Silicon Laboratories BRD4257A, EFR32FG14, BRD4257B Reference Manual

EFR32FG14 2400/915 MHz Dual Band
19 dBm Radio Board
BRD4257A Reference Manual

The BRD4257A Flex
wireless applications. The board contains a dual-band Flex Gecko Wireless System-on­Chip and it is optimized for operating at 19 dBm output power. For the 2.4 GHz band
with the on-board printed antenna and UFL connector, radiated and conducted testing is
supported. For the 915 MHz band, the on-board SMA connector enables conducted
testing and attachment of external whip antenna for radiated tests.

The BRD4257A Flex Gecko Radio Board plugs into the Wireless Starter Kit Mainboard,
which is included with the Flex Gecko Starter Kit and gives access to display, buttons
and additional features from Expansion Boards. With the supporting Simplicity Studio
suite of tools, developers can take advantage of graphical wireless application develop­ment and visual energy profiling and optimization. The board also serves as an RF ref­erence design for applications targeting 2.4 GHz and 915 MHz dual band wireless oper­ation with 19 dBm output power.

This document contains a brief introduction and description of the BRD4257A Radio
Board features, focusing on the RF sections and performance.

Gecko Radio Board enables developers to develop proprietary

RADIO BOARD FEATURES

• Wireless SoC:
EFR32FG14P233F256GM48

•

CPU core: ARM

• Flash memory: 256 kB

• RAM: 32 kB

• Operation frequency: 2.4 GHz + 915 MHz

• Transmit power: 19 dBm

• 2.4 GHz: Integrated PCB antenna, UFL
connector (optional)

• 915 MHz: Single SMA connector both for
transmit and receive

• Crystals for LFXO and HFXO: 32.768 kHz
and 38.4 MHz

• 8 Mbit low-power serial flash for over-the­air updates

Cortex®-M4 with FPU

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Table of Contents

1. Introduction ................................

4

2. Radio Board Connector ...........................5

2.1 Introduction...............................5

2.2 Radio Board Connector Pin Associations.....................5

3. Radio Board Block Summary .........................6

3.1 Introduction...............................6

3.2 Radio Board Block Diagram .........................6

3.3 Radio Board Block Description ........................6

3.3.1 Wireless MCU............................6

3.3.2 LF Crystal Oscillator (LFXO) .......................6

3.3.3 HF Crystal Oscillator (HFXO).......................6

3.3.4 Matching Network for Sub-GHz ......................6

3.3.5 Matching Network for 2.4 GHz ......................7

3.3.6 SMA Connector ...........................7

3.3.7 UFL Connector ...........................7

3.3.8 Radio Board Connectors ........................7

3.3.9 Inverted-F Antenna ..........................7

3.3.10 Serial Flash ............................7

3.3.11 Serial EEPROM ...........................7

4. RF Section ................................8

4.1 Introduction...............................8

4.2 Schematic of the RF Matching Network .....................8

4.2.1 Description of the Sub-GHz RF Matching ..................8

4.2.2 Description of the 2.4 GHz RF Matching ...................8

4.3 RF Section Power Supply ..........................8

4.4 Bill of Materials for the sub-GHz Matching ....................8

4.5 Bill of Materials for the 2.4 GHz Matching ....................9

4.6 Inverted-F Antenna ............................9

5. Mechanical Details .............................11

6. EMC Compliance ............................. 12

6.1 Introduction...............................12

6.2 EMC Regulations for 915 MHz ........................12

6.2.1 FCC15.247 Emission Limits for the 902-928 MHz Band ..............12

6.3 ETSI EN 300-328 Emission Limits for the 2400-2483.5 MHz Band ............12

6.4 FCC15.247 Emission Limits for the 2400-2483.5 MHz Band ..............12

6.5 Applied Emission Limits for the 2.4 GHz Band ...................12

7. RF Performance ............................. 13

7.1 Conducted Power Measurements .......................13

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7.1.1 Conducted Measurements in the 915 MHz Band ................13

