unterminated case does, however, allow the load signal to exhibit
pronounced overshoot and undershoot around the steady state voltage. If the load signal undershoot
places the receiver in its threshold uncertainty region, data errors result. There is a way to “civilize” the
voltage waveform of the unterminated line load by trading off signal rise time versus line time delay. This
is discussed later.
The final case of RS < R0 and R L < R0 is not generally useful in terms of voltage signals produced
(Figure 17). Systems using this case consume more power than the previous two cases and have no
particular advantage for voltage mode communications.
RL > R0
RL = R0
RL < R0
Figure 14. RS < R0
RL > R0
RL = R0
RL < R0
Figure 15. RS = R0
RL > R0
RL = R0
RL < R0
Figure 16. RS > R0
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Figure 17. Source and Load Voltage Waveforms for Various R S and RL
10
Reflection Effects for Matched-Source Drivers
In all three cases under discussion here, the initial voltage produced by the driver onto the line is
(55)
since RS = R0. The voltage at the source at time t = 0+ is
(56)
Assume, for clarity, that initial voltage (V0−) is zero, thus Equation 56 simplifies to
(57)
Since RS = R0, ρSis equal to zero. This means that load-generated reflections due to load mismatch are
absorbed at the source when, at time t = 2τ, the reflected wave arrives back at the source. The line then
assumes a steady state throughout. This back match or series termination effect of a matched source
allows a wide latitude in choice of load resistance without sacrificing the signal fidelity of the load voltage
waveform.
If the load resistance equals the characteristic line resistance RL = R0, then ρL equals zero and no load site
reflections are generated. The initial voltage wave arrives at the load at time t = τ (one line delay) and
voltages (and currents) on the line immediately assume steady state conditions (see Figure 17). The
optimum receiver threshold here is one-half the steady state voltage or V0+/4. The main advantage over
the voltage source type driver with matched load case (RS < R0, RL = R0) is that RSand RL resistance
tolerances may be relaxed without incurring much signal ringing. This effect is due primarily to the
termination provided by both line ends, rather than just one line end. Any reflected voltage wave on either
system is attenuated by the product of ρS and ρL for each round trip line delay time. Since the ρSρL product
for the fully matched case is smaller than the ρSρLproduct for the single matched case, the reflections are
attenuated and die out in fewer round trips. For example, if 20% tolerance resistors are used in both
cases, ρS and ρL values for the fully matched case become 0.0 ±0.0909, which is a ρSρ L product of
±0.0033. This means that after one round trip (2τ), the reflection amplitude starting back toward the load
would be less than 0.33% of the initial wave.
Using RS = 10Ω, RL = 100Ω, and R0 = 100Ω as for Figure 17, shows the same 20% tolerances applied to
the single matched case
—0.8519 ≤ ρS ≤ —0.7857
—0.0909 ≤ ρL ≤ +0.0909
(58)
(59)
|ρS ≤ ρL |≤ 0.0909
(60)
and
The voltage reflection amplitude after one round trip is a maximum of 7.7% of the initial wave.
The choice between using the single and fully matched system should be carefully considered because
the fully matched system does sacrifice signal voltage magnitude to get a decreased dependence on
absolute resistor values.
If the load resistance for a matched driver circuit is made much greater than the line resistance, the initial
wave arriving at the load at time t = τ will be almost double since ρL will be close to +1.0. Because source
resistance is set equal to line resistance, ρ S becomes zero, the reflected voltage wave from the load is
absorbed by the source at time t = 2τ, and steady state conditions prevail. Waveforms for this case are
shown in Figure 17. This is called back matching or series termination.
