Load Pull for RF PA for STM32WL series Low band version.
I am designing a custom board using the STM32WL55 in UFBGA73, operating at 433 MHz, and I am currently facing higher-than-expected TX current consumption together with lower RF output power.
I would like to know whether ST can provide load-pull data, optimum complex load impedance, or PA matching information at 433 MHz, particularly for approximately 0 dBm and +14 dBm TX power.
I have compared my custom board against an STM32WL Nucleo board using the same firmware and RF settings.
My measurements so far are:
Nucleo board TX current: approximately 65.8 mA
Custom board TX current: approximately 72 mA
Difference: approximately 6–7 mA higher on my custom board
Conducted RF power measurement on my custom board indicates approximately 10 dB lower output power than expected
My next step is to measure the impedance presented to the PA at the beginning of the RF matching network and determine whether the PA is seeing the correct load at 433 MHz.
Ideally, I would like to compare my measured impedance with the optimum PA load impedance from ST load-pull characterization.
I am particularly interested in the optimum load for:
433 MHz
0 dBm TX power, where my priority is minimum current consumption / maximum efficiency
+14 dBm TX power, where my priority is maximum output power with reasonable efficiency
Any intermediate power level if the optimum impedance changes significantly with output power
One reason I am unsure whether this is purely a matching issue is the TX current consumption I measure on the Nucleo board itself. Based on the STM32WL55 datasheet, I understand that TX current can be around 40 mA under 1.8 supply voltage / 14dbm power conditions, whereas I am measuring approximately 65.8 mA on the Nucleo board and approximately 72 mA on my custom board.
Could you please clarify:
Is load-pull data or optimum PA load impedance versus frequency/power available for the STM32WL55 at 433 MHz?
What complex impedance should the PA ideally see at 433 MHz for approximately 0 dBm and +14 dBm?
Are the reference matching values used on the Nucleo board optimized primarily for maximum output power, efficiency, or 50 Ω output matching?
What exact test conditions should be used when comparing TX current against the datasheet values — supply voltage, PA path, modulation, frequency, regulator mode, etc.?
Could a significant mismatch at the PA output explain both a roughly 10 dB reduction in conducted RF power and the additional TX current consumption I am seeing?
edit: I have checked application note AN5457 and all of the PA or TX load pull and impedance measurements are for High band version of the Chip.
