Hey everyone!
I’m developing an IoT device based on the STM32WL5CCU6 which should be able to operate over a wide range of protocols and frequencies. I would like to tune the device to both the 470-510 MHz and the 862-928MHz bands. While at it, I would also like to be able to use the whole transmit power range (both LP and HP). Originally, I also wanted to have both RX and TX but soon realized this would need way too many RF switches so instead I opted to having RX/TX on the high band and only TX on the low band.
I have created a draft design with this functionality, please check the schematics below:
There are several concerns with this design I would like to discuss:
1. Switching VDDPA supply
According to the datasheet and reference designs, the VDDPA supply needs to be connected to VDDRF1V55 when using RFO_LP and directly to VDD when using RFO_HP. Is this necessary or could I use a single power source in both cases? In the design, I use a TS5A3157 SPST analog switch IC to switch between the different supply nets. Is this a viable solution?
2. Loss of impedance matching in RF switch
I’m using the STM produced IPD baluns for the matching, specifically the BALFLB-WL-08D3 and BALFHB-WL-05D3. The RFI lines are connected directly into the IPD as intended, so this should work normally. The RFO lines pass through a DPDT RF switch, in order to be able to switch the outputs to the corresponding IPD. The question is, will the signals running through the switch instead of directly from the RFO to the IPD (which is matched to the RFO) affect performance? How severe can this impact be?
3. Connection of VR_PA
I still haven’t figured out how I can connect the VR_PA to the corresponding RFO output. Would shorting the 2 outputs together not work? What is the pin state if it’s not currently used? One idea I have would be to throw out the DPDT RF switch completely and instead route the RFO_LP and RFO_HP to the corresponding IPDs directly. Then, I would connect the VR_PA output, or rather the output of the choke coil to an SPDT RF switch which would connect to either of the RFO outputs. This would of course remove the ability to choose which frequency band will be LP and HP. Is there any other way to implement this?
Is there something I’m missing? Can this be done in a simpler fashion? Do you think a setup like this is viable?
Thanks in advance.
