Datasheet

−25
−20
−15
−10
−5
0
100 1000 10000
t
r
20%
80%
1875 MHz With
80 ps Rise Time
375 MHz With
80 ps Rise Time
20 dB/dec
40 dB/dec
40 dB/dec
1/(pi x 100/60 t
r
) = 2.4 GHz
f − Frequency − MHz
Signal Amplitude − dB
20 dB/dec
t
Period
= 1/f
SN65LVCP408
SLLS842A –JUNE 2009 –REVISED JUNE 2010 www.ti.com
Figure 27. Approximate Frequency Spectrum of the Transmit Output Signal With 80 ps Rise Time
The spectrum analysis of the data signal suggests building a backplane with little frequency attenuation up to
2 GHz. This is achievable only with expensive, specialized PCB material. To support material like FR4, a
compensation technique is necessary to compensate for backplane imperfections.
EXPLANATION OF EQUALIZATION
Backplane designs differ widely in size, layer stack-up, and connector placement. In addition, the performance is
impacted by trace architecture (trace width, coupling method) and isolation from adjacent signals. Common to
most commercial backplanes is the use of FR4 as board material and its related high-frequency signal
attenuation. Within a backplane, the shortest to longest trace lengths differ substantially – often ranging from
8 inches up to 40 inches. Increased loss is associated with longer signal traces. In addition, the backplane
connector often contributes a good amount of signal attenuation. As a result, the frequency signal attenuation for
a 300-MHz signal might range from 1 dB to 4 dB while the corresponding attenuation for a 2-GHz signal might
span 6 dB to 24 dB. This frequency dependent loss causes distortion jitter on the transmitted signal. Each
LVCP408 receiver input incorporates an equalizer and compensates for such frequency loss. The
SN65LVCP408 equalizer provides 5 dB of frequency gain between 375 MHz and 1.875 GHz, compensating
roughly for 20 inches of FR4 material with 8-mil trace width. Distortion jitter improvement is substantial, often
providing more than 30-ps jitter reduction. The 5-dB compensation is sufficient for most short backplane traces.
For longer trace lengths, it is recommended to enable transmit preemphasis in addition.
SETTING THE PREEMPHASIS LEVEL
The receive equalization compensates for ISI. This reduces jitter and opens the data eye. In order to find the
best preemphasis setting for each link, calibration of every link is recommended. Assuming each link consists of
a transmitter (with adjustable pre-emphasis such as LVCP408) and the LVCP408 receiver, the following steps
are necessary:
1. Set the transmitter and receiver to 0-dB preemphasis; record the data eye on the LVCP408 receiver output.
2. Increase the transmitter preemphasis until the data eye on the LVCP408 receiver output looks the cleanest.
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