FPGA可编程逻辑器件芯片XC2S15-6FGG456C中文规格书 - 图文

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Chapter12

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 chapter 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 Equation12-1:

f ≈ 0.35 / T

Where:

f = Frequency in GHz

T = The smaller of signal rise (Tr) or fall (Tf) time in ns

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 Equation12-1. For example, a 10Gb/s signal with a 10ps rise time has a bandwidth from 10GHz to 35GHz.

Equation 12-1

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 εr (also known as the dielectric constant) and loss tangent. Skin effect is also a contributor 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 εr is almost always preferred.Although the relative permittivity varies with frequency in all materials, FR4 exhibits wide variations in εr with frequency. Because εr affects impedance directly, FR4 traces can have a spread of impedance values with increasing frequency. While this spread can be less significant at 3.125Gb/s, it can be a concern at 10Gb/s operation.

RocketIO GTX Transceiver User GuideUG198 (v3.0) October 30, 2009

Chapter 12:PCB Materials and Traces

Loss Tangent

Loss tangent is a measure of how much electromagnetic energy is lost to the dielectric as it propagates down a transmission line. A lower loss tangent allows more energy to reach its destination with less signal attenuation.

As frequency increases, the magnitude of energy loss increases as well, causing the highest frequency harmonics in the signal edge to suffer the most attenuation. This appears as a degradation in the rise and fall times.

Skin Effect and Resistive Losses

The skin effect is the tendency for current to flow preferentially near the outer surface of a conductor. This is mainly due to the larger magnetic fields in higher frequency signals pushing current flow in the perpendicular direction towards the perimeter of the conductor.

As current density near the surface increases, the effective cross-sectional area through which current flows decreases. Resistance increases because the effective cross-sectional area of the conductor is now smaller. Because this skin effect is more pronounced as frequency increases, resistive losses increase with signaling rates.

Resistive losses have a similar effect on the signal as loss tangent. Rise and fall times increase due to the decreased amplitude of the higher harmonics, with the highest frequency harmonics being most affected. In the case of 10Gb/s signals, even the fundamental frequency can be attenuated to some degree when using FR4.

For example, an 8mil wide trace at 1MHz has a resistance on the order of 0.06Ω/inch, while the same trace at 10Gb/s has a resistance of just over 1Ω/inch. Given a 10 inch trace and 1.6V voltage swing, a voltage drop of 160mV occurs from resistive losses of the fundamental frequency, not including the losses in the harmonics and dielectric loss.

Choosing the Substrate Material

The goal in material selection is to optimize both performance and cost for a particular application.

FR4, the most common substrate material, provides good performance with careful system design. For long trace lengths or high signaling rates, a more expensive substrate material with lower dielectric loss must be used.

Substrates, such as Nelco, have lower dielectric loss and exhibit significantly less

attenuation in the gigahertz range, thus increasing the maximum bandwidth of PCBs. At 3.125Gb/s, the advantages of Nelco over FR4 are added voltage swing margin and longer trace lengths. At 10Gb/s, Nelco is necessary unless high-speed traces are kept very short.The choice of substrate material depends on the total length of the high-speed trace and also the signaling rate.

What-if analysis can be done in HSPICE simulation to evaluate various substrate

materials. By varying the dielectric constant, loss tangent, and other parameters of the PCB substrate material. The impact on eye quality can be simulated to justify the use of higher cost materials. The impact of other parameters such as copper thickness can also be explored.

RocketIO GTX Transceiver User Guide

UG198 (v3.0) October 30, 2009

Traces

RocketIO GTX Transceiver User GuideUG198 (v3.0) October 30, 2009

Chapter 12:PCB Materials and Traces

The same W/S ratio also must be less than 0.8, otherwise strong coupling between the

. To clarify, with Z0O at 50Ω, an even traces requires narrower, lossier traces for a Z0O of 50Ω

mode impedance (Z0E) of 60Ω or below is desired.

Figure12-1 through Figure12-4 show example cross sections of differential structures.

X-Ref Target - Figure 12-1hd=2h+twswtErhUG198_c12_01_062507Figure 12-1:

X-Ref Target - Figure 12-2Differential Edge-Coupled Centered Stripline

hhhworthogonal linestwtErUG198_c12_02_062507sd=3h+2tFigure 12-2: