While system-on-chips (SoCs) mostly do the heavy lifting in wireless networks, the last inches of the broadband signal path to the antennas are still crucial. Spanning those gaps falls to the RF front-end (RFFE), also known as RFE or FEM for front-end module.
What’s included in an RFFE varies. Basic RFFEs typically have an RF filter, a low-noise amplifier (LNA), and power amplifiers (PAs). Some add functions like a mixer for a local oscillator or analog switching elements. RFFEs up to about 7 GHz come in CMOS, while faster parts are available in gallium nitride (GaN) and gallium arsenide (GaAs) designs.
RFFEs can help extend range, improve receiver sensitivity, manage antennas, and address different requirements for regional markets. This blog examines RFFEs for four major wireless applications.
Low-power wireless networks
For sub-GHz, GNSS, Bluetooth, Thread, Zigbee, and cellular IoT designs, minimizing power consumption and size is important. More and more, signals on these networks are received and converted directly to digital in a zero IF architecture. RF is sometimes integrated in an SoC; however, using an external RFFE can usually increase link budget and lower noise figure.
An example of a low-power front-end module is the Skyworks SKY68000-31 for LTE-M and NB-IoT devices (Figure 1). In a 4 x 5 mm package, it supports LTE dual-band operation in Band 13 and Band 4 with receive SAW filters, transmit power amplifiers and harmonic filters, and an antenna switch. Out-of-band emissions are optimized for 3GPP compliance.
Figure 1 The dual-band SKY68000-31 module integrates the entire RF front-end necessary for an LTE dual-band radio operating in Band 13 and Band 4. Source: Skyworks
One significant trend seen here is the use of MIPI RFFE. It standardizes the control interface for an RFFE while keeping pin count low. That, in turn, allows up to 15 slaves on a single control bus.
Antenna diversity and dense constellations are big features in Wi-Fi 6E designs, spurring a new round of RFFE innovation. A key metric for Wi-Fi 6E parts is transmitter long-packet error vector magnitude (EVM), a measure of actual constellation points versus their ideal theoretical positions.
In a 2.5 x 2.5 mm 16-lead package, pSemi’s PE65211 device packs an FEM for 5-7 GHz 802.11ax designs (Figure 2). The LNA is bypassable, and separate PAs are optimized for 5 GHz and 6 GHz. The EVM on the 6-GHz side is -41dB at 13.0 dBm power out.
Figure 2 PE565211, an all-silicon die, facilitates a compact form factor to minimize PCB layout area in high-density MIMO applications. Source: pSemi
Not to be outdone, NXP has released the WLAN7205C front-end IC in a 2 x 2 mm package with a 6 GHz EVM of -44 dB at 14.5 dBM power out. The chipmaker is targeting the front-end IC at Wi-Fi 6E connectivity in mobile handsets.
Speaking of mobile, competition for RFFEs in 5G handsets is heating up. While players like Broadcom, Qorvo, and Skyworks offer 5G RFFE components, Qualcomm is rapidly gaining ground with its “modem-to-antenna” approach after buying all of its joint venture with TDK in 2019.
A July 2020 teardown study from ABI Research looked at nine flagship 5G phones and found Qualcomm’s RFFE components in everyone. The study identifies power amplifiers, envelope trackers, and diversity modules; following those part numbers finds few details on the Qualcomm website.
Next, Skyworks has a 5G diversity module with its SKY53728-11 device in a 2.8 x 2.6 mm 21-lead package (Figure 3). We see LNAs in two 5G NR bands—3.2 GHz to 4.2 GHz and 4.4 GHz to 5.0 GHz—along with filtering for out-of-band signals, plus the MIPI RFEE interface.
Figure 3 SKY53728-11, a highly integrated LNA filter module, integrates all the required filtering to attenuate unwanted out-of-band blockers in adjacent and far-out frequency bands. Source: Skyworks
It’s a big market. Broadcom, Qorvo, and Skyworks along with Intel, MediaTek, Murata, and Samsung are teaming up for 5G handset RFEE interoperability with the launch of OpenRF consortium. In a complex 5G landscape with regional requirements and different price points, there’s lots of room for innovative RF solutions.
Infrastructure has even more RF complexity, and 5G mmWave is pushing RFFE technology toward GaN and GaAs. Growth in 5G private networks is setting up smaller RF players with unique solutions on a more even playing field with Qualcomm and Skyworks.
One of these firms is arQana Technologies with its ARQSF3753-RX-A dual-channel receiver FEM (Figure 4). It’s one of a family of pin compatible 6 x 6 mm parts handling different bands in 5G NR; this part covers n77 and n79. Each channel has a two-stage GaAs LNA and an antenna switch.
Figure 4 The LNA in ARQSF3753-RX-A dual-channel receiver front-end module provides high and low gain modes with a 1.45 dB noise figure. Source: arQana
Qorvo also brings its heritage in GaN to the QPF4005, a dual-channel 37 GHz to 40.5 GHz FEM (Figure 5). Each channel has an LNA, a transmit-receive (T/R) switch, and a PA, all in a 4.5 x 6.0 mm package. It targets phased array 5G base stations. Its air cavity laminate package features an embedded copper heat slug for extended temperature operation.
Figure 5 QPF4005’s compact form factor is designed to meet the tight lattice spacing requirements and support multi-channel/dual-polarization phased array applications. Source: Qorvo
When everything goes wireless
With more products incorporating wireless connectivity, the chances an analog engineer runs across some RF component increases.
We have already seen a cycle where lower frequency RF paths are integrated directly into digital SoCs with RF IP blocks. Now, we see faster, noise-sensitive RF designs captured in channel-matched RFEE components with proven performance. Combining an RFEE designed for the job, a faster analog front-end, and the right software in the digital domain can get many wireless applications run quickly.
After spending a decade in missile guidance systems at General Dynamics, Don Dingee became an evangelist for VMEbus and single-board computer technology at Motorola. He writes about sensors, ADCs/DACs, and signal processing for Planet Analog.
- What’s in an RF Front End?
- RF front end adapts for increased mobile data demand
- 5G to Alter RF Front-End Landscape
- RF-SOI Engineered Substrates at the Heart of Modern RF mmWave Front-ends
- Digital Tuning for RF Analog Front End