Audio susceptibility

Of primary importance in any converter design is not what happens to the output when we perturb the reference (which is what the closed loop transfer function really is), but what happens at the output when there is a line disturbance. This is often referred to as audio susceptibility (probably because early converters switching at around 20 kHz would emit audible noise under this condition). 

Proper ground-system installation is the key to minimizing noise currents on signal-carrying cables. Audio, video, and data lines are often subject to AC power noise currents and RFl. The longer the cable run, the more susceptible it is to disturbances. Unless care is taken in the layout and installation of such cables, unacceptable performance of the overall system can result. 

The two most popular classic detector circuits for FM are the Foster-Seeley discriminator and the ratio detector. Other types of FM detection used today include phase-locked loop circuitry. The basic function of the discriminator is to convert the frequency swings in the FM signal back into amplitude variations for further audio processing. The discriminator is, therefore, susceptible to both amplitude and frequency variations. For this reason, the Foster-Seeley detector is always preceded by a limiter stage. The ratio detector on the other hand, acts like a limiter, and so there is no need for the use of a separate limiter stage. 

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