Noise and its Effects


The effect of noise is that the output of a synchronizer becomes nondeterministic with the input close to the balance point, and an individual output time no longer depends primarily on the inputs. This only occurs with very small timing differences, and to understand the effect of a noise signal measured in millivolts it is first necessary to find the relationship between voltage displacement and time. By measuring the initial voltage difference between the two latch outputs in a simulated Jamb latch resulting from very small changes in the overlap of clock and data, the trade-off θ, between time overlaps and equivalent voltage difference can be found.

Actual measurement of the noise is best done using a small DC bias to create a given voltage difference between the nodes rather that a time difference between data and clock signals because external jitter and noise can be filtered out from the DC bias, where it cannot be easily removed from high-bandwidth inputs.

The measurements can be made with the circuit of Figure 4.1. A constant current is fed into one node of a flip-flop and out of the other, so that a small variable bias is imposed between the nodes. Then the flip-flop is activated by continuously clocking while the bias is slowly varied.

When V0 is very close to V1 at the time the clock goes high, the flip-flop output is determined mainly by thermal noise, since the RMS noise voltage on these nodes is greater than the offset due to the ...

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