Question

I see high radiated spur within the frequency range of the Si4x6x RF chip VCO signal. How can I suppress this?

Answer

In RX only boards due to the lack of additional filtering the VCO signal might be radiated via the antenna too. In that case some filtering between the antenna and the matching balun network is recommended.

Moreover, the filtering of the traces connected to the SDN, GPIO 2, 3 pins (and the DC supply line of a TCXO) is also suggested at the VCO frequency (not only in case of RX only boards).

Use cases where applying additional filtering on SDN, GPIO 2, 3 (and the DC supply line of a TCXO) is strongly recommended:

• 2 layer PCBs (the problematic traces cannot be routed on inner layers, they can operate as hidden antennas)
• High-power (≥20dBm) applications (the level of possible spur is higher for high power parts)

The actual level of this radiated spur can vary with the actual board design.

Note: Not all Si4x6x reference designs use this additional filtering on SDN, GPIO 2, 3 (and TCXO VDD supply) lines. If possible, it is recommended to apply the additional filtering inductors on all customer designs. If BOM cost is critical, these elements can be eliminated from the design, but be aware that depending on the actual PCB layout structure, increased radiated spur in the frequency range of the VCO signal might appear.

The SDN and GPIO 2, 3 filtering on Silicon Labs reference designs is shown in the following figure:

The inductor value should be selected in a way to ensure enhanced suppression at the VCO frequency, so inductors that have SRF at the ~ 3.5 GHz frequency range. 

Question

What Si4x6x reference designs need external power supply even if being connected to the Wireless Motherboard (WMB-930)?

Answer

Si4x6x reference designs with additional external FEM or FET require external power supply even if connected to the Wireless Motherboard, WMB-930. It means to use an external 4xAA battery pack connected to the RF Pico Board. The reference Pico Boards also use an external LDO in order to ensure the required 3.3 or 3.6V power supply for the radio and FEM/FET.

The external power supply is required due to the higher current consumption.

This KBA provides a brief summary about and highlights the possible need for board-level ESD protection for RF devices.

Radio chips are designed for and tested against different chip-level ESD standards, such as Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM). These chip-level test results are summarized in the RF IC’s Qualification Reports.

However, in a real-world application a final module/board has to resist and stand against an ESD shock. For this purpose, the final electronic product has to be tested against a different, more stringent standard that simulates and replicates the real world ESD stress conditions. This system-level ESD standard is the IEC 61000-4-2, for instance. System/module designers should take care to comply with the IEC 61000-4-2 system-level ESD standard. This KBA provides some board-level insights about how to make an RF design more immune against ESD.

For more technical details, please refer to Silicon Labs' application note AN895. This application note provides recommendations on ESD protection circuits and shows test results measured with the Si4x6x chip family, however the suggested protection circuits can also be utilized with any other RF chip families. 

https://www.silabs.com/documents/public/application-notes/AN895.pdf

For an RF design the most ESD-sensitive part is the RF path, including the antenna, matching network and RF ports. Secondly, the supply and GND paths are also sensitive, and lastly any GPIO or other paths connected to the RF chip directly. 

So, the antenna definitely needs special care during design and assembly into the end product. ESD protection can be enhanced by: 

- antenna placement: end-user shouldn't be able to touch it in any case.

- design an antenna with direct GND connection, e.g. inverted-F antenna. 

- protection circuit elements in the RF path: parallel inductors, capacitors, TVS diodes. 

Many circuit designs have the supply trace connected to the PA externally, for which cases the supply trace may also need care and ESD protection. 

Lastly, any push-button or interface, that can be touched by the user of the end-product during normal usage, may also need to be ESD-protected. These are, typically, GPIO ports of RF devices. 

Please see AN895 application note for recommended ESD protection circuits and for more technical details. 

This KBA provides some hardware tips about how to maximize the isolation between multiple antennas mounted on the same PCB. 

- Maximize the distance(s) between the antennas on the PCB in order to minimize the radiated coupling effects. 
- Place the antennas in opposite orientation, i.e. opposite polarization in order to maximize the isolation.

- The radiation patterns of antennas can also be taken into account during placement, i.e. try to place one antenna in the null point (or less radiating) direction of the other one. 
- Place the antennas on different layers, i.e. put one antenna on the top and the other on the bottom layer of the carrier PCB.

- Select antenna types which have a bit more concentrated and localized RF currents close to the antenna input ports. E.g. inverted F-antenna (over a simple monopole antenna). 

- If each antenna is placed at the PCB edge, then GND slot(s) - one slot between two antennas - on the common GND plane can be ensured between the antennas in order to minimize the mutual current and thus decrease the coupling effects. 

The shape and dimensions of these board-edge current-blocker slots can be the followings:
1. Simple straight slot with 3mm width and quater-wavelength length on the given PCB.
2. For a wider band approach, slot line radial stubs or even diamond-shaped slots can also be utilized - as shown in the design of 4455-LED-868 RF Stick, for instance (see the diamond-shaped slot in the printed balun area): 
https://www.silabs.com/documents/public/schematic-files/4455-LED-868.zip

Si4x6x radios offer High-performance and Low-power operation modes. Selecting one over the other affects 3 RX mode parameters: RX current, RX Sensitivity, Adjacent Channel Selectivity. The following table summarizes the actual differences between these 2 modes:

The desired performance mode can be selected in GLOBAL_CONFIG API property:

- GLOBAL_CONFIG[0] = 0 --> High-performance mode

- GLOBAL_CONFIG[0] = 1 --> Low-power mode

Si4x6x radios can operate in High-performance or Low-power modes, which can be set in GLOBAL_CONFIG property. In order to save some current consumption (typically 2-3mA), low-power mode can be enabled, however it has some tradeoff in RX performance (sensitivity, linearity).

If the crystal parameters meet the requirements listed in AN785 section 1, both high-performance mode and low-power mode should result robust crystal operation on a properly designed PCB layout (crystal placed as close to the XTAL pins as possible).

In high-performance mode there is some margin on the required crystal parameters, but still it is recommended to stay within the ranges listed in AN785 section 1 for safe operation. Crystal start-up issues might occur if the parameters are not met.

In low-power mode meeting the required crystal parameters is more critical, i.e crystal start-up issues might happen even if the RF crystal parameters are near the edge. Low-power mode can be used robustly with a crystal that easily meets the recommended parameters. AN785 Table 1 provides a list of recommended crystals that will ensure safe operation both in high-performance and low-power mode.

If crystal start-up problem occurs in low-power mode, there are 2 possible options to avoid the issue:

1. Use the radio in High-performance mode. This will increase RX current by 2-3mA, but sensitivity and range will improve.
2. Run only the crystal oscillator block in High-performance mode, all other blocks can remain in Low-power mode. This will increase RX current only by 100-200uA, sensitivity will be the same as in Low-power mode, but crystal start-up issues will disappear. In order to do this, after enabling low-performance mode in GLOBAL_CONFIG property, the following bytes should be sent to the radio via SPI interface: "F1 F0 01 5A".