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PCB Materials and Traces
The choice of PCB materials and cable type can have a large impact on system
performance. Although any transmission medium is lossy at gigahertz frequencies, this
section provides some guidelines on managing signal attenuation so as to obtain optimal
performance for a given application.
How Fast is Fast?
Signal edges contain frequency components called harmonics. Each harmonic is a multiple
of the signal frequency and has significant amplitude up to a frequency determined by
Equation
Where:
Because dielectric losses in a PCB are frequency dependent, a bandwidth of concern must
be determined to find the total loss the PCB. Frequencies must start at the operation
frequency and extend to the frequency in
with a 10 ps rise time has a bandwidth from 10 GHz to 35 GHz.
Dielectric Losses
The amount of signal energy lost into the dielectric is a function of the materials
characteristics. Some parameters used to describe the material include relative permittivity
ε
to energy loss at line speeds in the gigahertz range.
Relative Permittivity
Relative permittivity is a measure of the effect of the dielectric on the capacitance of a
conductor. The higher the relative permittivity, the slower a signal travels on a trace and
the lower the impedance of a given trace geometry. A lower ε
Although the relative permittivity varies with frequency in all materials, FR4 exhibits wide
variations in ε
a spread of impedance values with increasing frequency. While this spread can be less
significant at 3.125 Gb/s, it can be a concern at 10 Gb/s operation.
Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
12-1:
f = Frequency in GHz
Tr = Signal Rise or Fall time (T
(also known as the dielectric constant), and Loss Tangent. Skin effect is also a contributor
r
with frequency. Because ε
r
www.xilinx.com
f = 0.35 / Tr
) in ns
f
Equation
12-1. For example, a 10 Gb/s signal
affects impedance directly, FR4 traces can have
r
Chapter 12
Equation 12-1
is almost always preferred.
r
231
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