Using a 2-layer PCB in the application hardware is usually preferred by customers due to cost saving purposes. Still, based on the RF output power level, unwanted radiation of top or bottom layer traces (mostly VDD or digital) can occur, which causes that the application can violate the harmonic limits of the related standards. In order to minimize the possibility of unwanted trace radiations, Silicon Labs recommends to use multilayer PCBs in the following cases:

• >=16dBm output power at sub-GHz frequencies

• >=10dBm output power at 2.4GHz

Note that the actual recommendation depends on which standard (ETSI, FCC, ARIB, etc.) the application has to be compliant with.

One can notice that not all Silicon Labs reference design follows the above listed recommendations. EZRadioPRO reference design boards are made on 2 or 4 PCB layers based on the output power, while all EZR32 and EFR32 reference design boards are using 4 or 6 PCB layers due to the complexity of the design. In the latter case, the layout routing could not be realised on a 2-layer PCB if all digital traces were intended to be used. Of course, on a custom design where the complexity of the design is much less, 2-layer PCBs can be used for EZR32 or EFR32 applications as well considering the above listed recommendations.

Silicon Labs provides RF range calculators for customers to help estimating the actual range of their wireless applications. Simple RF Range Calculator is available to download from the following link below.

http://www.silabs.com/documents/public/software/RF-Range-Calculator.zip

RF range depends on the following parameters:

• Conducted TX output power: the power driven to the antenna input [dBm]
• TX antenna gain [dBi]
• Conducted receiver sensitivity [dBm]
• RX antenna gain [dBi]
• Frequency [MHz]
• Propagation factor (depends on the environment)

Simple RF Range Calculator

Simple RF Range Calculator is for those customers who don’t want to deal with difficult RF questions, just simply would like to get fast and reasonable results for both outdoor and indoor environments.

Key features:

• Fast and simple while accurate
• Built in propagation factors, based on field measurements
• Antenna height fixed to 1 to 1.2 meters
• Supports all the unlicensed bands and custom frequency channels as well

Usage

Simple RF Range Calculator provides fast and accurate result as the customer selected the frequency band and set TX and RX parameters.

Frequency bands and custom frequency channels can also be selected.

TX Output Power and RX Sensitivity need to set up based on the radio device’s actual link parameters based on the data sheet.

If the exact antenna parameters are unknown notes at the right side can help to determine the closest values.

The achievable RF range depends on many other factors as well. See the following KBA article for further details on RF range factors:

https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2017/12/07/rf_range_factors-fnT1

Question

How should I connect the pins which are not intended to be used on Si4x6x / EZR32?

Answer

• TX: Connect to VDD (PA should be powered to avoid leakage currents)
• RXP, RXN: Connect to ground
• GPIO pins: No connect

Question

How can I make the frequency bandwidth of PCB antennas wider?

Answer

In some cases/applications the BW of printed antennas might not be sufficient. This article summarizes some design tricks on how to make a printed antenna wider bandwidth.

- Increase the board size (e.g. GND plane in the case of monopole-type antennas). Avoid using RF modules that have smaller size than quater-wavelength. Small modules generally have poor antenna gain and narrow bandwidth (due to the high Q factor).

- Increase the board thickness. Of course, it's typically limited by design.

- Decrease the dielectric constant of the PCB. Select PCB material with low epsilon value. 

- Use wider and/or tapered traces in the PCB antenna structure.

- Use coupled traces in the PCB antenna structure. Coupled structures typically have wider frequency bandwidth. 

- Do some tricks in the external antenna matching network. I.e. use more components to do the match (to stay within a given constant Q ellipse on the Smith Chart); create resonators in the matching network. Also, see Bode-Fano, Youla matching techniques.

A number of antenna types can inherently be matched to the desired input impedance (typically, 50-ohm single-ended) without using any external tuning component (e.g. printed inverted-F antenna). However, board size, plastic enclosures, metal shielding, and components in close proximity to the antenna can affect antenna performance. For best performance, the antenna might require tuning that can be realized by two ways:

• Dimension changes in the antenna layout structure, or
• Applying external tuning components.

It is typically a preferred solution when layout modification is not required on a custom design. To accomplish this, Silicon Labs generally recommends to ensure SMD placeholders for external antenna tuning components, where the suggested external antenna matching structure is a 3-element PI network. You can achieve a good match using as a maximum of two elements (with one series and one shunt component) of the PI network. Any unknown passive impedance can get matched to 50 ohms on this PI network, since all L, C, L-C, C-L combinations can be realized on it and therefore any de-tuning effect can be compensated out.

Note that every implementation of an antenna design might require different combinations of inductors and capacitors.

Recommended 3-element PI network for external antenna matching purposes:

Question

What is the recommended distance between antennas in an antenna diversity application?

Answer

Antennas in a product that implements antenna diversity have their antennas mounted at a distance of at least ¼ wavelength apart. This amount of spatial separation improves the probability that at least one antenna is NOT in a deeply faded signal condition. Another typical recommendation is ½ wavelength antenna distance, however, it can result quite large board sizes at low frequencies (e.g. for 434 MHz frequency ½ wavelength is 34,5 cm).

Question

Can I use the same matching network for EZRadioPRO and an EZR32 wireless MCU that is based on the same radio? 

Answer

The answer depends on the actual PCB layer stack-up as the distance between the top and the first inner (ground) layer determines the PCB parasitic capacitances, which plays part in the matching network.

If the same PCB layer stack-up is used (or at least the distance between the top and the first inner layer is similar), in that case the same matching network component values should result very similar TX and RX performance for an EZRadioPRO radio and an EZR32 wireless MCU. If the PCB layer stack-up deviates significantly, it is not recommended to use the same matching network as the detuning can cause lower output power, higher harmonics, higher current consumption and sensitivity loss.