AN873

Si446X AND FCC PART 15.247 DIGITAL TRANSMISSION

SYSTEM COMPLIANCE AT 902-928 MHZ

1. Introduction

This application note demonstrates the compliance of Si446x RFICs with the regulatory requirements of FCC Part

15.247 for Digital Transmission Systems (DTS) operating in the unlicensed 902-928 MHz band. Within this

frequency band, the FCC allows a transmit power level of up to 1 watt if certain operational conditions are met.

The primary intent of these restrictions is to minimize interference to other operators in the band through

“spreading” of the signal energy. This spreading may be accomplished in both the frequency and time domains

through use of a Frequency Hopping Spread Spectrum (FHSS) system, or only in the frequency domain through

use of a Wideband Digital Transmission System. In a DTS device, the signal energy may be spread across a wide

bandwidth by use of digital modulation with a combination of a high data rate and/or high frequency deviation. As a

result, a high level of transmit power level may be used while maintaining a relatively low power spectral density

(PSD) at any point in the modulation spectrum.

The FCC previously required a DTS device to spread the signal energy through a modulation technique known as

Direct Sequence Spread Spectrum (DSSS). In a DSSS system, the binary data of the transmit data stream is

multiplied with a higher-rate pseudo-random chip sequence. The resulting data stream takes on the rate of the chip

sequence and thus the bandwidth of the modulated signal on the air interface increases accordingly. The reverse

process is performed on the receive side of the link; the received signal is again multiplied with a local copy of the

pseudo-random chip sequence to “collapse” the signal down to its original bandwidth.

The FCC has recently eliminated the requirement that DTS devices must use DSSS techniques and now allows

any form of wideband digital modulation to be used, as long as the resulting 6 dB bandwidth is above a minimum

value (500 kHz) and the PSD is below a maximum level (+8 dBm / 3 kHz). Part 15.247 does not require use of a

specific type of modulation (e.g., OOK, FSK, BPSK, QPSK, QAM, etc.)

This application note is concerned only with DTS devices, and does not discuss or demonstrate operation as a

FHSS device. As will be shown, compliance is readily obtained when the high-power members of the Si446x family

(e.g., Si4463/Si4464) are configured for their maximum transmit power level of +20 dBm. As a result, any of the

chips within the Si446x family (including lower-power members such as Si4460 and Si4461) may also be used to

achieve compliance. The measurements presented within this document were taken with a Si4463-B1 RFIC

mounted on a 4463-PCE20C915-EK RFSW Pico RF Test Card (see section "5. Reference Design Schematic" on

page 28).

The Si4463 chip was configured to transmit at its maximum power level by setting the PA_PWR_LVL property =

0x7F. Various data rates and deviations were chosen in order to comply with the required minimum 6 dB bandwidth

and required maximum power spectral density. The tests were performed at room temperature with a supply

voltage of V

=3.3 V.

1.1. Reference Documents

It is expected that readers will familiarize themselves with the following documents:

Code of Federal Regulations (CFR) Title 47, Part 15 (with emphasis on Sections 15.247, 15.35, 15.205,

15.209, and 15.109)

FCC Office of Engineering and Technology (Laboratory Division), “Guidance for Performing Compliance

Measurements on Digital Transmission Systems (DTS) Operating Under Section 15.247”, June 5, 2014.

Part 15.247 contains the required levels of performance but does not provide details of the test methodology

recommended to verify the performance (i.e., spectrum analyzer settings). The document from the Office of

Engineering and Technology (hereafter, referred to as the “Compliance Measurements Guide”) discusses the

recommended test methodology in detail; this test methodology was followed meticulously in development of this

application note.

AN873

2 Rev. 0.2

1.2. Summary of Measured Results

A summary of measured results is provided in Table 1.

Table 1. Summary of Measured Results

Spec Par Parameter Condition Limit Measured Margin

15.247(b)(3) Maximum

Conducted Power

RBW > DTS BW

Method (Peak)

+30 dBm +20.59 dBm 9.41 dB

AVGSA-1 Method +30 dBm +20.59 dBm 9.41 dB

15.247(a)(2) 6 dB Bandwidth 2GFSK

DR=150 K

Dev=200 K

500 kHz (min) 557.4 kHz 57.4 kHz

2GFSK

DR=4.8 K Dev=190 K

500 kHz (min) 512.6 kHz 12.6 kHz

15.247(e) Power Spectral

Density

AVGPSD-1 Method

2GFSK

DR=150K Dev=200K

Pout = +20.59 dBm

+8 dBm/3 kHz +4.19 dBm/3kHz 3.81 dB

AVGPSD-1 Method

2GFSK

DR=500K Dev=175K

Pout = +20.59 dBm

+8 dBm/3kHz -0.36 dBm/3kHz 8.36 dB

PKPSD Method

2GFSK

DR=150 K

Dev=200 K

Pout = +20.59 dBm

+8 dBm/3 kHz +15.06 dBm/

3 kHz

-7.06 dB

15.247(d) Spurious Emissions Non-Restricted Bands -30 dBc -70.96 dBc 40.96 dB

Restricted Bands -43.2 dBm -50.64 dBm 7.44 dB

Unintentional

Radiation (RX Mode)

-43.2 dBm -87.05 dBm 43.85 dB