Digital Signal Processing Reference
In-Depth Information
The return path determines the signal trace capacitance, inductance, and
resistance.
•
A transmission line can be modeled as a chain of very small RLCG elements.
•
The transmission line circuit elements are usually given as per unit length.
•
The transmission line capacitance and inductance cause the line to possess
impedance and impose a delay.
•
The transmission line impedance is independent of length, but the length
determines the line's propagation delay time.
•
Current flows in the return path simultaneously with the signal current as a
mirror image.
•
The signal rise time determines if a line will behave as a transmission line or
as a lumped circuit.
•
A line will act as a transmission line if its electrical length is greater than
half the signal rise time. In some circumstances even shorter lines will act as
transmission lines.
•
Voltage divider action between the driver impedance and the transmission
line impedance determines the value of the launched signal voltage.
•
Problems
Answers to these problems are available on the Artech House Web site at http://
www.artechhouse.com/static/reslib/thierauf/thierauf1.html.
6.1
An ASIC drives a 50
Ω
transmission line with a driver having an output
impedance of 30
. The driver is connected to a 2.2-V supply. What current
is drawn from the supply when the driver switches from a logic low to a
logic high?
Ω
6.2
In the above setup, what voltage gets launched?
6.3
An ASIC has 30
drivers in its library, and they can be
connected to either a 3.3-V or a 2.2-V supply. Which driver impedance and
supply voltage should be specified if the goal is to launch no less than 1.2V
but no more than 1.7V down a 50
Ω
, 50
Ω
, and 75
Ω
Ω
transmission line?
6.4
An ASIC drives a 32-bit bus from a 3.3-V supply. The driver impedance can
be set to either 50
, and the circuit board traces can be created with
either of these impedances. When the drivers switch, it is necessary to limit
the total current drawn from the I/O power supply to under 750 mA. Using
only the ASIC total I/O current as the design consideration, what driver and
transmission line impedance should be chosen?
Ω
or 75
Ω
6.5
Using the results from Problem 6.3, which combination of driver and power
supply creates the least amount of on-die heat?
6.6
Find the propagation delay and impedance for a half-meter-long transmis-
sion line having
L
=
280 nH/meter and
C
=
118 pF/meter.