16
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The main advantage of series termination is a great reduction in steady state power consumption when
compared with the parallel terminated case (RS ≪ R0, RL = R0). At the same time, series termination
provides the same signal fidelity to a receiver placed at the line end. Compare the load voltage waveforms
for the two cases in and Figure 17. The main disadvantage to series termination is that receivers placed
along the line see a waveform similar to that shown for the source in Figure 17. That is, receivers along
the line see the V 0+/2 initial wave as it passes that point on the line, and do not see a full signal swing until
the load end reflection passes that point. Consequently, receivers along the line do not see a signal
sufficient to produce the valid logic state output until the load reflection returns. Depending on actual line
length and receiver characteristics, the receiver may even oscillate, having been placed in its linear
operation region. With the benefit, then, of reducing system power, the series termination method has a
constraint of allowing only one line receiver located at the line load end. The parallel termination method
should be used if other receivers along the line are required.
The final case of matched source drivers is with the use of a load resistance less than the characteristic
line resistance. The waveforms for this case are shown in Figure 17. A line receiver with a threshold of
V0/4 placed at the source responds like a positive, edge triggered one-shot and produces a pulse in
response to a +V/2 initial wave of 2τ duration. Aside from its use as a one-shot, this circuit doesn't seem
to offer any advantages for voltage mode communications.
11
Reflection Effects for Current-Source Drivers
The name current source drivers is somewhat of a misnomer, and might be more properly called currentlimited voltage source drivers. True current source drivers such as the DS75110A are normally used in
conjunction with parallel termination resistors to create a matched source.
The current source drivers (RS > R0) discussed resemble true current sources in the respect that their
output resistance is usually much greater than the characteristic line resistance. The initial voltage step
produced on the line is thus usually small vi(1) = (iS(1)R 0). This is due to the voltage divider action of the
driver source resistance and the characteristic line resistance.
Voltage waveforms for a current source type driver either step up to VSS, reach steady state after 2τ, or
execute a dampened oscillation around VSS, depending on whether the load resistance RL is greater,
equal, or less than R0, respectively. The second case RL = R0 provides signals much the same as the
other two cases where RL = R0, that is, the source voltage steps immediately to VSS, with the load voltage
following after one line time delay. Here the amplitude of the signal is much smaller than previous
matched load cases. Since the current source type drivers (DS75110A) have high off-state impedances,
they allow multiple drivers on the line to produce data bus or party line. Waveforms for the matched load
case are shown in Figure 17.
The case RL < R0 really provides no useful advantage for voltage mode communications. The negative
sign for ρ L and the positive sign for ρS lead to dampened oscillatory behavior, or ringing. The maximum
perturbation takes place at the source end of the line. Waveforms for this case are similar to those shown
in Figure 17, and are shown to scale in Figure 17. With the given values used to produce the figure, the
maximum amplitude ringing appears at the source line end.
The RL > R0 case is of interest because it is representative of DTL driving a transmission line with the
output going from LOW to HIGH. DTL has a high value RS, (2 kΩ or 6 kΩ) in the HIGH logic state. Since
both RS and RL are greater than R0, both ρS and ρL are positive. A small voltage step starts from the
source at t = 0+; its magnitude is
(61)
NOTE: Since the input diode is not represented, the representation of DTL input as a single resistor
to ground is not strictly correct. For purposes of approximation, this simple representation is
used. Treatment of non-linear current/voltage sources and loads is covered by Metzger &
Vabre. (op. cit.)
Upon arrival at the load at time t = τ, this initial wave generates a positive voltage reflection since ρL > 0.
The voltage reflection arrives back at the source site at time t = 2τ. Since ρS is also positive, another
positive voltage reflection is launched back toward the load. The process repeats, and the source and load
voltages both execute a step-up approach toward steady state voltage VSS. These waveforms are shown
in Figure 17.
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In examining voltage at the line midpoint (x = ℓ/2), a step-up type waveform is seen which is the sum of all
the incident voltage waves passing the line midpoint up to the time of examination. The midpoint voltage is
expressed as follows.
vm(t) = VSS(1 − exp[−(t + 0.5τ)/T])
(62)
NOTE: This equation is presented without derivation, but a procedure similar to that used by Matick
(Ref. 2, AN-806) can be used.
for t = n + 0.5τ with n = 0, 1, 2, 3, etc. VSS in Equation 62 is the steady state line voltage
(63)
and T is a time constant given by
(64)
with τ being one line delay (τ = ℓδ).