7.1.2 Conducted Measurements in the 2.4 GHz Band ................14

7.2 Radiated Power Measurements ........................15

7.2.1 Radiated Measurements in the 915 MHz Band .................16

7.2.2 Radiated Measurements in the 2.4 GHz Band .................16

8. EMC Compliance Recommendations ..................... 17

8.1 Recommendations for 915 MHz FCC 15.247 Compliance ...............17

8.2 Recommendations for 2.4 GHz ETSI EN 300-328 Compliance .............17

8.3 Recommendations for 2.4 GHz FCC 15.247 Compliance ...............17

9. Board Revision History .......................... 18

10. Errata................................. 19

11. Document Revision History ........................ 20

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BRD4257A Reference Manual

Introduction

1. Introduction

The EFR32 Flex Gecko Radio Boards provide a development platform (together with the Wireless Starter Kit Mainboard) for the Silicon
Labs EFR32 Flex Gecko Wireless System-on-Chips and serve as reference designs for the matching networks of the RF interfaces.

The BRD4257A Radio Board supports dual-band operation with its integrated sub-GHz ISM band and 2.4 GHz band transceivers. The
sub-GHz section is designed to operate in the US FCC 902-928 MHz band with an external whip antenna, the 2.4 GHz section is de­signed to operate at the 2400-2483.5 MHz band with the on-board printed antenna. The matching networks are optimized for operating
at 19 dBm output power.

To develop and/or evaluate the EFR32 Flex Gecko, the BRD4257A Radio Board can be connected to the Wireless Starter Kit Main­board to get access to display, buttons and additional features from Expansion Boards and also to evaluate the performance of the RF
interfaces.

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GND

F9 / PA3 / VCOM.#RTS_#CS

3v3

NC / P36

P200

Upper Row

NC / P38
NC / P40
NC / P42
NC / P44

DEBUG.TMS_SWDIO / PF1 / F0

DISP_ENABLE / SENSOR_ENABLE / PD15 / F14

UIF_BUTTON0 / PF6 / F12

UIF_LED0 / PF4 / F10

VCOM.#CTS_SCLK / PA2 / F8

UIF_BUTTON0 / PF6 / P4

DEBUG.TDO_SWO / PF2 / F2

DISP_SI / PC6 / F16

VCOM.TX_MOSI / PA0 / F6

PTI.DATA / PB12 / F20

DISP_EXTCOMIN / PD13 / F18

USB_VBUS

5V

Board ID SCL

GND

Board ID SDA

USB_VREG

F7 / PA1 / VCOM.RX_MISO

F5 / PA5 / VCOM_ENABLE

F3 / PF3 / DEBUG.TDI

F1 / PF0 / DEBUG.TCK_SWCLK

P45 / NC

P43 / NC

P41 / NC

P39 / NC

P37 / PD15 / DISP_ENABLE / SENSOR_ENABLE

F11 / PF5 / UIF_LED1
F13 / PF7 / UIF_BUTTON1
F15 / PC8 / DISP_SCLK
F17 / PD14 / DISP_SCS
F19 / PB13 / PTI.SYNC
F21 / PB11 / PTI.CLK

GND

VMCU_IN

VCOM.#CTS_SCLK / PA2 / P0

P201

Lower Row

VCOM.#RTS_#CS / PA3 / P2

UIF_BUTTON0 / PF6 / P4
UIF_BUTTON1 / PF7 / P6

GND

VRF_IN

P35 / PD15 / DISP_ENABLE / SENSOR_ENABLE

P7 / PC9

P5 / PC8 / DISP_SCLK

P3 / PC7

P1 / PC6 / DISP_SI

P33 / PD14 / DISP_SCS

P31 / PD13 / DISP_EXTCOMIN

P29 / NC

P27 / NC

P25 / NC

P23 / NC

P21 / NC

P19 / NC

P17 / NC

P15 / NC

P13 / PC11

P11 / PA1 / VCOM.RX_MISO

P9 / PA0 / VCOM.TX_MOSI

NC / P34

UIF_LED1 / PF5 / P32

NC / P30

DEBUG.TDO_SWO / PF2 / P28

DEBUG.TMS_SWDIO / PF1 / P26

DEBUG.TCK_SWCLK / PF0 / P24

PTI.SYNC / PB13 / P22

PTI.DATA / PB12 / P20

PTI.CLK / PB11 / P18

VCOM_ENABLE / PA5 / P16

PA4 / P14

PC10 / P12

DEBUG.TDI / PF3 / P10

UIF_LED0 / PF4 / P8

BRD4257A Reference Manual

Radio Board Connector

2. Radio Board Connector

2.1 Introduction

The board-to-board connector scheme allows access to all EFR32FG14 GPIO pins as well as the RESETn signal. For more information
on the functions of the available pins, see the EFR32FG14 data sheet.

