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Signal Integrity Tips and Techniques Using TDR, VNA and Modeling

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Signal Integrity Tips and Techniques Using TDR, VNA and Modeling

Article Reprint

Signal Integrity Tips and Techniques Using TDR, VNA and Modeling

Heidi Barnes, Jeff Most and Mike Resso

Keysight Technologies, Santa Rosa, Calif.

Signal integrity (SI) is all about the losses and types of signal degradation that can happen along the path (channel) between a transmitter and a receiver. In a perfect world, transmitter communication would instantaneously be heard at the receiver and with no change in the signal. Equalization methods exist both in the transmitter and the receiver to help correct for channel losses, but they have their limitations, and the channel must still have some minimal level of performance. SI engineers are faced with the challenge of how to characterize the signal losses that exist in the channel and identify the key elements that are controlling the performance. The use of time and frequency domain analyses for both simulation and measurements is a fast way of becoming an expert on a given channel design. 

Simulation Modeling

Starting with simulation, one can build a distributed model of the channel with measurements, EM simulations and/or algorithmic models that are cascaded together to predict channel performance. One can look at the output eye diagram to see the aggregate performance and do brute force simulations by varying hundreds of variables to find the best performance. The better option is to run quick time and frequency domain analyses to gain insight and reduce the design space that needs to be simulated. Figure 1 shows how time domain reflectometry (TDR) and transmissivity (TDT) can be used to get spatial information on what is happening to the signal as it travels through the channel. The TDR shows where reflections are occurring, which reduces the amount of signal that reaches the transmitter. The TDT shows how the rise time is degraded by material losses in the channel. The near end cross talk (NEXT) on adjacent channels shows which component is the likely source of noise coupling, by being coincident in time with the component’s TDR reflection.

This is a very high level look at the power of time domain analysis. To become an expert at reading the TDR/TDT and frequency dependent losses, some very simple simulations can help. The two basic types of impedance discontinuities encountered in a channel are a series change in impedance and a stub that branches off the signal path. Simulating the series impedance discontinuity from a length that is shorter than the rise time of the signal to a length that is much longer shows two very different responses in the time and frequency domain. As the length of the discontinuity gets shorter than the rise time of the signal, the reflection gets smaller and more of the signal transmits through (see Figure 2a). At longer lengths, the double refl ections off both ends of the series impedance discontinuity result in a forward traveling wave that is delayed in time and added back into the signal going to the receiver (see Figure 2b). This causes a rippling in the amplitude of the signal versus frequency. The ripple valleys are located at frequencies where the forward traveling waves are 180 degrees out of phase and deconstructively add (see Figure 2c).

The stub resonator exhibits some of the same behavior. When the stub is much shorter than the rise time, the reflection is reduced, and more of the signal goes through to the receiver (see Figure 3a). A stub longer than the rise time (see Figure 3b) can lead to signifi cant losses, where 100 percent refl ection from the end of the stub deconstructively adds with the forward traveling wave (see Figure 3c).

Simulation makes it easy to create a stub and series impedance discontinuity with the same excess capacitance and delta impedance change, to see how these two types of structures compare in the time and frequency domain. It is not just the TDR peak height that matters, but also the subtle information from the double refl ection occurring later in time. With these two simple simulations, an SI engineer can look at an eye diagram at the receiver, an S-parameter frequency response or a TDR/TDT time domain response and know whether the problem is a series or stub impedance discontinuity

Finding Causes of EMI

The spatial information that TDR/TDT provides can also be used for understanding and troubleshooting EMI problems coming from the physical channel. While there are many potential sources of EMI in high speed serial designs, the most typical is radiation caused by common currents generated by a differential channel. A common signal as small as 10 mV on an external twisted pair can cause an FCC certifi cation test failure. In theory, if the drivers produce a perfect differential signal and the signal passes through a perfect differential channel, there will be no common signal generated. Unfortunately, in practice that is seldom the case.

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