Computor Simulation of Narrowband Systems

Abstract—Any narrowband signal can be represented by two low-frequency functions, containing only frequencies lower than the bandwidth, which represent the envelopes of the in-phase (cosine) and quadrature ( sine) components of the carrier frequency. To obtain the transient response of a narrowband system efficiently by computer simulation, the system can be represented by a low-frequency equivalent system, avoiding time-consuming direct simulation of carrier frequencies. This paper presents techniques by which a wide variety of narrowband systems, expressible mathematically as block diagrams, can be converted into block diagrams consisting only of low-frequency functions. The resulting low-frequency equivalent systems can be simulated on either an analog or digital computer by well-known techniques. Although the low-frequency equivalent representation presented here includes an extension of the bandpass low-pass analogy, it is not subject to many of the restrictions normally applied to this analogy. It is not necessary that the modulation be simple AM, that the filtering be symmetrical, or that only the envelope velope of the output be of interest.