1. Key parameters impacting the RF output power and TX current consumption on STM32WL33xx
1.1. Topic
RF output power optimization, PA configuration, and supply impact.
1.2. Introduction
This wiki page describes the main parameters that influence RF output power and TX current consumption on the STM32WL33xx family. The objective is to help designers select the most appropriate configuration for a target output power while balancing:
- RF output power and RF range
- TX current consumption
This wiki page is based on measurements carried out on several ST evaluation and reference boards.
1.3. Scope and purpose
This document focuses on the following topics:
- RF output path selection using board-level schematic options
- PA registers configuration
- Internal SMPS voltage selection
- Impact of the external VDD supply
- Output power, TX current consumption, and harmonic emissions measurements.
1.4. System-level overview
The RF transmit chain of STM32WL33xx devices can be operated through three main output paths:
- TX
- TX_HP
- TX + TX_HP
These operating modes are combined with:
- Programmable internal PA settings: PA_DEGEN_ON and PA_LEVEL_7
- Internal SMPS supply control
The final output power and TX current consumption depend on the interaction of these elements.
To simplify configuration, these parameters can be programmed using STM32Cube Wise Radio Explorer.
1.5. Parameters influencing RF output power
1.5.1. RF output path selection
The RF schematic and populated components determine the active RF transmit path.
1.5.1.1. TX path
The TX path is generally used for moderate output power levels. It is typically suitable for output power up to approximately +10 dBm, depending on:
- Frequency band
- Matching network
- PCB stack-up
1.5.1.2. TX_HP path
The TX_HP path enables the high-power transmit chain and is typically suitable for output power up to approximately +14 dBm.
1.5.1.3. TX + TX_HP path
Using TX and TX_HP simultaneously enables the maximum power capability of the platform, up to approximately +20 dBm.
1.5.1.4. Board implementation
On ST reference boards, the selected RF topology is defined by resistor population.
| R1 | R2 | Target output power |
|---|---|---|
| 0 Ω | NC | 10 dBm |
| NC | 0 Ω | 14–16 dBm |
| 0 Ω | 0 Ω | 20 dBm |
| STM32WiseRadioExplorer panel: TX configuration |
|---|
 |
1.5.2. PA programmable settings
Two PA-related parameters are especially relevant:
- PA_DEGEN_ON
- PA_LEVEL_7
1.5.2.1. PA_DEGEN_ON bit
The PA_DEGEN_ON bit is located in the PA_REG register at address 0x490000A8.
It changes the PA operating point and improves the ability of the power amplifier to deliver higher output power in high-power configurations.
When PA_DEGEN_ON is enabled, burst ramp-up and ramp-down control cannot be adjusted. It introduces a pre-distortion mechanism to linearize the power control curve.
| STM32WiseRadioExplorer panel: PA_DEGEN_ON configuration |
|---|
 |
1.5.2.2. PA_LEVEL_7 field
The PA_LEVEL_7 field is located in the PA_LEVEL_7_4 register at address 0x49000424.
It controls the transmit power level used during the TX frame when the PA_LEVEL_MAX field is configured to 0x07 in the PA_CONFIG register.
The maximum useful setting is:
PA_LEVEL_7 = 81
| STM32WiseRadioExplorer panel: PA_LEVEL_7 configuration |
|---|
 |
1.5.3. Internal SMPS voltage
The internal SMPS is a central part of the STM32WL33xx power architecture.
It provides a regulated voltage between:
1.2 V ≤ VSMPS ≤ 2.4 V
with 100 mV increments.
The selected VSMPS directly affects the available headroom for the RF power stage.
- Lower VSMPS values reduce consumption but may limit maximum output power.
- Higher VSMPS values enable higher output power, especially in TX_HP and TX + TX_HP modes.
