Use selectivity to improve receiver intercept point - AN749

A receiver's intermodulation (IM) spurious response attenuation is a measure of the receiver's ability to receive a modulated-input RF signal frequency on its assigned channel frequency in the presence of two interfering continuous wave (CW) tones. The receiver's protection against spurious response interference is a measure of the its ability to discriminate between the input signal at the assigned frequency and an undesired signal at any other frequency to which it is responsive. This application note analyzes the sources of RF interference that affect receiver linearity. A dual-conversion receiver is examined for off-channel interference response. A similar version of this article appeared in the December 1997 issue of RF Design magazine. Introduction A receiver's secondand third-order intercept points (IPs) represent the two most important linearity specifications for a radio circuit or system. The intercept points allow us to predict the receiver's intermodulation (IM) performance, which describes the radio's susceptibility to interference from adjacent or nearby users. This article presents a modification of the traditional receiver intercept point cascading equation for both the secondand third-order IM cases. The mathematical derivation for each of the second-order (IP2) and third-order (IP3) intercept point cascading equations incorporates the effect of adding selectivity (S) between receiver stages to improve IIP2 and IIP3. Note: in this article all variables in uppercase letters refer to dB or dBm units and all variables in lowercase letters refer to linear units. Spurious response interference In wireless specifications related to the recommended minimum performance standards for the mobile station, the receiver's IM performance is specified technically under two main topics: the receiver's IM spurious response attenuation, and the receiver's protection against spurious response interference. The IM spurious response attenuation is a measure of the receiver's ability to receive a modulated-input RF signal frequency on its assigned channel frequency in the presence of two interfering continuous-wave (CW) tones. These tones are separated from the assigned input signal frequency so that the nth-order mixing of the two undesired signals can occur in the nonlinear elements of the receiver, thereby producing a third signal in the band of the desired signal. The receiver's protection against spurious response interference is a measure of its ability to discriminate between the input signal at the assigned frequency and an undesired signal at any other frequency to which it is responsive. Interference from third-order IM products As a result of third-order mixing in a receiver's front-end, two off-channel CW tones at frequencies f1 and f2 introduce a third-order IM product at a frequency equal to (2f1 f2), which can appear in the on-channel signal passband (Figure 1a). This in-band third-order intermodulation (IM3) product reduces the carrier-to-interference ratio (C/I) at the receiver's demodulator. Based on a linear 3:1 slope (Figure 1b), the level of this input IM3 product (IIM3, in dBm) can be determined using an equation that includes the receiver's overall input IP3 (IIP3, in dBm) and the input power level of each of the two off-channel CW tones (PI, in dBm):1 IIM3 = 3 × PI 2 × IIP3 (dBm) (Eq. 1)