Consider Overall Cascaded Performance When Comparing Integrated RF Frequency Mixers to Passive Mixer Solutions - AN2371

Passive mixers have insertion loss that requires compensation in either the RF or IF gain stages to establish the overall desired receiver noise figure performance. For these passive mixers, the user must not only consider the third-order input intercept points (IIP3), but should also consider the output third-order intercept point (OIP3) when comparing the performance to integrated mixers. Passive mixers seldom have second order linearity performance as good as an integrated balanced mixer design, which is important for receiver half-IF spurious considerations. Because a mixer's linearity performance is directly related to the LO drive level, rather large LO injection signals must be generated and then routed on the PC board to the passive mixer's LO port. External RF stages are needed to amplify these signals making the whole design susceptible to both LO radiation and pickup. Finally, passive mixers typically lead to an all-discrete design resulting in higher cost, larger PC board space, and larger performance variation due to discrete part-to-part tolerance.