2.2 Radio Board Connector Pin Associations

The figure below shows the mapping between the connector and the EFR32FG14 pins and their function on the Wireless Starter Kit
Mainboard.

Figure 2.1. BRD4257A Radio Board Connector Pin Mapping

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3. Radio Board Block Summary

EFR32

Inverted-F

PCB

Antenna

2.4 GHz RF

UFL

Connector

LF

Crystal

32.768k

HF

Crystal

38.4M

Radio
Board

Connectors

8 Mbit

MX25R

Serial Flash

I2C

24AA024

Serial EEPROM

Matching

Network &

Output

Selection

GPIO

UART

Debug

Packet Trace

AEM

I2C

SPI

SPI

2.4 GHz RF

2.4 GHz RF

SubGHz RF

Matching

Network &

DC Bias

SubGHz RF

SMA

Connector

EFR32

EFR32

Wireless SoC

3.1 Introduction

This section gives a short introduction to the blocks of the BRD4257A Radio Board.

3.2 Radio Board Block Diagram

The block diagram of the BRD4257A Radio Board is shown in the figure below.

BRD4257A Reference Manual

Radio Board Block Summary

Figure 3.1. BRD4257A Block Diagram

3.3 Radio Board Block Description

3.3.1 Wireless MCU

BRD4257A Flex Gecko Radio Board incorporates an EFR32FG14P233F256GM48 Wireless System-on-Chip featuring 32-bit Cor-

The
tex®-M4 with FPU core, 256 kB of flash memory, 32 kB of RAM, an integrated 2.4 GHz band and an integrated sub-GHz ISM band

transceiver with output power up to 19 dBm. For additional information on the EFR32FG14P233F256GM48, refer to the EFR32FG14
Data Sheet.

3.3.2 LF Crystal Oscillator (LFXO)

The BRD4257A Radio Board has a 32.768 kHz crystal mounted. For details regarding the crystal configuration, refer to Application
Note "AN0016: Oscillator Design Considerations".

3.3.3 HF Crystal Oscillator (HFXO)

The BRD4257A Radio Board has a 38.4 MHz crystal mounted. For details regarding the crystal configuration, refer to Application Note
"AN0016: Oscillator Design Considerations".

3.3.4 Matching Network for Sub-GHz

The BRD4257A Radio Board incorporates a sub-GHz matching network which connects both the sub-GHz TX and RX pins of the
EFR32FG14 to the SMA connector to be able to transmit and receive with one antenna. The component values have been optimized
for the 915 MHz band RF performance and current consumption with 19 dBm output power.

For a detailed description of the matching network see Chapter 4.2.1 Description of the Sub-GHz RF Matching.

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3.3.5 Matching Network for 2.4 GHz

BRD4257A Reference Manual

Radio Board Block Summary

The BRD4257A

Radio Board incorporates a 2.4 GHz matching network which connects the 2.4 GHz TRX pin of the EFR32FG14 to the
one on-board printed inverted-F antenna. The component values have been optimized for the 2.4 GHz band RF performance and cur­rent consumption with 19 dBm output power.

For a detailed description of the matching network, see Chapter 4.2.2 Description of the 2.4 GHz RF Matching.

3.3.6 SMA Connector

To be able to perform conducted measurements or mount external antenna for radiated measurements, range tests etc., Silicon Labs
added an SMA connector to the Radio Board. The connector allows an external 50 Ohm cable or antenna to be connected during de­sign verification or testing.

3.3.7 UFL Connector

To be able to perform conducted measurements, Silicon Labs added a UFL connector to the Radio Board. The connector allows an
external 50 Ohm cable or antenna to be connected during design verification or testing.

Note: By default, the output of the matching network is connected to the printed inverted-F antenna by a series component. It can be
connected to the UFL connector as well through a series 0 Ohm resistor, which is not mounted by default. For conducted measure­ments through the UFL connector, the series component to the antenna should be removed and the 0 Ohm resistor should be mounted
(see Chapter 4.2.2 Description of the 2.4 GHz RF Matching for further details).