1.6. ST REF DES & Nucleo platforms
Measurements have been performed on four ST boards covering different RF front-end implementations (PCB layers, IPD or SMD), output-power targets, frequency bands, package options, and XTAL frequencies.
| Board | Package | PCB layers | RF matching | XTAL frequency | Band | Target output power |
|---|---|---|---|---|---|---|
| NUCLEO-WL33CC1 | QFN48 | 4 | IPD | 50 MHz | 868/915 MHz | 14–16 dBm |
| STDES-WL3C2ILL | QFN48 | 2 | IPD | 50 MHz | 433 MHz | 10 dBm |
| STDES-WL3C2SLH | QFN48 | 2 | SMD | 48 MHz | 868 MHz | 10 dBm |
| NUCLEO-WL3RKB2 | QFN32 | 4 | IPD | 50 MHz | 433 MHz | 14–16 dBm |
These boards belong to the STM32WL3x ST REF DES portfolio. Additional variants were also evaluated to confirm the influence of each parameter.
1.6.1. Measurement methodology
Measurements were performed using:
- STM32Cube Wise Radio Explorer
- Rohde & Schwarz FSV3013 Spectrum Analyzer
- RBW = VBW = 30 kHz
- Span = 2 MHz
- STM32WL3xx configuration:
- External HSE and LSE enabled
- 16 MHz system clock
- HSIPLL mode
- HSE GMC = 0x0A
- SMPS frequency = 4 MHz
- Keysight DC power analyzer
- Current range = 100 mA
- Measurement includes complete STM32WL3 current consumption.
1.6.2. NUCLEO-WL33CC1
1.6.2.1. Board description
STM32 Nucleo-64 development board featuring STM32WL33CC, targeting 868/915 MHz operation.
Characteristics:
- 4-layer PCB
- 50 MHz crystal
- IPD RF matching network
- Optimized for 14–16 dBm operation
1.6.2.2. Key observations
- PA_LEVEL_7 = 81 is required to achieve the highest output power.
- Enabling PA_DEGEN_ON increases both output power and TX current consumption.
- Several combinations of VSMPS and PA_DEGEN_ON can provide similar RF output power with different current consumption.
- Harmonics remain within ETSI limits.
| PA_LEVEL_7 | PA_DEGEN_ON | VSMPS (V) | Output power (dBm) | TX Current (mA) | H2 (dBm) | H3 (dBm) | H4 (dBm) |
|---|---|---|---|---|---|---|---|
| 69 | 0 | 1.4 | 10.22 | 13.89 | |||
| 73 | 0 | 1.4 | 11.70 | 15.85 | -56.03 | -49.23 | -69.60 |
| 77 | 0 | 1.4 | 12.77 | 17.55 | |||
| 81 | 0 | 1.4 | 13.65 | 19.05 | -45.86 | -45.27 | -69.40 |
| 81 | 1 | 1.4 | 14.77 | 22.25 | |||
| 81 | 0 | 1.5 | 14.04 | 21.01 | |||
| 81 | 1 | 1.5 | 15.35 | 24.86 | |||
| 81 | 0 | 1.8 | 14.96 | 27.03 | |||
| 81 | 1 | 1.8 | 16.88 | 33.63 | |||
| 81 | 0 | 2.2 | 15.74 | 35.32 | |||
| 81 | 1 | 2.2 | 18.55 | 47.53 | |||
| 81 | 0 | 2.4 | 16.39 | 39.27 | |||
| 81 | 1 | 2.4 | 19.24 | 55.01 | -37.83 | -41.35 | -60.29 |
The row in bold refers to the STM32WL3 configuration used in the datasheet
1.6.2.3. Interpretation
The measurements show:
- Output power increases with both VSMPS and PA_DEGEN_ON.
- TX current rises significantly at the highest power settings.
- Trade-offs exist between RF output power and energy efficiency.
1.6.3. STDES-WL3C2SLH
1.6.3.1. Board description
2-layer reference design using SMD matching.
