4. RF Section
4.1 Introduction
This section gives a short introduction to the RF section of the BRD4257A Radio Board.
4.2 Schematic of the RF Matching Network
The schematic of the RF section of the BRD4257A Radio Board is shown in the following figure.
GND
GND
GND
GND
VBIAS
GND
C1
C6
C10
L2
C3
C7
P2
U.FL
3
2
1
C4
Ground
RF I/O
RF Crystal
RF Analog Power
PA Power
U1B
EFR32
2G4RF_IOP
20
2G4RF_ION
19
RFVDD
9
HFXI
10
HFXO
11
PAVDD
21
RFVSS
17
PAVSS
18
SUBGRF_OP
13
SUBGRF_ON
14
SUBGRF_IP
15
SUBGRF_IN
16
C8
C2
R2
0R
NM
L4
L6
TP1
R1
0R
L3
BAL1
0900BL15C050
SE
1
BAL2
4
BAL1
3
BIAS
2
GND
5
N/C
6
AT1
INVERTED_F
P1
SMA
3
2
1
4
5
L7
L1
L5
C5
Sub-GHz Matching Network
TRX Matching &
Filter
Filter
Inverted-F
Antenna
UFL
Connector
TRX Matching
Discrete Balun
GND
GND
GND
VDCDC
PAVDD
GND
VBIAS
X1
38.400 MHz
3
1
2
4
C107
10P
L102
BLM18AG601SN1
1
2
L103
BLM18AG601SN1
1
2
C102
100P
C103
10P
C106
220N
Supply Filtering
High Frequency
Crystal
2.4 GHz Matching
Network
2.4 GHz Path
Selection
SMA
Connector
Sub-GHz PA
Power Supply
Figure 4.1. Schematic of the RF Section of the BRD4257A
4.2.1 Description of the Sub-GHz RF Matching
The sub-GHz matching network connects the differential TX outputs and RX inputs of the sub-GHz RF port to the SMA connector while
transforming the impedances to 50 Ohm. Careful design procedure was followed to ensure that the RX input circuitry does not load
down the TX output path while in TX mode and that the TX output circuitry does not degrade receive performance while in RX mode.
The matching includes a differential impedance matching circuitry, a discrete balanced-unbalanced transformer and a filter section. The
targeted output power is 19 dBm at 915 MHz.
4.2.2 Description of the 2.4 GHz RF Matching
The 2.4 GHz matching connects the 2G4RF_IOP pin to the on-board printed Inverted-F Antenna. The 2G4RF_ION pin is connected to
ground. For higher output powers (13 dBm and above) beside the impedance matching circuitry it is recommended to use additional
harmonic filtering as well at the RF output. The targeted output power of the BRD4257A board is 19 dBm. Therefore, the RF output of
the IC is connected to the antenna through a four-element impedance matching and harmonic filter circuitry.
For conducted measurements, the output of the matching network can also be connected to the UFL connector by relocating the series
R1 resistor to the R2 resistor position between the output of the matching and the UFL connector.
4.3 RF Section Power Supply
On the BRD4257A Radio Board the supply pin of the radio (RFVDD) is connected directly of the output of the on-chip DC-DC converter
while the supply for the sub-GHz and 2.4 GHz power amplifiers (SUBGRF_ON, SUBGRF_OP and PAVDD pins) is provided directly by
the Motherboard. This way, by default, the DC-DC converter provides 1.8 V for the RF analog section, the Motherboard provides 3.3 V
for the PAs (for details, see the schematic of the BRD4257A).
4.4 Bill of Materials for the sub-GHz Matching
The Bill of Materials of the sub-GHz matching network of the BRD4257A Radio Board is shown in the following table.
BRD4257A Reference Manual
RF Section
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