3.3.8 Radio Board Connectors

Two dual-row, 0.05” pitch polarized connectors make up the BRD4257A Radio Board interface to the Wireless Starter Kit Mainboard.

For more information on the pin mapping between the EFR32FG14P233F256GM48 and the Radio Board Connector, refer to Chapter

2.2 Radio Board Connector Pin Associations.

3.3.9 Inverted-F Antenna

The BRD4257A Radio Board includes a printed inverted-F antenna (IFA) tuned to have close to 50 Ohm impedance at the 2.4 GHz
band.

For detailed description of the antenna, see Chapter 4.6 Inverted-F Antenna.

3.3.10 Serial Flash

The BRD4257A Radio Board is equipped with an 8 Mbit Macronix MX25R SPI flash that is connected directly to the EFR32FG14 to
support over-the-air (OTA) updates. For additional information on the pin mapping see the schematic of the BRD4257A.

3.3.11 Serial EEPROM

The BRD4257A Radio Board is equipped with a serial I2C EEPROM for board identification and to store additional board related infor­mation.

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4. RF Section

GND

GND

GND

GND

VBIAS

GND

C1

C6

C10

L2

C3

C7

P2

U.FL

3

2

1

C4

Ground

RF I/ORF 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

31

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

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.

BRD4257A Reference Manual

RF Section

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.

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BRD4257A Reference Manual

RF Section

Table 4.1. Bill of Materials for the BRD4257A Sub-GHz RF Matching Network

Component Name Value Manufacturer Part Number

L3 3.3 nH Murata LQW15AN3N3B80D

L4 3.3 nH Murata LQW15AN3N3B80D

L5 18 nH Murata LQW15AN18NG00D

L6 10 nH Murata LQW15AN10NJ00D

L7 10 nH Murata LQW15AN10NJ00D

C3 1.8 pF Murata GRM1555C1H1R8WA01D

C4 1.8 pF Murata GRM1555C1H1R8WA01D

C5 3.9 pF Murata GRM1555C1H3R9WA01D

C6 3.3 pF Murata GRM1555C1H3R3BA01D

C7 5.6 pF Murata GRM1555C1H5R6BA01D

C8 3.3 pF Murata GRM1555C1H3R3BA01D

C10 56 pF Murata GRM1555C1H560GA01D

4.5 Bill of Materials for the 2.4 GHz Matching

The Bill of Materials of the 2.4 GHz matching network of the BRD4257A

Radio Board is shown in the following table.

Table 4.2. Bill of Materials for the BRD4257A 2.4GHz RF Matching Network

Component Name Value Manufacturer Part Number

L1 1.8 nH Murata LQP15MN1N8W02D

L2 3.0 nH Murata LQP15MN3N0W02D

C1 2.0 pF Murata GRM1555C1H2R0WA01D

C2 1.0 pF Murata GRM1555C1H1R0WA01D

4.6 Inverted-F Antenna

The BRD4257A Radio

Board includes an on-board printed inverted-F antenna tuned for the 2.4 GHz band. Due to the design restric­tions of the Radio Board, the input of the antenna and the output of the matching network can't be placed directly next to each other.
Therefore, a 50 Ohm transmission line was necessary to connect them. The resulting impedance and reflection measured at the output
of the matcing network are shown in the following figure. As it can be observed, the impedance is close to 50 Ohm (the reflection is
around or better than -10 dB) for the entire 2.4 GHz band.

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BRD4257A Reference Manual

RF Section

Figure 4.2. Impedance and Reflection of the Inverted-F Antenna of the BRD4257A Board Measured from the Matching Output

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5. Mechanical Details

SMA Connector

LFXTAL

EFR32xx

HFXTAL

UFL

Connector

Printed

Inverted-F

Antenna

Sub-GHz

Matching

and Filter

Frame of

the

Optional

Shielding

Can

2.4 GHz Matching
and Filter

45 mm

30 mm

38.6 mm

4.4 mm

2.4 GHz Path
Selection

DC-DC

&

Supply

Filter

Caps.