Characteristics:
- STM32WL3 QFN48
- 868 MHz band
- Target output power: 10 dBm
| PA_LEVEL_7 | PA_DEGEN_ON | VSMPS (V) | Output power (dBm) | TX Current (mA) | H2 (dBm) | H3 (dBm) | H4 (dBm) |
|---|---|---|---|---|---|---|---|
| 53 | 0 | 1.4 | 5.91 | 5.64 | |||
| 57 | 0 | 1.4 | 6.09 | 6.14 | |||
| 61 | 0 | 1.4 | 6.31 | 6.79 | |||
| 65 | 0 | 1.4 | 5.19 | 7.61 | |||
| 69 | 0 | 1.4 | 6.96 | 8.45 | |||
| 73 | 0 | 1.4 | 8.38 | 9.12 | |||
| 77 | 0 | 1.4 | 9.55 | 9.52 | |||
| 81 | 0 | 1.4 | 10.38 | 9.73 | -44.50 | -58.40 | -64.00 |
| 81 | 1 | 1.4 | 11.07 | 10.06 | |||
| 81 | 0 | 1.6 | 10.84 | 11.96 | |||
| 81 | 1 | 1.6 | 12.09 | 12.40 | |||
| 81 | 0 | 1.8 | 11.49 | 14.40 | |||
| 81 | 1 | 1.8 | 13.33 | 15.18 |
The row in bold refers to the STM32WL3 configuration used in the datasheet
1.6.3.2. Interpretation
The measurements demonstrate:
- Lower operating power range than high-power platforms.
- Noticeable output-power increase when PA_DEGEN_ON is enabled.
- TX current consumption increases when VSMPS increases.
1.6.4. STDES-WL3C2ILL
1.6.4.1. Board description
2-layer reference design using IPD matching based on MLPF-WL-03D3.
Characteristics:
- STM32WL3 QFN48
- 433 MHz band
- Target output power: 10 dBm
| PA_LEVEL_7 | PA_DEGEN_ON | VSMPS (V) | Output power (dBm) | TX Current (mA) | H2 (dBm) | H3 (dBm) | H4 (dBm) |
|---|---|---|---|---|---|---|---|
| 53 | 0 | 1.4 | 5.91 | 5.64 | |||
| 57 | 0 | 1.4 | 6.09 | 6.14 | |||
| 61 | 0 | 1.4 | 6.31 | 6.79 | |||
| 65 | 0 | 1.4 | 5.19 | 7.61 | |||
| 69 | 0 | 1.4 | 6.96 | 8.45 | |||
| 73 | 0 | 1.4 | 8.38 | 9.12 | |||
| 77 | 0 | 1.4 | 9.55 | 9.52 | |||
| 81 | 0 | 1.4 | 10.38 | 9.73 | -44.50 | -58.40 | -64.00 |
| 81 | 1 | 1.4 | 11.07 | 10.06 | |||
| 81 | 0 | 1.6 | 10.84 | 11.96 | |||
| 81 | 1 | 1.6 | 12.09 | 12.40 | |||
| 81 | 0 | 1.8 | 11.49 | 14.40 | |||
| 81 | 1 | 1.8 | 13.33 | 15.18 |
The row in bold refers to the STM32WL3 configuration used in the datasheet
1.6.4.2. Interpretation
This board confirms the same trends observed on the other platforms:
- Increasing VSMPS increases achievable RF output power.
- PA_DEGEN_ON provides additional output-power headroom.
- TX current consumption increases accordingly.
1.6.5. NUCLEO-WL3RKB2
1.6.5.1. Board description
STM32 Nucleo-64 development board with STM32WL3RKB MCU.
Characteristics:
- Arduino connectivity
- ST Morpho connectivity
- 4-layer PCB
- 50 MHz crystal
- SMD RF matching
| PA_LEVEL_7 | PA_DEGEN_ON | VSMPS (V) | Output power (dBm) | TX Current (mA) | H2 (dBm) | H3 (dBm) | H4 (dBm) |
|---|---|---|---|---|---|---|---|
| 65 | 0 | 1.4 | 9.00 | 9.97 | |||
| 69 | 0 | 1.4 | 9.97 | 13.54 | |||
| 73 | 0 | 1.4 | |||||
| 81 | 0 | 1.4 | 13.09 | 18.68 | -56.89 | -60.05 | -66.16 |
| 81 | 1 | 1.4 | 14.22 | 21.59 | |||
| 81 | 0 | 1.5 | 13.48 | 20.61 | |||
| 81 | 1 | 1.5 | 14.73 | 24.10 | |||
| 81 | 0 | 1.8 | 14.38 | 26.24 | |||
| 81 | 1 | 1.8 | 16.13 | 32.45 | |||
| 81 | 0 | 2.2 | 15.06 | 33.46 | |||
| 81 | 1 | 2.2 | 17.54 | 44.87 | |||
| 81 | 0 | 2.4 | 15.33 | 36.99 | |||
| 81 | 1 | 2.4 | 18.06 | 51.46 | -52.14 | -54.86 | -65.24 |
The row in bold refers to the STM32WL3 configuration used in datasheet
1.6.5.2. Interpretation
The behavior is consistent with other high-power boards:
- Higher VSMPS increases RF output power.
