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.
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):
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:
|High-performance mode||Low-power mode|
|RX Current||~3mA higher||~3mA lower|
|RX Sensitivity||~2dB better||~2dB worse|
|~2dB better||~2dB worse|
The desired performance mode can be selected in GLOBAL_CONFIG API property:
- GLOBAL_CONFIG = 0 --> High-performance mode
- GLOBAL_CONFIG = 1 --> Low-power mode
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.
RF range depends on the following parameters:
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.
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:
Why is there a “6 dB” output power limitation in the older 0.5 mm thick PCB reference designs using the 434 MHz cross-tapped loop antennas (see Antenna Selection Guides) for the Si4021 and Si4421?
For both, there is a “6dB” output power limitation due to the antennas parallel resistance, which is larger than the optimal required load resistance (for maximum output power).
At the Pmax output power state the RF output becomes saturated due to the limited voltage swing. In a proper and reduced output power state, the RF current is lowered so the output is not in saturation, here there is a decrease in the radiated power but he range decrease is not substantial (under 10%). In addition, the reduced output power state provides a lower current consumption and reduced harmonics.
What happens if the output of a transmitter is open circuited during operation?
An open circuited transmitter output can malfunction after a short period of operation.
The reason is that the tail current generator of the output differential driver pair forces the output transistor into saturation and the emitter base diode opens. This high DC current (~12mA) is then forced to flow to the preceding emitter follower output stage, and from this point it becomes unpredictable, where it may become damaged.
Do baluns or any external components have to be used with a cross-tapped loop antenna?
The cross-tapped loop antennas are high impedance differential antennas.
Their impedance is adjusted to be close to the optimum differential antenna impedance and as such the antenna can be connected directly to the chip outputs. One caveat however is that the antenna needs to be connected exactly to the input points of the antenna as a transmission line will detune the impedance and degrade the radiation.
Do loop antennas require a ground plane?
Loop antennas do not require any ground plane, and actually perform slightly better without one.
In reality however, as the rest of the application circuitry is positioned beside the antenna the antenna properties are effected by this proximity in terms of impedance and radiation properties. To reduce the effects of the application circuit it is recommended to fill the gaps with ground metal, this in turn effectively becomes a small ground plane.