Download How Does Return Current Really Return?

NO MYTHS ALLOWED
How Does Return Current Really Return?
In ‘short,’ through the capacitance between the signal and return conductors.
However, suppose we were to change the voltage across
the capacitor. The only way we can increase the voltage
into the signal conductor of a transmisacross a capacitor is if we add some additional positive
sion line, it returns on the other conduccharge to the top conductor and add some negative charge
tor – the return path conductor. But how
to the bottom conductor.
does it get to the other conductor, espeAdding negative charge to the bottom conductor is the
cially if there is an insulating dielectric
same as pushing out some positive charge from the bottom
between the signal and return path?
conductor. When voltage is increased across a capacitor, it
Suppose our transmission line is a
DR. ERIC
BOGATIN
looks like some positive charge is added to the top conduc50Ω coax cable, 10’ in length. The onetor and some negative charge pushed out from the bottom
way time delay is about 15 nsec. If we
conductor. From the outside, it looks like current has gone
launch a 1 V signal into the line, the curthrough the capacitor.
rent will be I = V/R = 1 V/50Ω = 20 mA. This 20 mA of
Do we actually have positive charge carriers flowing
current goes into the signal line. If at the far end of the
through the insulating dielectric? No, but it sure acts that
cable, we short the center conductor and outer shield, how
way. The only way we get current through a capacitor is to
long do we have to wait for the return current to come out
change the voltage across it.
of the shield conductor at the front end?
When we launch a current into the signal conductor of a
Most engineers might think we have to wait one roundtransmission line, how could the current flow through the
trip time, or 30 nsec. After all, how can else can the current
insulating dielectric to get to the return conductor? It flows
launched into the signal line get back to the shield, acting as
through the capacitance between the
the return path, except by going down
signal and return conductors. After
to the end of the line and connecting
all, the signal is a changing voltage.
to the return path at the far-end’s
Wherever that changing voltage wave
short?
front is, the return current is flowing
For those that think the return curbetween the signal and return path
rent will come back out the shield at
conductors. For this reason, the signal
the same instant we launch the current
is not just a voltage transition propainto the signal line, think about how
FIGURE 1. The return current in a transmisgating down the line; it is also a curthe current could possibly flow
sion line returns through the capacitance
between the signal and return conductors.
rent loop between the signal and
through the insulating dielectric
return currents.
between the signal conductor and
The instantaneous impedance the signal sees is really the
outer shield. How could current flow (20 mA) possibly get
ratio of this transition voltage to the current loop, and for a
through this insulating dielectric? It’s all about how current
uniform transmission line, this ratio is constant as the signal
goes through a capacitor (FIGURE 1).
propagates down the line, which is why we refer to this
A capacitor is composed of any two conductors with
constant instantaneous impedance, this impedance that is
an insulating dielectric between them. Dump some plus
characteristic to the line, as the characteristic impedance of
charge on one conductor and an equal amount of negative
the transmission line. PCD&M
charge on the other, and a voltage is generated between
the conductors.
The capacitance between these two conductors that
Ed.: An online lecture on this topic can be viewed at
make up a capacitor is a measure of the capacity of these
BogEnt.com. Many of the details on this and other topics
conductors to hold charge, at the price of the voltage
can be found in Bogatin’s new book, Signal Integrity –
between them. The more charge they can hold for the same
Simplified (Prentice Hall).
voltage, the higher their capacitance.
Capacitance is a property of geometry. It depends only
upon the physical features of the conductors and the dielectric constant of the insulation between them. It is completely independent of the voltage between the conductors.
Because there is an insulting dielectric between the conDR. ERIC BOGATIN is the CTO of Synergetix (synergetix.com).
ductors at DC, current cannot flow between them. The
He is scheduled to speak at PCB Design Conference East in
impedance of the capacitor at DC is infinite.
October. He can be reached at [email protected].
WE KNOW THAT if a current is launched
20
PRINTED CIRCUIT DESIGN & MANUFACTURE
AUGUST 2004