TX Pre-emphasis address high frequency channel media insertion loss by applying a frequency-selective boosting to edges at the transmit end of the signal path as shown in the below example.
The DisplayPort specification for the main link allows four differential peak-peak voltage swing levels, four pre-emphasis (Post Cursor1) levels and four Post Cursor2 levels. Please note Post Cursor 2 is optional and may be supported by upstream devices at any or all bit rates.
Certain combinations of voltage swing levels and pre-emphasis levels result in differential peak-to-peak voltages which are outside the allowable range and thus, are not allowed. Below table listed the supported combinations of the voltage swing and pre-emphasis levels.
Please note while level 0 - 2 support is required, level 3 support is optional.
When designing in a DisplayPort Source with a linear re-driver such as TPD142, SN65DP141, or TDP2004, please note each of them has a certain linearity range as shown in below example.
The linearity range means the signal output is almost a 1 to 1 ratio or linear function of the input signal. But the linearity persists up to a certain input amplitude. When the input signal amplitude exceeds the linearity range, then it gets into the compression region which the output signal is no longer a linear function of the input signal, but compressed at a fixed voltage.
With most DisplayPort Source characterizes their SWING and Pre-emphasis for a receptacle environment, not for re-driver, they tend to use a large SWING and Pre-emphasis setting. While the large setting may be suitable for a receptacle environment, the combination of certain SWING and Pre-emphasis level can cause the overall input amplitude to be outside of the linear re-driver linearity range. This is especially true with the SWING Level 2 and Pre-emphasis Level 1.
When the DisplayPort electrical compliance testing evaluates the delta between two combinations of SWING and Pre-emphasis level of a DisplayPort Design. One SWING and Pre-emphasis combination may be within the linear re-driver linearity range, so the linear re-driver preserves the SWING and Pre-emphasis combination. But the second SWING and Pre-emphasis combination may be within the compression region, so the linearity re-driver no longer preserves the SWING and Pre-emphasis combination as shown in below example. While the DisplayPort Source itself is DisplayPort compliant, but when using with a re-driver, the re-driver output may fail the delta compliance testing since the Source SWING and Pre-emphasis is too large for the re-driver input and the delta can no longer be maintained.
Below are some solutions to resolve this issue and pass the compliance testing.
Thanks
David
