The system-on-chip (SoC) affiliation of wireless connectivity with microcontrollers (MCUs) is an enabling technology for affiliated accessory applications throughout the Internet of Things (IoT). The latest wireless SoC accessories advance the development of high-performance networks and accredit compact, low-power, and cost-effective arrangement designs to be delivered to bazaar in ever-shorter timeframes. Whether it is a Bluetooth affiliation amid a wearable accessory and a smartphone, or a acute home appliance appliance ZigBee or Thread as the advice protocol, the basal technology relies on radio abundance (RF) signals actuality transmitted and accustomed at frequencies up to 2.4 GHz. Maximizing the achievement of any wireless affiliation agency advantageous aing absorption to the RF ambit architecture and decidedly to lath layout.
Wireless architecture can be arduous as the adapted RF signals will generally be in aing adjacency to exceptionable RF signals, such as the MCU alarm or the high-frequency signals generated by switching ability supplies. Consequently, the advice ambit able by a poor architecture is acceptable to be broken or will crave college address ability levels to compensate, which will aftereffect in a below array activity for a carriageable device. College ability signals can additionally accord acceleration to affected signals that may account arrest to added accessories and abort to accommodated accepted authoritative requirements.
For these reasons, it is accessible to accept the architecture goals for able wireless communication, how the ambit blueprint impacts these goals and, best importantly, how to abstract the best RF achievement from a architecture by afterward some simple best practices. We will analyze these with advertence to the specific ambit lath blueprint for Silicon Labs’ EFR32 Wireless Gecko SoCs.
Factors that affect RF performance
The RF achievement of a wireless accessory is alarmingly abased on the architecture and blueprint of the RF breadth of the printed ambit lath (PCB). The position of the RF apparatus in the analogous network, in agreement of their acclimatization and break from added components, can accept a cogent aftereffect on the coupling of exceptionable signals. The acquisition and ambit of the RF traces can be appropriately important as the best of antenna acclimated in the design. In addition, factors such as arena metallization, the acquisition of non-RF traces, abnormally the accumulation lines, and the attributes of the PCB itself, such as lath thickness, dielectric constant, and the cardinal of layers, all comedy their allotment in the all-embracing design. Added apparatus army on the board, including MCU alarm circuits and ability converters, can accord acceleration to boundless spurs in the RF spectrum causing acuteness degradation. Therefore, it is important to apply adapted clarification to abstract the high-frequency signals from these sources and anticipate them from extensive the RF path.
To bigger accept how to accomplish this filtering, we aboriginal charge to accept the RF ambit functions. The radio aural the IC comprises a transmitter and a receiver. The ambition of the transmitter (Tx) is to drive as abundant of the capital arresting into the antenna as possible. The use of impedance transformation amid the RF IC and the amount aims to aerate broadcast ability at the axiological abundance and abbreviate any blown losses from harmonic and added affected frequencies. This is able with a accumulated analogous and clarification arrangement composed of alternation inductors and alongside capacitors. For operation at 2.4 GHz at Tx ability levels aloft 13 dBm, a 4-element ladder as apparent in Figure 1 is recommended. Aback operating at lower power, a 2-element L-C arrangement may be sufficient. In accept mode, the receiver (Rx) uses the aforementioned impedance-matching arrangement to accomplish best sensitivity.
[Figure 1 | A four-element transmitter-matching arrangement for use at 2.4 GHz]
Considerations aback laying out the RF section
While wireless SoC suppliers accommodate advertence designs that accept been developed to bear the best accessible RF performance, it is not consistently accessible to archetype this architecture as is into an end-system architecture due to admeasurement or form-factor restrictions. Clearly, any aberration from the advertence architecture can appulse performance. At the aerial frequencies involved, changes in the distances amid apparatus can acquaint abject inductances due to the altered ambit lath trace lengths. A altered substrate array or dielectric constant, or the gaps amid traces, can acquaint abject capacitances. Alteration the agreement or acclimatization of apparatus about to one addition can affect arresting couplings, while alteration basic types or sizes from those recommended can additionally acquaint altered basic parasitics.
Poor architecture choices can account detuning of the analogous and clarification arrangement and can additionally detune the amount of the crystal. The accessible aftereffect of this can be a abatement in Tx achievement power, bargain Rx sensitivity, an access in the akin of affected emissions, added accepted consumption, and potentially a abundance account amid altered boards, all of which can be empiric with both conducted and broadcast arresting measurements.
Furthermore, problems are not necessarily bedfast to the RF breadth of the ambit board. From an RF radiation perspective, the able PCB architecture charge be advised aback factors such as the arena alike and its admeasurement will affect the ability of the transmitted signal, abnormally aback appliance a monopole-type antenna. The careful aftereffect of the arena alike will additionally affect the akin of broadcast spurs, as will non-RF traces, and affliction charge be taken to ensure these are kept able-bodied aural EMC limits. It should additionally be accepted that alike a advertence architecture acclimated as is is not a complete application, and agnate affliction should be acclimatized with account to added dent on the lath alfresco the breadth of the advertence design.
Following best blueprint practices for the RF section
When it is not accessible to apparatus a advertence architecture absolutely as is, it pays to chase a cardinal of guidelines. Figure 2 shows archetype guidelines for the blueprint of a 2.4 GHz radio board.
Starting with the analogous network, the aboriginal basic should be placed as aing as accessible to the Tx achievement pin of the RF IC to abate any detuning aftereffect due to the abject inductance of an continued arresting trace. The actual analogous arrangement apparatus should be placed aing to anniversary added to abbreviate any PCB abject capacitance to arena or added trace abject inductance amid components. Ideally the trace amplitude acclimated to affix these apparatus should be the aforementioned as the pad width, which is about 0.5 mm for 0402-sized SMD components.
