Design of narrowband FM receiver with delay‐flat IF filter for mobile radio communication

Abstract

AbstractThis paper discusses the design of the IF filter for the mobile communication receiver, realizing narrowband and low‐distortion FM transmission characteristics. In the traditional mobile communication system with 25‐kHz channel spacing, a Chebyshev IF filter with a steep stop‐band roll‐off has been employed to prevent interference from adjacent channels. If the same design is applied to the narrowband mobile communication system with 12.5‐kHz channel spacing, the linear distortion due to the IF filter increases the demodulation distortion. First, this paper derives the expressions for the demodulation distortion and the adjacent channel selectivity in the narrowband FM transmission system, using the TBT filter as the model for the receiver. Then, based on the result, the condition for the narrowband IF receiver filter is derived to realize the low distortion and the required adjacent channel selectivity. Finally, a receiver was constructed experimentally on the breadboard, and it was verified that the theory and measurement are in good agreement.