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| 型號： | 21176 |
| 英文描述： | Designing 100BASE-TX? 86.8KB (PDF) System with the QFEX Family ? |
| 中文描述： | 設(shè)計100BASE - TX的？ 86.8KB（PDF格式）系統(tǒng)？與QFEX家庭 |
| 文件頁數(shù)： | 11/15頁 |
| 文件大?。?/td> | 86K |
| 代理商： | 21176 |
Designing 100BASE-TX Systems with the QFEX Family
11
RX_CLK, and TX_CLK. The effect of the termination
will also slow down any fast ramping CMOS signals.
Additionally, some QFEXr signals are pulled down with
weak pulldown resistors to pull down those signals
from a tristate level. RX_DV, RX_ER, and RX_CLK
should be pulled down with 20-K
resistors.
Termination is also needed at the PECL level. Signal
traces should be effective 50-
can be achieved using Thevenin termination or other
recommended schemes from transceiver and magnet-
ics vendors.
transmission lines and
Most PHYs, including the QFEXr, provide differential
Signal Detect (SD) pairs to the transceiver/MLT-3 de-
vices. However, some MLT-3 devices do not offer differ-
ential SD pairs, and to counter this, designers should
follow transceiver vendor termination guidelines, where
the PHY SD signal is terminated with a different divider
network.
The unused pairs of the RJ-45 jacks should also be ter-
minated to improve EMI performance. This can be
done in several ways, including 50-
networks. Consult magnetics vendor application notes.
Power Supply: Decoupling and Filtering
With a complicated high-speed design, noise can be
picked up and propagated easily through the devices.
resistors and RC
If this noise is in the power supply lines, device and sys-
tem performance is compromised.
DVDD signals should be decoupled as closely as pos-
sible to the IC. Ferrite beads can be used to isolate
power planes (see the
Plane Partitioning and EMI Con-
siderations section
), and an appropriate selection of
capacitors can be placed around the board to filter out
other frequencies. In general, 10-100
electrolytic capacitors filter line frequencies and act as
high speed filters. An additional ferrite bead can be
used to help the effectiveness of the capacitor on the
power input. 0.1
μ
F capacitors will help filter out higher
harmonics (100 MHz and higher).
μ
F tantalum or
Other recommendations are as follows:
I
0.1
decoupling
0.01
μ
tween each port’s power/ground signals
A large (10-30
μ
F) capacitor can be used between
the power and ground planes at the sides of the IC
package with many DVDD inputs
μ
F to 1.0
μ
F capacitors should be used for logic
I
F to 0.1
μ
F capacitors should be used be-
I
Use Table 1 to decouple the QFEXr power and ground
signals.
Table 1.
Decoupling Guidelines for QFEXr Devices
Plane Partitioning and EMI Considerations
100BASE-TX uses MLT-3 line coding (with a funda-
mental frequency of 32.5 MHz) to shift 90 percent of
spectral energy to below 40 MHz. Without it, most sys-
tems, even with good system design, would not be able
to pass standard EMI tests.
In general, digital areas should be partitioned from
analog areas, which are extremely sensitive to noise.
Digital signals alone suffer greatly since the fast switch-
ing times of digital components cause a significant
amount of energy to be dumped into power and ground
layers, generating significant overshoots and under-
shoots on the line. Any analog circuitry on the same
plane will also experience the energy fluctuation, which
can improperly bias the transistors, possibly causing
the circuits to malfunction.
A lowpass filter combination of a ferrite bead (inductor)
and capacitors provides a cleaner, filtered power plane
for analog considerations. Ferrite beads are more ef-
fective than coils. At DC, the ferrite bead acts as a
short, providing a low-resistance path for the power on
the analog plane. The unwanted higher frequency AC
noise sees the ferrites as inductors in a lowpass filter.
Ferrite beads generally allow an increase in impedance
as the frequency increases. Ferrite beads are quite
useful as filters. However, be aware that they do add
extra inductance to the plane and some added cost to
the system. Therefore, they should be used sparingly
and as needed.
Signal
Name
Function
Decoupling
Capacitor Value
Notes
DVDD
Digital power supply
0.1
μ
F
Connect to DVSS through decoupling capacitor.
DVDD_D
PDX power supply
0.01
μ
F
Connect to DVSS_D through decoupling capacitor.
Recommend connecting to DVDD through Ferrite Bead.
DVDD_Ex
PECL power supply
0.01
μ
F
Connect to DVSS_Ex through decoupling capacitor (x=0,1,2,3).
Recommend connecting to DVDD through Ferrite Bead.
DVSS
Ground
Extra grounds connected to DVSS plane.
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