This wikipedia explanation tells nothing. Anyone have a detailed explanation of the circuit. I didn't understand the underlined portion.
It uses a tuned RF transformer to convert frequency changes into amplitude changes. A transformer, tuned to the carrier frequency, is connected to two rectifier diodes. The circuit resembles a full-wave bridge rectifier. If the input equals the carrier frequency, the two halves of the tuned transformer circuit produce the same rectified voltage and the output is zero. As the frequency of the carrier changes, the balance between the two halves of the transformer secondary changes, and the result is a voltage proportional to the frequency deviation of the carrier.