NOTE: This equation is presented without derivation, but a procedure similar to that used by Matick
(Ref. 2, AN-806) can be used.
Equation 62 provides an exact solution for odd multiples of n (n = 1, 3, 5…, so t = 1.5τ, 3.5τ, 5.5τ…), while
it approximates vm(t) for even multiples of n (n = 0, 2, 4…, so t = 0.5τ, 2.5τ, 4.5τ…). The closer the ρSρL
product is to 1, the better Equation 62 predicts vm(t), particularly for even multiples of n. To illustrate the
fitting, Table 1 and Table 2 are generated by the BASIC language computer program Table 3 and their
data is plotted in Figure 18.
Designers familiar with DTL circuits should quickly recognize that the waveforms shown in Figure 18 are
very similar to the rising edge waveforms found when a DTL gate output goes from the LOW to HIGH
state. This characteristic waveform has usually been attributed to the series RC circuit (a gate output
resistance driving a lumped transmission line capacitance). The time constant for this approach, based on
the C(dv/dt) = i rule from simple circuit theory, provides only an approximation. The actual cause of the
waveform shape, however, is due to reflection effects. Unfortunately, the only way to speed up the rising
edge is to reduce source resistance, (providing an initial step greater than the receiving threshold) and
terminate the line to eliminate the load reflections.
DTL inability to drive transmission lines at high repetition rates is the direct result of the signal rise time
limitation caused by positive reflection coefficients for both the source and load. A transmitted positive
pulse may be missed if its duration is less than the time required for the load signal to reach the receiver
threshold.
The RS > R0 and RL > R0 case provides no definite advantages as voltage mode communication is
concerned. This case, in fact, poses a definite hazard to high speed data communications because the
reflections cause, in effect, a slow, exponential signal transition. Because line delay is a factor, longer
lines will only increase the effect.
Table 1. (RS = 2000Ω, R0 = 100Ω, RL = 4000Ω) (1)
(1)
18
TIME
VM(T)
VAPPX
%DIFF
0.5
0.04762
0.04820
+1.220%
1.5
0.09292
0.09292
+0.000%
2.5
0.13390
0.13440
+0.373%
3.5
0.17288
0.17288
+0.000%
4.5
0.20815
0.20858
+0.207%
5.5
0.24170
0.24170
+0.000%
RHOS = 0.904762
RHOL = 0.951220
TAU = −13.3250
V1(1) = 4.76190H-C2
VSS = 0.666667
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Table 1. (RS = 2000Ω, R0 = 100Ω, RL = 4000Ω) (1) (continued)
TIME
VM(T)
VAPPX
%DIFF
6.5
0.27206
0.27243
+0.136%
7.5
0.30093
0.30093
+0.000%
8.5
0.32705
0.32737
+0.097%
9.5
0.35190
0.35190
+0.000%
10.5
0.37439
0.37466
+0.073%
11.5
0.39577
0.39577
+0.000%
12.5
0.41512
0.41536
+0.057%
13.5
0.43353
0.43353
+0.000%
14.5
0.45018
0.45038
+0.045%
15.5
0.46602
0.46602
+0.000%
16.5
0.48035
0.48053
+0.036%
17.5
0.49399
0.49399
+0.000%
18.5
0.50632
0.50647
+0.030%
19.5
0.51805
0.51805
+0.000%
20.5
0.52867
0.52880
+0.024%
21.5
0.53877
0.53877
+0.000%
Table 2. (RS = 500Ω, R0 = 75Ω, RL = 10 kΩ) (1)
(1)
TIME
VM(T)
VAPPX
%DIFF
0.5
0.13043
0.13971
+7.112%
1.5
0.25893
0.25893
+0.000%
2.5
0.35390
0.36066
+1.909%
3.5
0.44746
0.44746
+0.000%
4.5
0.51661
0.52153
+0.952%
5.5
0.58473
