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Amplitude limiting and thresholding.

Subject: Priniciples of Communication Engineering

Difficulty : Medium

Marks : 05

1 Answer
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  • Ratio detector is a frequency de-modulator circuit in which amplitude limiting and thresholding is provided.
  • In ratio detector output voltage is equal to half of the difference between output voltages from individual diodes.
  • Output voltage is proportional to the difference between individual output voltages.

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  • Capacitor C5 in the circuit helps in amplitude limiting. It is a large value of capacitance.
  • Amplitude changes in FM signal applied at the input makes ratio detector circuit react in the following manner.
  • If FM input V1 tries to increase secondary voltage also increases. Due to this extra diode current increases. But voltage across capacitor C5 will not change instantaneously. It will increase very gradually.
  • Thus the load current has increased but load voltage is almost constant, Hence load impedance is decreased. Due to this secondary of input transformer is heavily clamped.
  • Due to damping, the Q decreases and therefore gain of amplifier driving the ratio detector will also decrease.
  • This will counteract increase in FM input voltage to the ratio detector. Similarly if amplitude of FM tries to decrease, the load impedance will now increase.
  • The clamping of input transformer is reduced, Q increases, increasing the gain of driving amplifier to compensate for reduction in amplitude.
  • In FM ratio detector provides amplitude limiting by means of process called diode variable clamping. As a result no additional limiter is required.
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