Digital Circuit Boards: Mach 1 GHz

5.4 Noise Budgets

ICs with the same type number can vary in performance. These variations can result from differences in manufacture or from differences in application. These latter differences result from reflections, cross coupling, various voltage drops, or power supply fluctuations. The designer must make sure that these variations do not cause the logic signal to arrive outside of the control envelope. The limits of signal variation are called the noise margin. For the TTL logic shown above, the peak-to-peak noise margin is limited to 1.5 V. An allocation of error might be reflection ± 0.05 V, cross coupling ± 0.1 V, power supply sag 0.3 V, and various logic voltage drops 0.15 V. These numbers would then be used in the design of the circuit board. If the reflection error is too high then a line termination adjustment might be required. The reflection error in this example would include permitted variations in the characteristic impedance of traces.

In analyzing a noise budget, losses subtract from a nominal signal. Cross coupling can add or subtract from the logic level, so it is necessary to assume that it is a worst case number. For example, cross coupling can come from both sides of a trace.

A list of the logic errors is given below. The important errors are discussed in the following sections. Each error must be interpreted as a signal voltage. They are as follows:

1. Driver variations, voltage, and source impedance

2. Characteristic impedance variation and effect on ...