Best Practices and Usage Tips
21
SNLU238A – August 2018 – Revised September 2019
Copyright © 2018–2019, Texas Instruments Incorporated
DS250DF230EVM User’s Guide
3
Best Practices and Usage Tips
The following is a general procedure that should be followed when using the DS250DF230EVM in a
system.
1. Set up your data source (either BERT TX or ASIC TX) to generate a PRBS pattern of the desired data
rate.
1. Not all BERT TX sources have FIR capabilities. The DS250DF230 receiver usually does not need
much de-emphasis applied by the link partner transmitter (i.e. the BERT TX or ASIC TX). Typically
3dB of de-emphasis or 0-15% post-cursor will be adequate. If the BERT/ASIC TX has pre-cursor
capabilities, then 0-15% pre-cursor should be adequate. Most links should be operable without any
TX de-emphasis.
2. Connect the EVM in to the system. Typically this will consist of the following topology:
BERT TX or ASIC TX
→
SMA cables
→
channel_1
→
Huber+Suhner cables
→
DS250DF230 EVM
RXn
→
DS250DF230 EVM TXn
→
Huber+Suhner cables
→
channel_2
→
SMA cables
→
BERT RX or
ASIC RX
1. After making your data rate selection, push the “RESET CDR” button on the CDR tab. You only
need to press this once, provided you do not change data rate or adapt mode.
2. Check the Signal Detect and CDR lock status indicators to see if the link is established.
3. If the CDR is in locked, the CDR lock indicator on the page will turn green and display "CDR
locked."
3. Check the Horizontal Eye Opening (HEO) and Vertical Eye Opening (VEO), displayed on the Device
Status tab. The user may also go the Eye Monitor Page to plot a full eye diagram.
4. Check the Retimer Receiver’s bits received and errors by clicking "Turn ON" in the PRBS pattern
checker section of the PRBS Gen/Checker" tab. If necessary, tune the link partner transmitter’s FIR
settings to achieve the target BER.
5. Tune the Retimer TX FIR settings on the TX FIR/VOD tab. It is best to demonstrate that the return path
(Retimer TX to BERT/ASIC RX) is working first before trying to optimize the Retimer RX parameters.
One way to do this would be to test over a simple channel_1 first to prove that the Retimer can drive
data error-free into the BERT/ASIC (optimizing the Retimer TX FIR as needed) then switch to the more
difficult channel_1 while keeping channel_2 unchanged.
Things to watch for:
1. At 25-28Gbps data rates, small imperfections in the channel can be problematic. Ensure that cables
are properly torqued (not over-torqued), paddle cards are properly mated with backplane connectors,
and the BERT RX is properly aligned to the incoming data stream.
2. When adding up the total channel loss, do not forget to include the loss of the test fixture and cables.
For example, the DS250DF230 EVM board plus Huber+Suhner cables have ~4dB of insertion loss
from the device output to the Huber+Suhner cable end; and another ~4dB from the Huber+Suhner
cable end to the device input.