Serial
Flash

DC-DC

Inductor

24 mm

27.3 mm

28.6 mm

5 mm

Interface

Connector

Interface

Connector

15 mm

Board

Identification

PAVDD

Supply

Selection

Display

Enable

Selection

WSTK
Sensor
Enable

Selection

The BRD4257A Radio Board is illustrated in the figures below.

BRD4257A Reference Manual

Mechanical Details

Figure 5.1. BRD4257A Top View

Figure 5.2. BRD4257A Bottom View

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BRD4257A Reference Manual

EMC Compliance

6. EMC Compliance

6.1 Introduction

Compliance of the fundamental and harmonic levels of the BRD4257A Radio Board is tested against the following standards:

• 915 MHz:

• FCC 15.247

• 2.4 GHz:

• ETSI EN 300-328

• FCC 15.247

6.2 EMC Regulations for 915 MHz

6.2.1 FCC15.247 Emission Limits for the 902-928 MHz Band

FCC 15.247 allows conducted output power up to 1 Watt (30 dBm) in the 902-928 MHz band. For spurious emmissions the limit is

-20 dBc based on either conducted or radiated measurement, if the emission is not in a restricted band. The restricted bands are speci­fied in FCC 15.205. In these bands the spurious emission levels must meet the levels set out in FCC 15.209. In the range form
960 MHz to the frequency of the 10th harmonic it is defined as 0.5 mV/m at 3 m distance (equals to -41.2 dBm in EIRP).

In case of operating in the 902-928 MHz band from the first 10 harmonics only the 2nd and 7th harmonics are not in restricted bands so
for those the -20 dBc limit should be applied. For the rest of the harmonics the -41.2 dBm limit should be applied.

6.3 ETSI EN 300-328 Emission Limits for the 2400-2483.5 MHz Band

Based on ETSI EN 300-328, the allowed maximum fundamental power for the 2400-2483.5 MHz band is 20 dBm EIRP. For the unwan­ted emissions in the 1 GHz to 12.75 GHz domain the specified limit is -30 dBm EIRP.

6.4 FCC15.247 Emission Limits for the 2400-2483.5 MHz Band

FCC 15.247 allows conducted output power up to 1 Watt (30 dBm) in the 2400-2483.5 MHz band. For spurious emissions the limit is

-20 dBc based on either conducted or radiated measurement, if the emission is not in a restricted band. The restricted bands are speci­fied in FCC 15.205. In these bands the spurious emission levels must meet the levels set out in FCC 15.209. In the range from
960 MHz to the frequency of the 5th harmonic it is defined as 0.5 mV/m at 3 m distance which equals to -41.2 dBm in EIRP.

Additionally, for spurious frequencies above 1 GHz, FCC 15.35 allows duty-cycle relaxation to the regulatory limits. For the EmberZNet
PRO the relaxation is 3.6 dB. Therefore, the -41.2 dBm limit can be modified to -37.6 dBm.

If operating in the 2400-2483.5 MHz band the 2nd, 3rd and 5th harmonics can fall into restricted bands. As a result, for those harmonics
the -37.6 dBm limit should be applied. For the 4th harmonic the -20 dBc limit should be applied.

6.5 Applied Emission Limits for the 2.4 GHz Band

The above ETSI limits are applied both for conducted and radiated measurements.

The FCC restricted band limits are radiated limits only. In addition, Silicon Labs applies the same restrictions to the conducted spec­trum. By doing so, in case of a custom board, assuming that an antenna is used which has 0 dB gain at the fundamental and the har­monic frequencies, the compliance with the radiated limits can be estimated based on the conducted measurement.

The overall applied limits are shown in the table below.

Table 6.1. Applied Limits for Spurious Emissions for the 2.4 GHz Band

Harmonic Frequency Limit

2nd 4800~4967 MHz -37.6 dBm

3rd 7200~7450.5 MHz -37.6 dBm

4th 9600~9934 MHz -30 dBm

5th 12000~12417.5 MHz -37.6 dBm

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BRD4257A Reference Manual

RF Performance

7. RF Performance

7.1 Conducted Power Measurements

During measurements, the BRD4257A Radio Board was attached to a Wireless Starter Kit Mainboard which was supplied by USB. The
voltage supply for the Radio Board was 3.3 V.