0.58473
+0.000%
6.5
0.63509
0.63867
+0.564%
7.5
0.68469
0.68469
+0.000%
8.5
0.72135
0.72396
+0.361%
9.5
0.75747
0.75747
+0.000%
10.5
0.78416
0.78606
+0.242%
11.5
0.81046
0.81046
+0.000%
12.5
0.82990
0.83128
+0.167%
13.5
0.84904
0.84904
+0.000%
14.5
0.86320
0.86420
+0.117%
15.5
0.87714
0.87714
+0.000%
16.5
0.88744
0.88818
+0.083%
17.5
0.89759
0.89759
+0.000%
18.5
0.90510
0.90563
+0.059%
19.5
0.91249
0.91249
+0.000%
20.5
0.91795
0.91834
+0.042%
21.5
0.92334
0.92334
+0.000%
RHOS = 0.739130
RHOL = 0.985112
TAU = −6.30356
V1(1) = 1.30435
VSS = 0.952381
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Table 3. BASIC Program Listing
PRINT'ENTER RS, R0, RL'1
100
INPUT R1, R0, R2
110
P1=(R1-R0)/R1+R0)
120
P2=(R2-R0)/R2+R0)
130
V1=R0/R1+R0)
140
K1=2./LOG(P1*P2)
150
V9=R2/(R1+R2)
160
PRINT'RHOS='; P1;'RHOL=';PS;'TAU=';K1
170
PRINT 'V1(1)=';V1;'VSS=';V9
180
V=V1
190
PRINT'TIME VM(T) VAPPX %DIFF'
200
FOR T=0.5 TO 20.5 STEP 2.
210
V2=V9*(1.-EXP((T+.5)/KL))
220
P=100.*(V2-V)/V
230
PRINT USING 250,T,V,V2,P
240
:##.# -#.##### -#.##### +###.###%
250
V1=V1*P2
260
V=V+V1
270
REM SOURCE END
280
V2=V9*(1.-EXP( (T+1.5)/k1 ) )
290
P=100.*(V2-V)/V
300
PRINT USING 250,T+1.,V,V2,P
310
V1=P1*V1
320
V=V+V1
330
NEXT T
340
PRINT
350
PRINT
360
PRINT
370
GOTO 100
380
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Table 3. BASIC Program Listing (continued)
END
390
Figure 18. Approximation of Midline Voltage with RS > R0 and RL > R0
(65)
12
Summary—Which are the Advantageous Combinations?
In examining the basic combinations of source, line and load resistances, and typical waveforms
characteristic of each case, advantageous combinations can be determined. The primary results are
tabulated in Table 4. those combinations generally used in voltage mode communications circuits are as
follows.
1. Unterminated case (RS ≪ R0, R L ≫ R0). This situation provides low steady state power dissipation and
large signal levels, but also shows pronounced “ringing” effects. The “ringing” can be reduced by
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controlling signal rise/fall time versus τ, or by clamping diodes to limit load signal excursions. This case
is representative of TTL circuits and is thus widely employed.
2. The parallel terminated case (RS ≪ R0, RL = R0) provides large signal levels, and excellent signal
fidelity. However, it is power consuming with most of that power dissipated in the load resistor. This
case is useful for cleaning up the reflection effects of Case 1 but, at the same time, does require a
driver circuit to have its internal current limits set at greater values than those required to produce the
desired signal level into the minimum line resistance used. Thus, this case requires specific line driver
devices such as the DS75114/DS9614. Ordinary TTL, except for the above mentioned circuits, has too
low a current limit point to adequately drive 50Ω lines.
3. The series terminated or backmatched driver case RS = R 0, RL ≫ R0 provides a low steady state
power dissipation system for use with one receiver located at the load end of the line. The positive
reflection coefficient of the load is used to approximately double the initial wave arriving at the load.
