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11/24/2004EE 42 fall 2004 lecture 361 Lecture #36: Transmission lines Last lecture: –Transmission lines –Balanced and unbalanced –Propagation This lecture: –Transmission line equations –Reflections and termination

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11/24/2004EE 42 fall 2004 lecture 362 Transmission Line Voltage Current in (+) signal line Current in (-) signal line, or current in ground If the signal is conducted in a pair of lines, it is called a balanced line. If the return path is through a ground, it is called unbalanced

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11/24/2004EE 42 fall 2004 lecture 363 Transmission line impedance The ratio of the voltage of propagating pulses to the current the carry is a constant, called the impedance of the line A typical transmission line impedance is 50-100 ohms. When a line is being used in this fashion, it can not be split into two, because for a given voltage, twice as much current would be needed So digital transmission lines do not branch, there is only one path from one end to another.

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11/24/2004EE 42 fall 2004 lecture 364 Transmission line impedance The impedance of a line depends on the width of the conductors and their separation. The greater the separation between the conductors, the higher the impedance The wider the conductors, the lower the impedance

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11/24/2004EE 42 fall 2004 lecture 365 Practical TL impedances If the separation is large compared to the width of the conductors, the impedance gets larger only logarithmically, so impedances much higher than 300 ohms or so are impossible to achieve without losses getting very large Low impedance lines can be made with wide conductors which are close together, but are usually undesirable because of the high current required.

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11/24/2004EE 42 fall 2004 lecture 366 Drivers for transmission lines A driver for a transmission line needs to put out a current I=V 0 /Z 0, of course, which is quite a bit of current for a logic driver. In general this requires several cascaded inverters, each with wider transistors that the previous ones.

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11/24/2004EE 42 fall 2004 lecture 367 Power The power required to transmit signals on a transmission line is P=IV, so: In order to reduce the power required, reduced voltages can be used. If voltages lower than logic levels are transmitted, then amplifiers (comparators) are needed at the inputs to convert the voltage levels.

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11/24/2004EE 42 fall 2004 lecture 368 Power (2) Notice that the power required to transmit information on a transmission line does not depend on the data rate. This means it is an advantage to reduce the total number of signal lines, by increasing the data rate. The data rate is limited by the CMOS drivers, and by the quality of the transmission line.

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11/24/2004EE 42 fall 2004 lecture 369 Transmission line model (from the driver’s perspective) Since transmitting on a long transmission line means providing a current I=V 0 /Z 0, to the driver it looks exactly like a resistance of R=Z 0.

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11/24/2004EE 42 fall 2004 lecture 3610 Transmission line formulas If a transmission line has the same cross section from one end to the other, then propagation on it can be written Where t is time, and x is distance along the line

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11/24/2004EE 42 fall 2004 lecture 3611 Transmission line formulas What this means is that f+ can be any function of a single argument, sin(t) for example. Plugging in the argument (t-x/v) shifts the function, so that at points some distance x down the line, you see a delayed version of the function, delayed by the time x/v

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11/24/2004EE 42 fall 2004 lecture 3612 Transmission line formulas Since the transmission line has the same cross section at each point along its length, it has a constant ratio of current to voltage, Z 0, and the current pulse has the same shape. Where t is time, and x is distance along the line

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11/24/2004EE 42 fall 2004 lecture 3613 Transmission line formulas Since pulses can propagate either way on the transmission line, we have: Notice that the current changes direction for the signal propagating in the reverse direction

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11/24/2004EE 42 fall 2004 lecture 3614 Transmission line into a resistor What happens at the end of a transmission line? For example an open circuit R=∞? Where does the power go? Z0Z0

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11/24/2004EE 42 fall 2004 lecture 3615 Transmission line into a resistor A voltage pulse coming down the transmission line and hitting the resistor can generate a reflected pulse traveling back up the transmission line. Z0Z0

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11/24/2004EE 42 fall 2004 lecture 3616 Transmission line formulas Since the voltage at the end of the transmission line is the same voltage which appears across the resistor, and since the current in each of the wires of the transmission line is the current in the resistor (don’t count it twice!) we can set up an equation to find the reflected pulse

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11/24/2004EE 42 fall 2004 lecture 3617 Transmission line formulas First, lets make our coordinate system so that x=0 occurs at the resistor.

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11/24/2004EE 42 fall 2004 lecture 3618 Transmission line formulas And we have V R =I R R And now we just need to solve for the reflected pulse f -

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11/24/2004EE 42 fall 2004 lecture 3619 Transmission line formulas So if the resistor has a resistance R=Z 0, then there will be no reflection.

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11/24/2004EE 42 fall 2004 lecture 3620 Transmission line termination Since a transmission line carries pulses of voltage and current, there must be somewhere for the current to go. FET devices are very high impedance, so they don’t absorb that current All that is required to absorb the current at the end of the transmission line is a resistor which has the same resistance as the impedance of the line R termination =Z 0 If a termination is not provided, then a reflection will be created, propagating back up the line

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11/24/2004EE 42 fall 2004 lecture 3621 Changes along a TL If a transmission line changes in impedance at a point along its length, then that will cause a reflection If a transmission line branches, that will cause a reflection Transmission lines must be build with uniform impedance along its length, from one end to another with no branches

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11/24/2004EE 42 fall 2004 lecture 3622 Branch in a transmission line Let’s see what happens if we just join from one transmission line into two

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11/24/2004EE 42 fall 2004 lecture 3623 Replace transmission lines with a resistor to model their input Each of the outgoing transmission lines need a current I=V/Z 0, so they can each be modeled by a resistor. The transmission line from the left now sees a termination into a resistance R=Z 0 /2.

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11/24/2004EE 42 fall 2004 lecture 3624 Reflected amplitude So there will be a reflected pulse, inverted, at 1/3 the amplitude, and each of the branches will see a pulse of 2/3 the voltage of the original.

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11/24/2004EE 42 fall 2004 lecture 3625 Transmission line bus To use transmission lines in a bus, each driver along the bus must be connected very close, and they must not consume significant current from pulses going by. Each end of the bus must have terminators for each transmission line

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