Eye diagram overshoot..

[groundhog1]

I have a digital data stream displayed as an eye diagram. The data is comming out of my circuit. When displayed on the scope, the overshoot (ringing) is high and a little out of spec. The spec is 10% or less and I am getting about 13%.

I have tried a number of things and cannot control this ringing except by cutting bandwidth down.

The ringing frequency looks like about 2GHz which is on the high end of my band.

When I put the signal into a long length of cable (which acts as an LPF), the ringing is filtered out.

THis is an HDTV application.

As an engineer, I am not too concerned about the ringing because I am thinking of the signal as being pre-emphasized. With the high frequency content being boosted.

The ringing doesn't reduce the size of the eye at all and maybe helps keep it open.

Am I being blissfully ignorant? Is there something I am missing? Will the overshoot hurt some other aspect of the system that I am not aware of?

THanks,
Groundhog1

 

[dspDad]

Are you using a scope probe with grounding pins close to the tip or does it use a ground lead that goes to the back of the probe? Getting the scope ground very close (1/4&quot to the test point will reduce ringing and overshoot that is an artifact of the inductance in the testing probe created by having a large enclosed area. Your scope probes probably also have an adjustment, usually where it connects to the scope, to balance out line capacitance. To measure a 3% error on a digital signal you should check that it is compensated properly.