Setting R S = R0 terminates the reflected wave when it arrives back at the source site after two line
delays, and the line then assumes steady state conditions. The use of other receivers located along
the line is not recommended, because they will not see the full driver signal swing until the reflection
from the load passes their particular bridging points Such receivers could malfunction, as they would
see a voltage very close to their threshold, and perhaps even place the line receiver in its linear
operating region. This could make the line receiver sensitive to oscillatory, parasitic feedback. If these
constraints are acceptable, the series termination method can be used to good advantage in providing
the same signal fidelity and signal amplitude as with the parallel termination method, while at the same
time, contributing a significant savings in steady state power consumption.
4. The fully matched case RS = R0, RL = R0 not only provides excellent signal fidelity all along the line, but
also has reduced signal amplitude over that of the parallel terminated case. Additionally, the power
consumption is somewhat less than the parallel termination case and the power is divided equally by
the source and load. The primary advantage of the fully matched system is that termination resistor
tolerances can be relaxed somewhat without incurring large amounts of ringing. This is because both
the source and load act as line terminations.
Table 4. Summary of Effects
Signal
Characteristics
Optimum
Receiver
Threshold
Line Receivers
Allowed at Other
Than Load End of
Line?
≫ R0
Ringing Pronounced
0.5 VSS
Yes
Undershoot May
Cause Errors
≪ R0
= R0
Excellent Fidelity
0.5 VSS
Yes
Load Resistor
Consumes Power
≪ R0
≪ R0
Awful—Different
Signals at Each
Point on the Line
NA
No
Not Generally Useful
Series
Terminated or
Backmatched
Driver
= R0
≫ R0
Load Signal
Excellent
0.5 VSS
No
Reduced Power
Consumption Over
Parallel Termination
Fully Matched
= R0
= R0
Excellent Fidelity
0.25 VSS
Yes
Greater Tolerances
on Resistors Allowed
for Same Fidelity as
Parallel Termination
= R0
< R0
Load Signal Like a
One-Shot
NA
NA
Not Generally Useful
for Data, is Useful as
Pulse Generator
≫ R0
≫ R0
Exponential Like
Signal Waveforms
0.5 VSS
Yes
Low Power
Consumption.
Increased Delay due
to Signal “Rise”
Times.
(Driver)
Source
Resistance
(Receiver)
Load
Resistance
Unterminated
≪ R0
Parallel
Terminated
Configuration
Name (if any)
Comments
(66)
22
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Table 4. Summary of Effects (continued)
Configuration
Name (if any)
13
(Driver)
Source
Resistance
(Receiver)
Load
Resistance
≫ R0
= R0
≫ R0
< R0
Optimum
Receiver
Threshold
Line Receivers
Allowed at Other
Than Load End of
Line?
Small Signal
Amplitude and
Excellent Fidelity
0.5 VSS
Yes
Produces Only Small
Signal Voltages
Compared with Other
Methods. Uses
Current Sinking
Drivers such as the
75110A.
Very Small Signal
Amplitudes, also
Ringing
NA
NA
Not Generally Useful
Signal
Characteristics
Comments
Effect of Source Rise Time on Waveforms
Previously, it was assumed that the source-produced signal rise time was always much less than the line
time delay (τ). Because the waveforms for the source and load voltage were the superposition of incident
and reflected waves occurring at their proper times, and because the shape of each wave was a square
edged step function, the resultant source and load waveforms were thus also square edged, or ideal in
nature. In many practical cases, particularly when line length is short, the source excitation possesses a
finite, and non-negligible, rise time. Therefore, depending on the ratio of rise time to line delay, it is
possible to have a new wave start arriving at the point of interest before the previous wave can reach its
final value. The net waveform for voltage or current at that point, then, would consist of the superposition
of two or more waves during their time of overlap. To study the superposition effect on signal waveforms,
the source excitation is represented as a simple linear ramp rise to its final value of V0+, so
e(t)— = 0 for t < 0
e(t)— = V0+•t/tr for 0 ≤ t ≤ tr
e(t)— = V0+ for t > tr
and where tr represents the 0-to-100% source rise time. The circuit model and its lattice diagram are
shown in Figure 19. The values of RS, R0 and RL were chosen to equal those of an actual circuit on hand,
allowing the theoretical waveforms, obtained by graphical superposition, to be compared with the
measured response of an actual circuit.
