AM7968-125JC datasheet(69/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

as baseline wander effect and is illustrated in Figure 5-5. In Figure 5-5a, the average DC

fluctuates between 40% and 60% of the maximum level (+10% of midpoint). After the

signal is capacitively coupled (Figure 5-5b), the average DC component is lost due to

high-pass filtering, causing an undesired shift in the signal levels. This shift in the signal

levels, coupled with non-zero rise and fall times of the serial stream cause pulse width

distortion and thus apparent jitter and possible increased error rates.

This DC shifting effect can be minimized if the values of the AC coupling components

are chosen appropriately. The DC level of the data will fluctuate at a data-dependent

frequency, fb, called the baseline wander frequency. The 3 dB corner frequency of the

AC coupling, f3dB=1/(2

πRC), should be chosen below the minimum baseline wander

frequency of the data. This allows most DC variations to pass through the AC coupling

high-pass filtering, minimizing the DC shift in the signal.

To minimize f3dB we must maximize R and C. The resistance R is generally determined

either by the termination required by the transmission line or by biasing requirements on

both sides of the link. Hence, only the coupling capacitor C can be maximized to keep

f3dB as low as possible. The largest value capacitor that can be used is limited by the fact

that it must be an RF capacitor. RF capacitors are generally of the ceramic type (NPO

and X7R dielectrics) and are limited to a maximum value of approximately 1.0

µF.

0.1

µF capacitors have proven to be sufficient in laboratory tests of TAXIchip set

systems.

For a 0.1

µF capacitor, we must verify that the capacitive reactance at the lowest

fundamental frequency possible is less than 1

Ω. The lowest fundamental frequency

possible is the frequency that results when the TAXIchip set is running at it’s lowest

BAUD rate (40 Mbaud) and the command or data pattern with the least number of

transitions is being sent. This pattern turns out to be the HQ command (FDDI terminol-

ogy) which has only 1 transition per command, or 1 transition per 10 bits when the

command is encoded. If a continuous stream of HQ commands are sent at 40 Mbaud,

the resultant fundamental frequency of the signal is 2 MHz. At 2 MHz, the capacitive

reactance of a 0.1

µF capacitor is calculated as follows:

XC =

πfC

π (2*106) (0.1*10-6)

0.8

Ω

Hence, in the worst case a 0.1

µF capacitor will give a reactance of less than 1 Ω, as

desired.

In summary, the largest value RF capacitor available should be used to optimize the

performance of the TAXlchip link.

Figure 5-5

Baseline Wander

12330E-12

Average DC Level Varies

with Data Pattern

a) Data Before AC Coupling

Varying DC is Filtered Out

Causing an Undesired DC

Shift in the Data

b) Data After AC Coupling

AM7968-125JC Datasheet(PDF) 69 Page - Advanced Micro Devices

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