Subject: Priniciples of Communication Engineering

Difficulty : Medium

Marks : 05

• 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.

• 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.