Figure 20 shows the load voltage vL, source voltage vS and source current iS waveforms versus time for a
circuit with a source rise time very much less than τ. The actual waveforms for vL, v S and iS are composed
of the superposition of both incident and reflected waves in their proper time sequence. In the figures,
these waves are shown as dotted lines. Each wave represents the sum of the incident wave plus its
reflection. The resultant vL, vS and i S waveforms (shown as solid lines) are the superposition of the waves
represented by the dotted lines. With the exception of a slight rounding of the edges, the actual waveforms
for the circuit, shown in the oscilloscope photograph in Figure 20, closely approximate the waveforms
predicted by theory.
Source
Load
t
vi + vr
ii + ir
vS
iS
t
vi + vr
ii + ir
vL
iL
in (τ)
(V)
(mA)
(V)
(mA)
in (τ)
(V)
(mA)
(V)
(mA)
0
0.9400
12.53
0.9400
12.53
1
1.8500
0.40
1.8500
0.40
2
0.1224
−22.64
1.0624
−10.10
3
−1.5500
−0.34
0.3000
0.06
4
−0.1026
18.97
0.9599
8.87
5
1.2986
0.28
1.5986
0.35
6
0.0859
−15.90
1.0458
−7.03
7
− 1.0881
−0.24
0.5106
0.11
8
−0.0720
13.32
0.9738
6.29
9
0.9116
0.20
1.4222
0.31
10
0.0603
−11.17
1.0341
−4.87
11
−0.7638
−0.17
0.6584
0.14
12
−0.0505
9.36
0.9836
4.48
13
0.6399
0.14
1.2983
0.28
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Effect of Source Rise Time on Waveforms
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Source
Load
t
vi + vr
ii + ir
vS
iS
t
vi + vr
ii + ir
vL
iL
in (τ)
(V)
(mA)
(V)
(mA)
in (τ)
(V)
(mA)
(V)
(mA)
14
0.0424
−7.84
1.0259
−3.36
15
−0.5362
−0.12
0.7622
0.16
16
0.0355
6.57
0.9904
3.21
17
0.4492
0.10
1.2114
0.26
Figure 19. Transmission Line Model and Its Lattice Diagram
Figure 20. Waveforms for tr = 2 ≪ τ
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If the source excitation is adjusted so that its 0-to-100% rise time tr is equal to 2τ, each of the vi + vr and ii
+ ir waveforms must be modified to include this rise time. The waves will have the same final value as
predicted by the lattice diagram, but they now require two line time delays to reach this final value. The vL,
vS and iS waveforms consist of the superposition of these linear ramps. Because each wave reaches its
final value just as a new wave arrives, their superposition converts the square edged vL, vS and iS
waveforms into triangular waveforms. This is shown in Figure 21. The accompanying oscilloscope plot
shows the close correspondence between the actual and theoretical waveforms whereas an additional
oscilloscope photograph in Figure 21 shows the actual waveforms for the case where tr = τ. Not
surprisingly, the tr = τ case changes the vL, vS and iS waveforms of the tr ≪ τ case into trapeziodal forms
because each arriving wave reaches its final value well before a new wave arrives.
If the source excitation is adjusted such that its rise time equals three line delays tr = 3τ, the vi + vr and ii +
ir waves overlap for a period of time equal to τ. That is, each wave reaches only ⅔ of its final value when a
new wave starts arriving. Considering the waveform, the load voltage from time τ to 3τ is
vi(1) (1 + ρL) e (t − τ)
(67)
Starting at t = 3τ, the wave
vi(3) = vi(1)ρ SρL e(t − 3τ).
(68)
begins arriving from the source, and the load voltage then is the superposition of these two waves.
