What is Controlled Impedance Trace ? A Simple Guide To PCB Design For Embedded System

What is Controlled Impedance Trace ? A Simple Guide To PCB Design For Embedded System

by cawan (cawan[at]ieee.org or chuiyewleong[at]hotmail.com)

on 14/11/2012

In embedded hardware design, the interconnects among SMDs on the PCB are mission
critical individually. For example, the trace in connecting clock source to the
DDR-RAM is very crucial in order to ensure the DDR-RAM can work in proper. Otherwise,
the jitter issue will be the root cause to stop the hardware from working properly.
Well, this is about the topic of signal integrity. In fact, any single trace on the
PCB has its own characteristic impedance, and the characteristic impedance is due to
inductance and capacitance, which are based on the factors of trace's length and
width, dielectric material of the PCB, thickness of PCB, and etc. In electromagnetic,
characteristic impedance is also known as Zo. Sometimes, it is easily to mix up the
characteristic impedance (Zo) with the intrinsic impedance (Zi), but both of them are
really different. Instead of considering inductance and capacitance in characteristic
impedance, the intrinsic impedance is based on permeability and permittivity, which
is more on the factors of electric and magnetic fields of transmission medium. Hence,
the intrinsic impedance has wider scope than the characteristic impedance, and it
covers any type of transmission medium such as transmission line, waveguide, or even
freespace. Well, the intrinsic impedance is out of the topic of embedded hardware
design, and it is better to refocus to characteristic impedance again.

So, we can make a simple assumption here, any single trace on the PCB has its own
characteristic impedance. But, there is another issue here, which is, when we talk
about impedance, it is closely related to the frequency. Then, what frequency we
are going to refer when measuring the characteristic impedance ? For example, in
doing impedance measurement of a loud speaker, it is always to refer the frequency
is at 1kHz. Fine, it is not the same issue as in characteristic impedance. Instead,
it is about the instantaneous voltage to current ratio of the transmission line, or
the trace, in our discussion here. Of course, the ratio is based on the inductance
and capacitance of the trace. In most of the time, the characteristic impedance (Zo)
is defined as SQRT(L/T), where L and T represent inductance and capacitance,
respectively. On the other hand, for your own info, the intrinsic impedance (Zi) is
defined as SQRT(u/e), where u and e represent permeability and permittivity,
respectively.

Then what is controlled impedance trace now ? We know the impedance is about the
characteristic impedance, how about "controlled" ? Yes, it means the characteristic
impedance of the trace need to be controlled at a specific value. But why we need to
do so ? Yes, it is based on the specification. For example, the characteristic
impedance for Compact PCI standard backplane is 65 ohm. How about DDR-RAM ? By
referring "Hardware and Layout Design Considerations for DDR2 SDRAM Memory Interface"
from freescale (http://www.freescale.com/files/32bit/doc/app_note/AN2910.pdf), it is
stated in checklist item 37 on page 5 about the differential clock should be routed
at 50-60 ohm of impedance for each single-ended with proper spacing. Why we need to
do so ? Can we simply ignore it ? No, it is really not recommended. The reason is the
impedance should be matched in order to prevent the occurrence of reflection. In
general, for those transmission lines without proper termination with matched
impedance, the incoming signal will be reflected in different phase, which is
dependent to the level of mismatching. This will affect the signal integrity of the
transmission line. But, our issue right now is the trace will be terminated at the
DDR-RAM which is about 50 ohm, and what will happen if the trace itself is not in
50 ohm. Well, the answer is simple, this will cause signal degradation and
distortion, and eventually incurring jitter and noise effects. But, how the PCB
manufacturer can make sure a trace has a controlled impedance, let's say 50 ohm,
accordingly ? For example, in the PCB design of our embedded hardware, we specify
all the traces with 5 mil (0.005 inch) width must have controlled impedance of 50 ohm
where the tolerance is +/- 5 ohm. So, when the PCB manufacturer start to fabricate
the board, in regarding to those traces with 5 mil width, they might adjust the width
of the traces based on the PCB conditions such as dielectric thickness, dielectric
material, length of individual traces, and etc, accordingly. Hence, for those traces
with 5 mil width in PCB drawing might have slightly changed individually on the real
board. As a result, each of the traces will have characteristic impedance within the
range of 45 ohm to 55 ohm. So, the resulting board should work well accordingly.

pdf version:

http://www.scribd.com/doc/113340928/What-is-Controlled-Impedance-Trace