7.1.1 Conducted Measurements in the 915 MHz Band

The BRD4257A Radio Board was connected directly to a Spectrum Analyzer through its SMA connector. The supply for the RF section
(RFVDD) was 1.8 V provided by the on-chip DCDC converter, the supply for the sub-GHz power amplifier (SUBGRF_ON,
SUBGRF_OP) was 3.3 V provided by the mainboard (VBIAS through the discrete balun); for details, see the schematic of the
BRD4257A. The transceiver was operated in continuous carrier transmission mode. The output power of the radio was set to 19 dBm.

The typical output spectrum is shown in the following figure.

Figure 7.1. Typical Output Spectrum of the BRD4257A

As shown in the figure, the fundamental is close to 19 dBm so it is compliant with the 30 dBm fundamental limit. The strongest unwan­ted emission
(-20 dBc) with large margin. The other unwanted emissions are under -50 dBm, so the conducted spectrum is compliant with the regu­lation limits.

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is the double-frequency harmonic, but with only around -38.9 dBm level it is compliant with the corresponding limit

7.1.2 Conducted Measurements in the 2.4 GHz Band

BRD4257A Reference Manual

RF Performance

The BRD4257A Radio

Board was connected directly to a Spectrum Analyzer through its UFL connector (the R1 resistor was removed
and a 0 Ohm resistor was soldered to the R2 resistor position). The supply for the RF section (RFVDD) was 1.8 V provided by the on­chip DCDC converter, the supply for the 2.4 GHz power amplifier (PAVDD) was 3.3 V provided by the mainboard; for details, see the
schematic of the BRD4257A. The transceiver was operated in continuous carrier transmission mode. The output power of the radio was
set to 19 dBm.

The typical output spectrum is shown in the following figure.

Figure 7.2. Typical Output Spectrum of the BRD4257A

As shown in the figure, the fundamental is slightly lower than 19 dBm and all of the unwanted emissions are under the -37.6 dBm ap­plied limit.

Note: The

conducted

measurement is performed by connecting the on-board UFL connector to a Spectrum Analyzer through an SMA

Conversion Adapter (P/N: HRMJ-U.FLP(40)). This connection itself introduces approximately 0.3 dB insertion loss.

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7.2 Radiated Power Measurements

BRD4257A Reference Manual

RF Performance

During measurements,

the BRD4257A Radio Board was attached to a Wireless Starter Kit Mainboard which was supplied by USB. The
voltage supply for the Radio Board was 3.3 V. The radiated power was measured in an antenna chamber by rotating the board 360 de­grees with horizontal and vertical reference antenna polarizations in the XY, XZ and YZ cuts. The measurement planes are illustrated in
the figure below.

Figure 7.3. Illustration of Reference Planes with a Radio Board Plugged into the Wireless Starter Kit Mainboard

Note: The radiated measurement results presented in this document were recorded in an unlicensed antenna chamber. Also, the radi-

ated

power levels may change depending on the actual application (PCB size, used antenna, and so on). Therefore, the absolute levels

and margins of the final application are recommended to be verified in a licensed EMC testhouse.

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7.2.1 Radiated Measurements in the 915 MHz Band

BRD4257A Reference Manual

RF Performance

For the

915 MHz radiated power measurements, an external whip antenna (P/N: ANT-915-CW-HWR-SMA) was used as a transmitter
antenna. It was connected to the SMA connector of the BRD4257A Radio Board. The supply for the RF section (RFVDD) was 1.8 V
provided by the on-chip DCDC converter, the supply for the sub-GHz power amplifier (SUBGRF_ON, SUBGRF_OP) was 3.3 V provi­ded by the mainboard (VBIAS through the discrete balun); for details, see the schematic of the BRD4257A. The transceiver was operat­ed in continuous carrier transmission mode. The output power of the radio was set to 19 dBm.

The measured radiated powers are shown in the table below.

Table 7.1. Maximums of the Measured Radiated Powers in EIRP [dBm]

Frequency EIRP [dBm] Orientation Margin [dB] Limit in EIRP [dBm]

Fund 20.8 YZ/H 9.2 30

2nd -30.1 YZ/H 30.9 -20 dBc

3rd -47.6 XY/V 6.4 -41.2

4th -49.5 YZ/H 8.3 -41.2

5th

<-50

*

-/- >10 -41.2

6th -48.5 YZ/H 7.3 -41.2

7th

8th

9th

10th

<-50

<-50

<-50

<-50

*

*

*

*

-/- >30 -20 dBc

-/- >10 -41.2

-/- >10 -41.2

-/- >10 -41.2

* Signal level is below the Spectrum Analyzer noise floor.

