High Speed Digital Design Considerations

The four main consideration of today’s high-speed design:

                                                                                                                            Ref(1)
  • We need to consider signal integrity, which includes parameters such as Overshoot and Ringing.
  • Need to look at crosstalk which is about EMI coupling between traces on PCB board
  • Timing is about calculating the flight time
  • EMI and radiation 

These parameters are related to each other and affect each other.
For e.g. by enhancing signal integrity and quality of signals, we also enhance the crosstalk and EMI in our design. Similarly, If excessive crosstalk, it will affect timing and EMI in our design.

Signal integrity analysis

The signal transmission from one IC on the board to other IC on the board. Everything that is in the path of signal transmission will affect signal integrity. For e.g. driver buffer, components Pins, receiver buffer, traces can affect the signal quality. Another element such as passive devices, coupled devices, traces, connectors, cables, other cards affect the signal integrity. In summary, we need to analyze all the factors in signal integrity analysis.

                                                                                                                            Ref(1)
Signal integrity
The potential problem that you can face during design such as Overshoot and Ringing. These effects signal quality and needs to be analyzed and addressed properly. 

                                                                                                                            Ref(1)
We have two types of overshoot; 
Rail overshoot where we measure the difference between high voltage level such as VCC and peak of the signal at receiver buffer.
SI overshoot where we measure the difference btw peak of a signal at receiver buffer and the steady-state level of the same signal.

Lossless transmission lines

A lossless line has certain properties
  • It includes at least two conductors, one provides for the signal path and others provide a current return path.
  • It has zero resistance
  • It extends indefinitely
  • Propagation delay associated with transmission lines
  • It has characteristics impedance

                                                                                                                            Ref(1)
Propagation of signal in transmission lines

Signals propagate down the transmission lines in the form of Electric and Magnetic field or TEM mode. The Electric and Magnetic fields are orthogonal to each other. The direction of propagation is in the Z-direction.

                                                                                                                            Ref(1)
In this picture, you look at  the cross-section of the board where transmission line resides on the top layer of the board, red lines are Magnetic field and blue is the Electric field.

Lossless transmission Line model

We use a distributed model to represent lossless transmission lines using inductance and capacitance distributed across the length of transmission lines.  Since this is a lossless model it doesn’t include the resistor.

                                                                                                                            Ref(1)
we can calculate the impedance and propagation velocity with the value of L & C.
Now one question should come to your mind that when does traces start to exhibit transmission line behavior?
Or when does it need to be model as a transmission line model?

The answer is when the trace length exceeds a critical length that we can calculate using certain parameters. 
                                                                                                                    Ref(1)
These parameters are drivers signal rise time & delay per unit length.

Transmission Line Effects

Signal exhibits undesired effects, Overshoot, Ringing wherein the signal transit from high state threshold to low state threshold and vice versa. This multiple crossing is an undesired effect.
                                                                                                                            Ref(1)
All these issues need to be identified, analyzed, and addressed on PCB design to improve signal quality.

Reflection
 Reflections are caused by impedance mismatches. Overshoot, Ringing and multi-crossing are caused by reflections. The Thevenin voltage is the driver buffer at the source and Load RL  is the receiver buffer.

                                                                                                                            Ref(1)
In the picture, if RL does not match Zo, there will be reflections from the load back to the source.
We can define reflection coefficients at load and source as ρL  and  ρS. They Characterize the amount of reflections and degree of impedance mismatch.

Reflections illustrated - Mismatched

                                                                                                                            Ref(1)
-ve means a signal is reflected back from the source to the load
In steady-state after reflection is settled, no transmission line and circuit behave like a voltage divider.

Lattice diagram
We use the first vertical as timeline output of driver buffer and second vertical as timeline input of the receiver buffer.

                                                                                                                            Ref(1)
Simple voltage driver (Vi) at the source. This voltage travels down the trace and after a delay of  TD which is transmission line propagation delay, it reached the receiver buffer, gets multiplied by the load reflection coefficient, and results in the amount which reflects back from the original 0.67V, it reaches the source after 2TD. Let's see what happens at load, the voltage at the load is a sum of  0.67 V and 0.54V.

                                                                                                                            Ref(1)
You are looking at the same data graphically. At steady-state, both Driver and Receiver voltage becomes equal.

What happens when if we match load resistance to a characteristic impedance of the transmission line?

                                                                                                                    Ref(1)
At the steady-state, a smaller value than a mismatched case. This obviously reduces the output voltage and noise margins.

Crosstalk

When two nets close together, a change in current flow in one net can introduce current in an adjacent.
                                                                                                                    Ref(1)
  • Aggressor Net: A neighboring net with changing the current
  • Victim Net: A net at a steady-state
  • Aggressor net contributes the mist crosstalks in the victim net when a driver in the aggressor net is switching.

Crootall due to Electric field

When crosstalk happens we say two traces are couple and coupling happens due to the Electric and Magnetic field. The Electric field can be represented by mutual capacitance which injects current to victim proportional to the current change in Aggressor net. There is the mutual capacitance between two nets due to their proximity. This capacitance represents the Electric field that gets created because of changing current in the aggressor net.
                                                                                                                            Ref(1)

The effects of Electric field and mutual capacitance inject energy in the victim net and causes current flow in both directions. These unwanted effects we called crosstalk.
The crosstalk that happens closer to the aggressor driver called near end and the opposite is far end.

Crosstalk due to Magnetic field

The coupling between two conductors due to a magnetic field is represented by mutual inductance.
Similar to the Electric field, we have the near and far end.

                                                                                                                            Ref(1)

Forms of crosstalk

We have an Aggressor net and a victim net. Because of current flowing in aggressor, Magnetic and Electric field get generated and they coupled energy to the victim and because of that current starts flowing both towards the far end of a victim which is called far and near the end which causes near the end.

                                                                                                                            Ref(1)
Uncoupled and Coupled transmission Lines

Uncoupled transmission lines
  •  Wher voltage nd current on line1 does affect the current and voltage on line 2

Coupled transmission lines
  • Energy is coupled from one transmission line into another  through Electric and Magnetic which are represented by mutual Capacitance and inductance.
                                                                                                                            Ref(1)


We can analyze the reflections problem and crosstalk in the HyperLynx tool.

References: 
1. Mentor Graphics, Hyperlynx tool library online courses accessed on 26-04-2020. All the figures are taken from Mentor Graphics High-speed design online courses.





Comments

  1. Unknown26 April 2020 at 09:42

    Verry well summarize.... complex problem in simple words...well done Praveen

    ReplyDelete

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