Subject : Principle of Communication Engineering

Topic : Radio Transmitters and Receivers

Difficulty : Medium

• In the Foster-Seeley discriminator, the amplitude of the resulting output voltage varies as per varying magnitude of the input signal.

• This discriminator circuit is modified is to provide amplitude limiting of the incoming signal and is termed as Ratio detector. The Ratio detector circuit is given below in Fig1.

  

  Fig.1 Radio detector circuit

Operation and phase diagram:

• The Ratio detector circuit is almost same as Foster-Seeley discriminator, except for the below modifications:

• Diode D2 is reversed in direction.

• A large capacitor C9 is connected across the output voltage of the two diodes.

• The output voltage of the detector is taken across P and Q.

• The primary and secondary circuits are tuned to carrier frequency and C3 is the coupling capacitor while L3 is the Rfc element.

• The two generated frequency dependent output voltages are applied to the two diodes D1 and D2. The capacitor C9 has large value, typically 10 μF which charges to the peak value of voltage across L2 and due to the large time constant, it holds this voltage.

• The effect of any amplitude variations due to noise and other interference is minimal on the charge of capacitor C9 and the voltage remains constant.

• Additional limiter circuit is not required since the output voltage is not affected by amplitude variations in the incoming signals.

• Conversely, when D1 conducts more than D2,V01 exceeds V02 again resulting in the sum of these voltages as constant. Thus the output voltage V_out is negative.

• Therefore in this circuit the sum of the voltages V01 and V02 always remains constant, but their ratio changes depending on the signal frequency. Hence the circuit is called ratio detector.

Advantages of ratio detector:

• Easy to align.

• Very good linearity.