Because vi(3) is a negative wave (ρS < 0), the algebraic sum of the last third of the first wave and the first
third of the second wave vi(3) arriving at the load causes the load voltage to reduce in amplitude from the
(t r ≪ τ) case. Likewise, the source voltage and source current show reduced amplitudes over the ideal
case, due to the overlap period of the waves arriving at the source.
Theoretical and actual waveforms for the tr = 3τ case are shown in Figure 24. Notice that load voltage
perturbations and source current iS requirements are reduced from those of the tr ≪ τ case. Similarly, the
ratio of tr to τ can be successively increased. This results in reduced ringing on the load voltage and
reduced source current due to the overlapping of more and more vi + vr (or i i + ir) waves. Actual and
theoretical waveforms for tr equal to 4τ, 6τ, and 8τ are shown in Figure 26, Figure 28 and Figure 30,
respectively. In each case, as the tr to τ ratio is increased, the instantaneous source and load voltages
become more equal. The source current is also reduced so that the circuit exhibits fewer reflection effects
and the transmission line itself can be considered as a simple interconnection from dc circuit theory.
Figure 21. Waveforms for tr = 2τ
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Figure 22. Waveforms for tr = 2τ
Figure 23. Waveforms for tr = 2τ
Figure 24. Waveforms for tr = 3τ
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Figure 25. Waveforms for tr = 3τ
Figure 26. Waveforms for tr = 4τ
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Figure 27. Waveforms for tr = 4τ
Figure 28. Waveforms for tr = 6τ
Figure 29. Waveforms for tr = 6τ
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Figure 30. Waveforms for tr = 8τ
Figure 31. Waveforms for tr = 8τ
Using the tr to τ ratio to reduce reflection effects has many practical advantages in digital design. The low
source and high input resistance of TTL or ECL circuits allows one gate to drive many receiving gates.
The reflection effects of this unterminated combination, however, can cause data errors or at least lead to
reduced noise immunity due to the pronounced load voltage undershoot. Since the rise and fall times of
these devices are easily measured, a maximum line length can be set such that the resulting tr to τ ratio
provides the desired reduction in ringing. This is the primary basis for the wiring rules of each logic family
and, usually, the tr to τ ratio is chosen somewhere between 3:1 and 4:1. As an example, the rise and fall
time for normal TTL is t10%–90% = 6 ns. When this is converted to an equivalent linear 0% to 100% time, t r =
8 ns. A common propagation delay of 1.7 ns/ft, in combination with the requirement that tr = 3τ, gives the
maximum line length of approximately 18 inches. This corresponds with the published recommendation of
the various manufacturers for the 74 series TTL circuits. A similar computation of the rise and fall times for
other logic families yields their respective line length recommendations. The faster families require shorter
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line lengths for the same trto τ ratio, and slower logic families allow relatively longer line length. This ratio
can also be used to make stubs or taps on lines “disappear”. In other words, if the stub's time delay is
made very short when compared to the tr of the signal at the stub line location, the stub reflections will
have a minimal effect on the line signals. A stub length to generate a tr to τ ratio of greater than 8:1 is
usually considered adequate to negate the stub reflections.
The third primary application of the tr to τ ratio for controlling reflection effects is that used in some
standard data communications interfaces such as EIA/TIA-232-E (RS-232). Here, driver slew rate is
explicitly controlled. This, along with the implied maximum interconnect cable length serves to produce a tr
to τ ratio of 3:1 or greater. This, in turn, reduces the reflection effects inherent in a voltage source driver,
unterminated line system. The main disadvantage of using the tr to τ ratio to control reflection effects is in
the overall time for the signal representing the data to rise above the receiver threshold level. With the
parallel terminated method, the minimum time delay was τ or one line delay. When the tr to τ ratio is used,
an additional delay time of approximately 0.5 tris added to the line delay yielding, therefore, a greater
effective signal propagation delay. This increased delay may or may not be acceptable in the desired
system so the trade-off between ease of usage of the unterminated case must be weighed against the
increased effective signal delay over that delay obtainable with the terminated case.
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