FM Threshold Effect

FM THRESHOLD EFFECT:

In an FM receiver, the effect produced when the desired-signal gain begins to limit the desired signal, and thus noise limiting (suppression). Note: FM threshold effect occurs at (and above) the point at which the FM signal-to-noise improvement is measured. The output signal to noise ratio of FM receiver is valid only if the carrier to noise ratio is measured at the discriminator input is high compared to unity. It is observed that as the input noise is increased so that the carrier to noise ratio decreased, the FM receiver breaks. At first individual clicks are heard in the receiver output and as the carrier to noise ratio decreases still further, the clicks rapidly merge in to a crackling or sputtering sound. Near the break point eqn8.50 begins to fail predicting values of output SNR larger than the actual ones. This phenomenon is known as the threshold effect.The threshold effect is defined as the minimum carrier to noise ratio that gives the output SNR not less than the value predicted by the usual signal to noise formula assuming a small noise power. For a qualitative discussion of the FM threshold effect, Consider, when there is no signal present, so that the carrier is unmodulated. Then the composite signal at the frequency discriminator input is

Where nI(t) and nQ(t) are inphase and quadrature component of the narrow band noise n(t) with respect to carrier wave Accos2 fct. The phasor diagram of fig8.17 below shows the phase relations b/n the various components of x(t) in eqn (1).This effect is shown in fig below, this calculation is based on the following two assumptions:

1. The output signal is taken as the receiver output measured in the absence of noise. The average output signal poweris calculated for a sinusoidal modulation that produces a frequency deviation Iequal to 1/2 of the IF filter bandwidth B, The carrier is thus enabled to swing back and forth across the entire IF band.

2. The average output noise power is calculated when there is no signal present, i.e.,the carrier is unmodulated, with no restriction placed on the value of the carrier to noise ratio.

Assumptions:

Noise Characterisation - APPLICATION & ITS USES:

·        Tape Noise reduction.

·        PINK Noise or 1/f noise.

·        Noise masking and baby sleep.