An analysis of a super wide–band FM line discriminator

Abstract

This paper presents a dynamic analysis of an extremely wide-band FM line discriminator consisting of a pair of transmission lines and coupling resistors. The relationship between the harmonic distortion of the output waveform of the discriminator and the permissible frequency deviation of the input signal is analyzed in detail by the method of Fourier transform. It is shown that the coupling resistor constant γ (see Fig. 1) should be set at unity from the viewpoint of the sensitivity of the discriminator and the constant input impedance of the discriminator over the frequency deviation of the input signal. Theoretically, an FM signal having 100 per cent frequency deviation could be detected by this discriminator with 2.67 per cent harmonic distortion while the input impedance of the discriminator remains constant (equal to characteristic impedance of the transmission line Z <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> ) during the frequency deviation. Experimental results are presented to verify the theoretical results obtained. The characteristic curve of the discriminator is linear for 40 per cent frequency deviation from the carrier frequency 85.5 Mc, and its input impedance remains within ±14 per cent of the idea value Z <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> .