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This article explores sufficient headroom in linear systems used for white noise measurements. 
White noise is caused by a random process that generates a voltage that varies randomly over time. The noise voltage is a normally distributed random variable.
Fig 1 shows the probability distribution of instantaneous noise voltage of RMS value one. The likelihood of high instantaneous voltage is low, but not zero.
It is the square of instantaneous noise voltage that quantifies the contribution to the average power measurement, so larger instantaneous voltages make a larger contribution to the average power.
To assess the impact of a voltage limiter on noise power measurement, both effects must be considered.
Fig 1 shows the reduction in power caused by a symmetric limiter on white noise. For the purpose of a measurement system, the reduction in power is an error, so to ensure that the contribution of limiting to error is less than say, 0.001dB, headroom of at least 11.7dB is required. A working value of 12dB is convenient, and it represents a limiter threshold of four times the RMS voltage. For lower accuracy measurements, 10dB headroom affording 0.011dB error may be quite adequate.
Noise Figure Meter (NFM) software
Version  Date  Description 
1.01  24/08/2007  Initial. 
1.02  
1.03 
Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.
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