The available reference matching networks for the EFR32 wireless Gecko family at the sub-GHz frequency region utilize the so-called direct-tie topology where the TX and RX paths are directly connected to each other without using external RF switch. In order to be able to insert a SAW filter / RF switch or FEM into this matching structure it is recommended to separate the TX and RX paths, since it is not suggested utilizing SAW filter in the TX path, because of:
- the expected power efficiency degradation in TX mode due to the considerable insertion loss of the SAW filters,
- SAW filters are typically designed for low power levels, i.e. would yield TX power limitation,
- SAW filters typically do have weaker attenuation at the higher frequencies, i.e. at the RF harmonics, so discrete LPF would always be recommended.
The recommended schematic structure for EFR32 with SAW filter is shown below.
- TX path match is kept from the reference matches available, recommended component values are shown in AN923 or refer to the existing reference designs.
- The order of the LPF section may be changed based on the power level, harmonic suppression requirements.
- RX path match utilizes a standard 4-element discrete matching balun network, similarly as detailed in AN643. Simulated component values are shown in the table below.
- RF switch is also being used in order to separate the TX and RX paths' matches, while being connected to the same antenna port.
- SAW filter's separate matching network may not be needed (LW1, LW2, CW1, CW2, CW3 and CW4) - refer to the given SAW filter datasheet.
For designs that use RF switch only, then the above schematic can be used without having the SAW filter and its matching components (LW1/2, CW1/2/3/4) mounted. However, a series RF bypass / DC blocking capacitor can be suggested between the 4-element discrete balun SE port and RF switch.
A typical design with FEM is being shown in the figure below. Here, the RF switch can basically be replaced by the FEM while the SAW filter is not shown below, but can be utilized between the FEM RX ports, if applicable. The recommended placement of SAW filter in FEM (with LNA) designs is between the separate RX ports of FEM that ensures the following order of RF blocks in the receiver path: Antenna --> discrete LPF (required due to TX harmonic suppression reasons) --> SAW filter --> FEM LNA --> 4-element discrete matching balun network --> EFR32 LNA. This approach will ensure robust receiver operation in an even noisy environment. Despite the fact that in a non-noisy environment, better link budget could be achieved if the SAW filter were placed between the 4-element matching balun and FEM LNA, Silicon Labs do recommend to use the approach described above, since introducing LNA in the receiver path in general yields however better sensitivity, but worse linearity and blocking performance. If the non-noisy environment can be ensured then the SAW filter is not necessary in the design.
For discrete matching solution on the TX path as well (i.e. eliminating the external ceramic balun between the EFR32 and FEM), please refer to the following KBA link reference.
Could you explain what is the RF switch intended for ? Isn't it possible to add the SAW filter (in the RX path only) in a direct tie matching architecture ?
Having the SAW filter in direct-tie configuration would likely cause RF performance degradation, since in TX / RX mode you cannot ensure high-impedance on the other RX / TX path connected directly, and thus it would provide an unwanted "sniffing load" in the TX / RX path.
Re: the SAW + RF Switch layout routing, I cannot see any extra challenge. If needed the biasing can be done through vias and more-layer routing. You can also refer to the full discrete solution mentioned in the end of the KBA post. We always recommend having LPF section between the RF switch and antenna as well to attenuate the harmonics possibly re-generated by the switch. Of course, all is power level dependent as well as harmonic suppression requirements. So, the 5-element LPF can first be reduced down to a 3-element one to reduce BOM. The LM1 / LM2 / CB components ensure having the proper match between the PA and ceramic balun's 50-ohm diff. ports. The external ceramic balun does not do any matching, since it is a 50-to-50 ohms balun, but does the balanced to unbalanced conversion.