International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064 Index Copernicus Value (2016): 79.57 | Impact Factor (2017): 7.296 Volume 7 Issue 7, July 2018 www.ijsr.net Licensed Under Creative Commons Attribution CC BY Minimization of the Noise Figure of RC Polyphase Filters J. Ladvánszky Ericsson Telecom Hungary Ltd., 1117 Budapest, Magyar Tudósok Körútja 17., Hungary Abstract: The sideband suppression ratio of RC polyphase filters is independent of source and load impedances. This property is valid for any number of stages and for any detuning between the stages. Due to this property, noise figure minimization can be done independently. Two formulas for noise figure of passive two-ports are obtained, and noise figure of polyphase filters are minimized. The noise figure of an RC polyphase filter is strongly dependent on the source resistance and capacitance. For a two-stage filter providing better than -25 dB sideband suppression over two octaves, a minimum noise figure of 10.18 dB was found. 1. Introduction RC polyphase filters are widely used in communication systems [1]. Their typical applications are modulation and demodulation of single sideband signals. Due to their simplicity, they are popular in IF stages of application- specific integrated circuits (ASICs). Noise in polyphase filters is qualitatively analyzed in [2], where the relation between resistor values in different stages is discussed for reducing noise. A pioneering paper on polyphase filters is [3]. Derivation of the structure and its sideband cancellation property are explained there in detail. In a receiver system, polyphase filter usually follows the mixer stages. Therefore noise contribution of the polyphase filter to the system noise may be significant. For this reason, noise minimization of the polyphase filter is important. However, none of the mentioned publications consider terminations of polyphase filters for minimum noise figure. In this paper we show that the sideband suppression ratio of an RC polyphase filter is independent of the impedance of the source and the load under very general conditions. As an application of this property, we show that noise figure optimization can be done independently, without degradation of the sideband suppression ratio. Two new formulas for noise figure of passive two-ports are obtained and applied for our case. It is shown that noise figure of an RC polyphase filter is strongly dependent on the source resistance and capacitance, and independent of the load impedance. As an example, noise figure of a two-stage RC polyphase filter, designed for sideband suppression lower than -25 dB over two octaves, is minimized to 10.18 dB. 2. Transfer functions of an RC polyphase filter The one-stage RC polyphase filter is an 8-node structure shown in Fig. 2.1 [3]. Figure 2.1: A one-stage RC polyphase filter In Fig. 2.1, all resistors and capacitors are identical. In a multistage filter, nodes 5-6-7-8 are connected to nodes 1-2- 3-4 of the next stage, respectively. Resistor and capacitor values of different stages are not necessarily identical. In this report, we consider the case when all capacitors are identical and only resistors of different stages may differ from each other. From the many possibilities, input and output ports can be configured from the nodes in two meaningful ways (see please Fig. 2.2ab). Paper ID: ART201922 DOI: 10.21275/ART201922 1440