- PA_DEGEN_ON enables the highest output levels.
- TX current consumption rises significantly at maximum output power.
1.6.6. Influence of VDD supply
The VDD sensitivity study was performed on the NUCLEO-WL33CC1 platform.
1.6.6.1. Test conditions
The following external supply voltages were evaluated:
- 3.6 V
- 3.3 V
- 3.2 V
- 2.8 V
| PA_LEVEL_7 bits | PA DEGEN bit | VSMPS (V) | 3V6 | 3V3 | 3V | 2V8 | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Output power (dBm) | Consumption (mA) | Output power (dBm) | Consumption (mA) | Output power (dBm) | Consumption (mA) | Output power (dBm) | Consumption (mA) | |||
| 67 | 0 | 1.4 | 9.78 | 12.54 | 9.84 | 13.28 | 9.82 | 14.45 | 9.75 | 15.59 |
| 72 | 0 | 1.4 | 11.91 | 14.92 | 11.94 | 15.96 | 11.94 | 17.42 | 11.86 | 18.62 |
| 79 | 0 | 1.4 | 13.92 | 17.99 | 13.96 | 19.20 | 13.96 | 20.99 | 13.65 | 21.97 |
| 81 | 1 | 1.5 | 16.20 | 24.46 | 16.23 | 26.33 | 16.25 | 28.82 | 16.20 | 30.85 |
1.6.6.2. Interpretation
The measurements show:
- RF output power remains essentially unchanged over the tested VDD range.
- TX current consumption increases when VDD decreases
- VDD has limited influence on RF power but impacts overall efficiency.
1.7. Recommendations
1.7.1. For minimum TX current consumption
Use:
- The lowest RF path compatible with the target link budget.
- The lowest VSMPS value meeting the performance requirements.
- PA_DEGEN_ON = 0 unless additional power is required.
- The lowest practical PA_LEVEL_7 value.
1.7.2. For intermediate power targets
Use:
- TX_HP when TX is insufficient.
- Moderate VSMPS settings.
- PA_DEGEN_ON only when necessary.
1.7.3. For maximum output power
Use:
- TX + TX_HP
- PA_LEVEL_7 = 81
- High VSMPS settings
- PA_DEGEN_ON = 1
1.7.4. For best energy efficiency
When several configurations provide the same RF power level:
- Select the configuration with the lowest TX current.
- Ensure acceptable harmonic compliance.
- Maintain sufficient process, temperature and supply margin.
- Consider PA_DEGEN_ON when it improves efficiency.
1.8. Conclusion
RF output power on STM32WL33xx devices is controlled by a combination of:
- RF output path selection
- PA register configuration
- Internal SMPS voltage
- External VDD supply
Characterization results demonstrate that:
- TX is appropriate for moderate output-power levels.
- TX_HP enables higher output power.
- TX + TX_HP provides the maximum power range.
- PA_DEGEN_ON increases output power at the expense of increased current consumption.
- PA_LEVEL_7 controls the transmission power level.
- VSMPS is a key parameter for high-power operation.
In all tested configurations, harmonic levels remained within expected compliance limits.
1.9. References
- [1] STM32WL3xCC Datasheet
- RM0511 STM32WL3xxx reference manual
- [2] Reference designs for STM32WL3x microcontrollers
- [3] ETSI RF Requirements
- [4] Technical note STM32WL3x MCU reference designs: RF performance in TX and RX operation