As shown in the table, the fundamental is below the regulation limit by more than 9 dB, the harmonic levels are also compliant.

7.2.2 Radiated Measurements in the 2.4 GHz Band

For the

transmitter antenna, the on-board printed inverted-F antenna of the BRD4257A Radio Board was used (the R1 resistor was
mounted). The supply for the RF section (RFVDD) was 1.8 V provided by the on-chip DCDC converter, the supply for the 2.4 GHz
power amplifier (PAVDD) was 3.3 V provided by the mainboard; for details, see the schematic of the BRD4257A. The transceiver was
operated in continuous carrier transmission mode. The output power of the radio was set to 19 dBm based on the conducted measure­ment. During the measurement the sub-GHz antenna (P/N: ANT-915-CW-HWR-SMA) was attached to the SMA connector.

The results are shown in the table below.

Table 7.2. Maximums of the Measured Radiated Powers in EIRP [dBm]

Frequency EIRP [dBm] Orientation Margin [dB] Limit in EIRP [dBm]

Fund 21.4 YZ/H 8.6 30

2nd -46.7 XZ/H 9.1 -37.6

3rd

4th

5th

<-50

<-50

<-50

*

*

*

-/- >10 -37.6

-/- >10 -30

-/- >10 -37.6

* Signal level is below the Spectrum Analyzer noise floor.

As shown in the table, due to the high gain of the inverted-F antenna, the level of the fundamental is higher than 19 dBm. The strongest
harmonic is the double-frequency one and it is compliant with the -37.6 dBm applied limit with ~9 dB margin.

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BRD4257A Reference Manual

EMC Compliance Recommendations

8. EMC Compliance Recommendations

8.1 Recommendations for 915 MHz FCC 15.247 Compliance

As it was shown in the previous chapter, the BRD4257A Flex Gecko Radio Board with 19 dBm output power is compliant with the emis­sion limits of the FCC 15.247 regulation. Although the BRD4257A Radio Board has an option for mounting a shielding can, it is not
required for the compliance.

8.2 Recommendations for 2.4 GHz ETSI EN 300-328 Compliance

As it was shown in the previous chapter, the power of the fundamental of the BRD4257A Flex Gecko Radio Board with 19 dBm output
exceeds the 20 dBm limit of the ETSI EN 300-328 regulation during the radiated power measurement due to the high antenna gain. In
order to be compliant, reduction of the fundamental power is required by approximately 2.4 dB. The harmonic emissions are under the

-30 dBm limit with a large margin. Although the BRD4257A Radio Board has an option for mounting a shielding can, it is not required
for the compliance.

8.3 Recommendations for 2.4 GHz FCC 15.247 Compliance

As it was shown in the previous chapter, the power of the fundamental of the BRD4257A Flex Gecko Radio Board with 19 dBm output
is compliant with the 30 dBm limit of the FCC 15.247 regulation. The harmonic emissions are under the -37.6 dBm applied limit with
margin. Although the BRD4257A Radio Board has an option for mounting a shielding can, it is not required for the compliance.

silabs.com | Building a more connected world. Rev. 1.00 | 17

BRD4257A

Reference Manual

Board Revision History

9. Board Revision History

Table 9.1. BRD4257A Radio Board Revisions

Radio Board Revision Description

A01 Updating EFR revision to Rev. B.

A00 Initial revision.

Note: The silkscreen marking on the board (e.g. PCBxxxx A00) denotes the revision of the PCB. The revision of the actual Radio Board
is laser printed in the 'Board Info' field on the PCB. Also, it can be read from the on-board EEPROM.

silabs.com | Building a more connected world. Rev. 1.00 | 18

10. Errata

There are no known errata at present.

BRD4257A Reference Manual

Errata

silabs.com | Building a more connected world. Rev. 1.00 | 19

11. Document Revision History

Revision 1.00

2017-10-26

Initial document revision.

BRD4257A Reference Manual

Document Revision History

silabs.com | Building a more connected world. Rev. 1.00 | 20

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