AN369: Si4010 Antenna Interface and Matching Network ... - wless.ru

AN3694.1.2. 868MHz SOIC Matching BalunThe schematic of the 868MHz balun with element values for the SOIC packaged Si4010 is shown in Figure 16.This matching is ETSI-compliant. The Si4010 tunes the internal cap. bank to state 17 automatically with thismatching.Figure 16. Si4010 Rev B SOIC Matching Network for 868 MHzThe top layer of the Silicon Labs testcard is shown in Figure 17. It is recommended to copy and use the RF layoutaround the chip as it is. The gerbers of the board can be found on the Silicon Labs website.Unfortunately, around the 3th harmonic frequency the 50 balun core output (at the C1 capacitor, before the filter)has a very low impedance and thus the filter could not attenuate properly. To avoid this a 8.5 mm long50 coplanar transmission line is introduced between the C1 capacitor and the filter. This line transforms theimpedance at the 3th harmonic to higher values. In this way, significant reduction of the 3rd harmonic level isachieved.Rev. 0.1 17