Use FEMs for Extended Range and Robust Links in Space-Constrained BLE Designs

Technical Overview: Bluetooth Low Energy (BLE) devices can require more transmit power, receiver sensitivity, and link margin than a system-on-chip (SoC) can provide. Front-end modules (FEMs) address these requirements by integrating a power amplifier (PA), low-noise amplifier (LNA), harmonic filtering, transmit/receive switching, control circuitry, and a bypass path. The Skyworks SKY66411-11 and SKY66415-11 provide efficient 2.4 gigahertz (GHz) operation in CSPs for space- and power-constrained designs. Their different supply and output-power characteristics support battery-powered applications. Associated evaluation boards provide RF and DC connections, schematics, layout files, and performance resources for device characterization and adaptation to custom hardware. (Overview courtesy of ChatGPT)

Designers of Bluetooth Low Energy (BLE)-enabled devices such as wearables, smart-home devices, asset tracking, proximity beacons, and gaming controllers increasingly face the challenge of maximizing range and link reliability while minimizing power consumption, space, cost, and time to market. While many BLE systems-on-chip (SoCs) can address these challenges for general use cases, some design targets require dedicated external circuitry to further optimize range, efficiency, and sensitivity.

These augmentations add extra components, increase RF routing complexity, consume board space, raise cost, and extend development time. However, designers can meet specific application goals and mitigate these issues using front-end modules (FEMs).

This article briefly discusses the challenges designers face with BLE devices. It then introduces FEMs and associated boards from Skyworks and shows how they can augment BLE SoCs while simplifying development.

Why use FEMs in BLE designs

The adoption of BLE has enabled substantially lower power consumption for many short-range wireless applications, but extended range often requires higher output power and greater receive sensitivity to maintain the link budget and ensure a sufficient link margin for reliable communication. The extra power runs counter to the need to preserve battery energy in devices such as smartwatches, especially as higher-throughput modes place greater demands on transmit power and linearity. In many cases, a BLE SoC by itself cannot achieve the required performance, so external circuitry is needed.

However, adding discrete RF components adds cost, complicates routing, consumes board space and power, and delays development. FEMs are a good way to find balance. These highly compact RF ICs replace or consolidate discrete front-end elements into a single, tightly integrated package optimized for efficiency and sensitivity.

Placed between the Bluetooth SoC and the antenna (Figure 1), the FEM pairs with the BLE SoC. While the SoC handles much of the signal processing and wireless functionality, the FEM provides system-level advantages in range, link stability, and power efficiency while minimizing cost, design complexity, and physical footprint.

Figure 1: The FEM integrates external amplification and routing components into a compact device positioned between the SoC and the antenna to extend range and improve signal reliability. (Image source: Skyworks)

A typical BLE FEM comprises a power amplifier (PA), low-noise amplifier (LNA), control interface, low-loss bypass path, and transmit/receive switches (Figure 2). The PA boosts outgoing transmit (Tx) signals from the SoC, while the LNA amplifies weak incoming receive (Rx) signals with minimal added noise. This approach can increase link margin and potentially extend a device's connection range. Moreover, the improved link margin enables more reliable communication, with fewer retransmissions and shorter active radio time, improving overall efficiency.

Figure 2: A typical BLE FEM integrates a PA, an LNA, harmonic filtering, Tx/Rx switching, and a bypass path, along with sleep or shutdown modes. (Image source: Skyworks)

Next, the Tx/Rx switches route the signal between the PA and LNA, depending on whether the BLE radio is transmitting or receiving. Finally, the control interface uses digital GPIO from the BLE SoC to turn the FEM's internal stages on or off, with microsecond (µs) switching times. Low-power or shutdown operating modes are typically included.

FEMs for BLE SoCs

On the surface, adding a FEM may seem to increase power consumption, but at the system level, it makes sense. Although a FEM consumes power, its efficient PA can let the SoC transceiver operate at a lower RF output level, while the complete system still achieves the required antenna output power, potentially improving overall efficiency.

