The design and simulation of the impedance match between the MAX2338 mixer stage and aSchematic Diagram hp MAX2338 Mixer IF SAW Filter Match at 183.6MHz for CDMA 183.6MHz SAW filter in a CDMA application. The SAW filter S-parameter is used to simulate the interface with the mixer. A worked example is provided with a SAWTEK 855893 SAW filter.

An optimum impedance matching circuit between the MAX2338 mixer differential IF ports and a 183.6MHz CDMA IF SAW filter is presented in this application note. A practical HP-ADS simulation schematic file along with test measurement results are included. Optimization criteria is based on minimum filter insertion loss, peak-to-peak amplitude ripple, and highest mixer IIP3 performance.

Typically, an IF SAW filter wants to see a source differential impedance in the order of several hundred ohms. The MAX2338 CDMA mixer output trans conductance stage’s optimum load is in the order of 2kΩ to 3kΩ as a function of IF frequency. Therefore, a high to low impedance transformation topology is needed for an optimum match.Schematic Diagram hp MAX2338 Mixer IF SAW Filter Match at 183.6MHz for CDMA

Initial S-parameter linear simulation is performed based on the mixer differential equivalent circuit at the intended IF frequency of operation and the four-port S-parameter of the SAW device (without any external matching circuits) supplied by the SAW manufacturer. Actual product platform PCB information such as embedded transmission line lengths, substrate thickness, dielectric material, and size characteristics, can all be incorporated in the simulation for accurate modeling. Component values derived from simulation and subsequently implemented on circuit board on the first attempt demonstrated very good correlation between the MAX2338 mixer gain and SAW filter insertion loss. Final circuit implementation may require slight deviation from simulated component values to account for board that have not been modeled.