As data rates increase, it is increasingly important that entire signal paths are modeled and designed at the same time. The entire signal path is generally called a “channel” or “signal channel and this activity is called “channel design.” The figure below illustrates a differential channel, but this also applies to single ended channels as well. The simplest channel models includes 1 signal path, from the transmitter to the receiver, and utilizes a simplified driver and receiver model.  At higher speeds, and as design margins become tighter, more accurate channel models are required.  These models will include adjacent aggressor channels, so that crosstalk is modeled---crosstalk reduces both the voltage eye opening and the time opening of the receiver eye.

Figure 1.  Signal Channel.

 

 

 

 

 

 

 

 

 

 

 

Figure 2 illustrates a typical receiver eye. This plot is made by dividing up the receiver waveform into 400 ps sections (because the data rate is 2.5 Gb/s and 1/2.5G is 400 ps) and overlaying them. The white center of the eye is where the receiver will receive good data, and its voltage sensitivity needs to be at least this good. Note the eye does not open fully in the horizontal direction---the time axis---in this case the total opening is about 320 ps out of 400, meaning the jitter is 80 ps.

Some customers specify a rectangle or hexagon that must fit in the eye opening.  This often depends on how the receiver is specified and measured. The receiver measurement technique greatly influences how the channel eye is specified.  FasTest has lots of experience with these parameters, and can advise the customer the optimum methodology for their application.

 

Figure 2. Receiver Eye Diagram.