The effect of noise upon polar-display instantaneous frequency measurement

This system's rms frequency-measurement error due to noise is calculated, and the effect of a limiter upon IFM performance at high input signal-to-noise ratios (SNR's) is determined. At high SNR's, IFM performance with or without a limiter is similar to that of a limiter and discriminator. However, the IFM threshold can be higher (when
BT
=1) or lower (when
BT
≫1) than that of a limiter and discriminator, and IFM accuracy exceeds that of the latter at SNR's below the limiter-discriminator threshold. The IFM threshold is not due to Rice's `click' phenomenon but, rather, to second-order noise×noise effects like those in the square-law demodulation of an AM signal, which result in a more gradual decline in performance with decreasing SNR.