Reference design

AN3206

4/14

Doc ID 17406 Rev 3

2.1

RF design guidelines

2.1.1

STM32W108 RF ports

The STM32W108's bi-directional RF port (pins RF_P and RF_N) provides access to a low

noise amplifier (LNA) and a power amplifier (PA) through a shared differential interface. The

disabled high-Z state of either the PA or the LNA enables transmitting and receiving through

the shared port without the need for a conventional T/R switch.

An ‘alternate’ PA-only port (pins RF_TX_ALT_P and RF_TX_ALT_N) is also provided. This

function is achieved via an additional PA. Design considerations for this alternate PA are not

covered in this document.

Note:

The on-chip parasitic capacitance is different between the two ports and will slightly alter the

off-chip matching component values.

2.1.2

Optimizing for TX (Boost mode)

The STM32W Reference Design has been optimized for maximum transmitter (TX) output

power in Boost mode using practical components and readily available printed circuit board

(PCB) substrates. The optimum load at the PA on the silicon die is approximately 700

Ω

when the PA/LNA device capacitance is resonated-out. This maximizes the voltage swing at

the drains of the PA devices within the available supply voltage. Too large a load causes

voltage limiting whereas too small a load does not make full use of the available supply

voltage. Figure 2 illustrates a die-level simulation of the load impedance and its effect on PA

output power.

Figure 2.

Simulated Boost output power with PA load resistance

Optimizing the TX output performance in Boost mode does not cause degradation in either

Normal mode or RX sensitivity. Because the receiver is CMOS-based, it amplifies the input

signal voltage, rather than its current. The receiver noise is a fixed value of noise voltage

that may be referred to its input. It might be expected that increasing the transformation