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IPCControlled Impedance
Speeding Edge, Spring 2003 SLIDE # 1
THE HOW AND WHY OF OBTAINING
ACCURATE IMPEDANCE CALCULATIONS
SESSION W-17
PRESENTED AT SPRING IPC
CONFERENCE
LONG BEACH, CALIFORNIA
PRESENTER LEE RITCHEY
COPYRIGHT, MARCH 2003 SPEEDING EDGE
Speeding Edge, Spring 2003 SLIDE # 2
WHAT IS IMPEDANCE?
? Impedance is the resistance to the flow of energy in a
transmission line.
? At low frequencies, it is primarily the DC resistance of
the bulk copper and is relatively small in PCB traces.
? At high frequencies it is primarily reactive and
substantially higher than the DC or low frequency
value.
? Reactance is both capacitive and inductive.
Bogatin, Eric, “What is Characteristic Impedance?” Printed Circuit
Design, January 2000. Very good article.
Speeding Edge, Spring 2003 SLIDE # 3
WHAT IS CONTROLLED IMPEDANCE?
? Controlling impedance is keeping all sources of
impedance mismatch in a transmission line within
limits that don’t result in malfunctions caused by
reflections at impedance changes.
? Perfect matching is not necessary. “Good enough” is
all that is needed. Good enough can and needs to be
calculated by the creator of the design rule set.
? Perfect impedance matching wastes time and money
and does not contribute to better performance.
Speeding Edge, Spring 2003 SLIDE # 4
WHY IS CONTROLLED IMPEDANCE
NEEDED?
? Energy in the form of an electromagnetic
wave propagates down transmission lines.
? At points along the transmission line where
there are impedance changes, some of that
energy reflects back to the source.
? This reflected energy can destructively
degrade a signal.
Speeding Edge, Spring 2003 SLIDE # 5
A SIGNAL WITH CONTROLLED
IMPEDANCE
Rs = 25 ohms
50 ohm 2.0 nSEC/ft 12 TL
Passive
Receiver
Linear
Driver
Zout = 25 ohms
Vout = 5V
SERIES TERMINATED TRANSMISSION LINE
Comment: Simple Series Terminated Transmission Line
8.000 volts
-1.000 volts
0.0 volts
0.000ns 20.000ns
1 V/div
2 nsec/div
Transmission Line
Output
Transmission Line
Input
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