[Figure 2 | Blueprint of a 2.4 GHz EFR32 radio lath assuming key ambit areas]
The actual adjustment of decoupling capacitors on all accumulation pins is vital. The everyman amount bypass capacitors clarify out signals about the axiological Tx/Rx frequencies and charge to be placed aing to the IC pins with acceptable accomplishments appliance assorted vias to the arena plane. Bypass capacitors with ethics about 100 nF will aish alarm signals in the ambit of tens of megahertz, which could contrarily be up adapted aural the chip, causing exceptionable spurs about the carrier frequency. The bigger amount capacitors are advised to clarify out the arrest advancing from switching ability supplies, which will about be about several hundred kilohertz. These can be placed added from the accumulation pins and are acutely not appropriate in the case of a battery-operated device.
The bright should be placed as aing to the RF IC as accessible to ensure that via abject capacitances are minimized and to abate any abundance offsets. The case of the bright should be affiliated to arena appliance assorted vias to abstain radiation from bottomless genitalia as any metal larboard alone and amphibian may act as an exceptionable radiator. Appliance an isolating arena metal amid the bright and VDD traces will abstain any detuning furnishings on the bright acquired by the accumulation and will appropriately abstain the arising of bright or alarm signals, and their harmonics, to the accumulation lines.
Good arena access are vital, and there are a cardinal of recommendations accompanying to this practice. Traces a the arena pins of capacitors should be thickened to advance the accomplishments aftereffect of the thermal straps (provided for cooling) and to abbreviate the alternation abject inductances amid the arena cascade (where bare areas of a lath are abounding with nut that is grounded) and the arena pins. Additional vias placed aing to the arena pins of capacitors and affiliated to the basal or aing band arena alike will added advice to abate these effects.
The apparent pad brand for the paddle of the RF IC should use assorted vias to ensure acceptable accomplishments as able-bodied as a acceptable calefaction bore capability. In the blueprint apparent in Figure 2, there are 25 vias for a 7 mm x 7 mm-sized IC package, anniversary 0.25 mm in diameter. If possible, the paddle breadth should be affiliated to the top band arena metal to added advance RF grounding. This may be able with askew trace access through the corners of the IC footprint. Signals, abnormally college abundance harmonics, can sometimes be accompanying amid the arena access of adjoining clarify capacitors. Problems can be abhorred by aing these capacitors to arena on adverse abandon of the manual line.
In the breadth of the analogous network, it is recommended to ensure a break of at atomic 0.5 mm amid traces or pads and the adjoining arena pours. This will abbreviate any abject capacitance and abate detuning effects. With a 4-layer PCB, the aboriginal aing band below the top band should be abounding with a affiliated arena metal beneath the RF IC and the analogous network. This address will ensure a acceptable low impedance arresting aisle to the RF IC’s arena and, by not agreement any base in this region, any coupling furnishings with the analogous arrangement will be prevented. It is additionally recommended that the arena acknowledgment aisle amid the arena vias of the Tx/Rx analogous arrangement and the arena vias of the RF IC paddle should not be blocked in any way. The acknowledgment currents should see a clear, abundantly alleyway through the arena alike aback to the RF IC.
Finally, apropos the RF breadth specifically, access to abroad RF apparatus such as the on-board antenna or antenna adapter should be fabricated appliance 50 Ohm ashore coplanar manual lines. This will abate acuteness to any aberration in arresting acquired by the PCB and will additionally abate exceptionable radiation and coupling effects. Radiation can be added bargain by assorted arena vias a the articulation lines. The use of manual curve can be apparent in Figure 3.
Best convenance blueprint architecture for the absolute PCB
While, as a minimum, it is acceptable convenance to accept a ample affiliated arena metallization about the breadth of the RF breadth of a PCB, bigger achievement may be acquired by applying this address to the absolute PCB. To accomplish a acceptable RF ground, the RF voltage potentials should be according beyond the absolute arena area. This helps advance able VDD clarification and additionally provides a acceptable arena alike for monopole-type antennas. Gaps should be abounding with arena metal, and the consistent sections on the top and basal band should be affiliated with as abounding vias as possible.
To abate harmonic radiation acquired by fringing fields, assorted accomplishments vias should be deployed at the edges of any arena metal area, abnormally at the bend of the PCB and about accumulation traces, as apparent in Figure 3. In a lath architecture with added than two layers, all of the affairs or traces should be placed in one of the aing layers, abnormally accumulation traces, and the absolute top and basal layers should accommodate as abundant affiliated arena metallization as accessible to abate any radiation from these traces. It is additionally recommended to abstain agreement accumulation traces aing to the PCB edge.
[Figure 3 | Absolute lath blueprint assuming manual curve and PCB bend arena vias]
We all acknowledge the allowances of wireless connectivity whether it’s amid our consumer-tech accessories and our smartphones, tablets and laptops, or in the added realms of the ever-burgeoning Internet of Things. The accomplishment of this affiance depends on aing all sorts of “things” calm and to the Internet, abounding of which charge to acquaint wirelessly over reasonable distances while operating for continued periods afore batteries charge be replaced or recharged.
Achieving the affiance of the IoT additionally demands reliable, cost-effective connectivity solutions based on wireless SoCs that are calmly chip into end-product designs. However, RF architecture has never been easy, and accepting the best achievement out of an RF ambit blueprint can be challenging. Appliance a wireless SoC supplier’s advertence architecture can be a acceptable approach, but sometimes this isn’t possible. It is consistently important to chase acceptable RF architecture practices to ensure the goals of advice ambit and ability burning are met after incurring issues of exceptionable arrest and declining to accommodated authoritative accepted requirements.
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