For digital circuits, when the rise time of a signal is less than a fixed multiple of the wire delay, the signal is a high-speed signal. The HDMI 4K video capture card chip solution must consider signal integrity (SI) issues when designing high-speed circuits.
1. Signal integrity
The so-called signal integrity refers to the quality of the signal on the signal line. Signal integrity in a broad sense refers to all the problems caused by interconnection lines in high-speed circuits. It mainly studies the impact of signal lines' electrical characteristics on product performance when they interact with current signals and voltage signals. .
Signal integrity problems are usually not caused by a single factor, but by a combination of factors in the design. The main problems that need to be solved for signal integrity are reflection, crosstalk, overshoot and undershoot, ringing, ground bounce, synchronous switching noise, power distribution, timing issues and EMI. The so-called reflection refers to the phenomenon that when a signal travels along a transmission line and encounters an impedance discontinuity, part or all of the energy is reflected back. Reflection is a basic effect of transmission lines. Typically, reflection is caused by impedance discontinuities.
The so-called crosstalk refers to the undesired voltage noise interference caused by electromagnetic coupling to adjacent signal lines when a signal propagates on a transmission line. Crosstalk is caused by capacitive coupling and inductive coupling between signal lines. Capacitive coupling induces coupling current, and inductive coupling induces coupling voltage. Crosstalk noise is transmitted from the interfering object to the interfered object through cross-coupling. When a signal passes through a signal line, a related signal will be induced on the adjacent signal line on the PCB.
When the edge rate of the signal is lower than the set value, crosstalk problems are inevitable and may even cause high-frequency resonance. Overshoot and undershoot refer to the phenomenon that a peak or valley value of the received signal exceeds the set voltage. For the rising edge, overshoot refers to a peak value exceeding the set voltage; for the falling edge, undershoot refers to a valley value exceeding the set voltage. The main cause of overshoot and undershoot is the wiring Too long or the signal changes too quickly.
In the HDMI 4K video capture card chip solution, ringing manifests itself as repeated oscillation of the signal, also known as oscillation. Oscillations are the result of excess energy due to reflections in the circuit that cannot be absorbed, so oscillations can be suppressed by proper termination. For high-speed devices, a large number of data signals flip rapidly at the same time, which will cause large changes in the current through the ground loop. When there is an inductance on the return path, a voltage drop will occur during the return flow.