For example, the SKY66411-11 (Figure 3), from Skyworks’ line of BLE FEMs for the 2.4 gigahertz (GHz) band, delivers 8 decibels referenced to 1 milliwatt (dBm) of output power with a typical 40% power-added efficiency (PAE) at VCC2 = 2.4 V. It also provides a typical receive noise figure (NF) of 2.3 dB, a typical sleep current of 0.1 microampere (µA), and an of/off switching time of under 1 microsecond (µs), all within a compact 1.32 × 1.32 × 0.381 millimeter (mm) CSP.

Figure 3: With VCC2 at 2.4 V, the SKY66411-11 delivers 8 dBm output power with 40% typical PAE. (Image source: Skyworks)

The SKY66411-11’s LNA supply, VCC1, operates from 1.1 V to 2.0 V, while the PA and switch supply, VCC2, operates from 1.7 V to 3.6 V, supporting battery-powered operation over a wide portion of the discharge curve. The CTX and CRX inputs connect to the BLE SoC’s control outputs.

The SKY66411-11’s typical PAE of 40% supports efficient conversion of battery power into RF energy. This helps designers extend battery life even under higher link-budget demands. Moreover, its low-loss bypass path lets signals bypass the active amplification stages when full boost is not needed, saving battery life during short-range communication.

The built-in harmonic filter helps suppress unwanted harmonics generated during amplification, supporting system-level compliance with applicable global radio regulations. Finally, its compact form factor makes it well suited for Bluetooth-enabled devices such as sensors, beacons, smart meters, thermostats, wireless cameras, smoke and CO₂ detectors, and wearables.

The SKY66415-11 FEM, which offers size and performance benefits similar to the SKY66411-11, is optimized for SoCs operating around 1.8 V. It delivers 11 dBm output power while maintaining 40% typical PAE and drawing 2.5 mA (typical) in receive mode to further maximize battery life. Its VCC1 supply operates from 1.1 V to 2.0 V, while VCC2 operates from 1.7 V to 2.0 V, supporting operation over a range of battery conditions. Like the SKY66411-11, its compact form factor suits space-constrained Bluetooth designs such as wearables and asset-tracking beacons.

BLE system designs must also account for losses along the path from the SoC to the antenna, including printed circuit board (pc board) trace losses, switch and matching-network losses, and filter insertion losses.

Overcoming system-level losses is critical to maintaining robust wireless performance across all operating conditions. Here, the SKY66411-11 and SKY66415-11, supported by accompanying evaluation boards, compensate for these losses to boost output power, improve receiver sensitivity, and stabilize performance across a wide range of usage scenarios.

FEM evaluation boards

BLE devices are designed for compact spaces, so they require careful RF trace routing, proper grounding, and matching networks to maintain harmonic suppression and filter performance. The accompanying evaluation boards, the SKY66411-11EK3 (Figure 4) for the SKY66411-11 and the SKY66415-11EK3 for the SKY66415-11, provide layout resources that demonstrate proven implementations of RF building blocks, including PAs and LNAs.

Figure 4: Shown is the SKY66411-11EK3 evaluation board for the SKY66411-11 FEM. (Image source: Skyworks)

Both evaluation boards have their respective FEM onboard and the necessary RF and DC connections for external power and control signals. They let designers verify how integrated RF blocks, such as the PA and LNA, perform and measure key metrics, including gain, NF, output power, and current consumption under selected operating conditions.

The specifications cover pin configurations, recommended operating conditions, and performance data, while the accompanying schematics, bill of materials (BOM), board layouts (GBR/BRD files), and S-parameters help designers copy or adapt the reference layout for custom hardware.

Both the SKY66411-11EK3 and SKY66415-11EK3 expose control pins that align well with the GPIO or dedicated FEM control features found on BLE SoCs. As a result, once designers have compatible voltage levels and reference layout guidelines, integration largely reduces to correct pin mapping and careful RF routing.

Conclusion

Some short-range wireless applications require greater transmit output, receiver sensitivity, or link margin than a standalone BLE SoC can provide. Skyworks FEMs integrate the PA, LNA, bypass path, and transmit/receive switching needed to address these requirements while minimizing component count and simplifying RF layout. The accompanying evaluation boards simplify device characterization and implementation.