Digital transmission systems designed with margin to compensate for impairments and it is important to determine that a system does in fact possess adequate margin. Margin is difficult to measure because it is based on beginning and end of life bit error ratios (BERs), which are usually too low to measure. The solution is to measure margin indirectly by increasing the BER in a controlled way to measurable levels. The current method~1 uses the dependence of BER on the decision circuit threshold voltage, V_th. This method requires a receiver with a linear amplifier, which is not universally available.
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