Below you’ll find a listing of the most frequently asked questions that Linx receives. Should you have any questions that are not found here, or need help with any of Linx’s products, please reach out to one of our technical service representatives at firstname.lastname@example.org. Also, be sure to check out our Wireless Made Simple blog for answers and updates to the latest innovations in the world of wireless.
General Radio Frequency
RF interference generally comes from two sources, internal and external. Internal sources are things in the product that generate electromagnetic noise in the same frequencies as the radio. This can include things like crystal oscillators, switching power supplies and regulators and motors. Internal sources usually reduce the sensitivity of the receiver, resulting in a reduction in system range. When this happens, the noisy components need to be shielded and filtered to knock down the noise both conducted through traces and radiated around the board. This can get very complicated very quickly, so it is important to follow good PCB and system design rules for reducing the potential for Electromagnetic Interference (EMI). In our experience, switching regulators and motors have the biggest impact in our customer’s designs, so take extra care if your design uses these components.
External interference is typically other transmitters on the same frequency in the same environment at the same time. This generally results in corrupted packets and reduces range since the two sides need to be closer to successfully pass messages. This is somewhat like trying to talk to your friend at a loud concert. Since it is rare that the installer can affect other transmitting systems in the area, having methods to repeat messages and receive acknowledgements as well as the ability to have many frequencies (or channels) to transmit on helps reduce the impact of external interference.
There are several factors involved in determining the range of an RF link. The primary ones are frequency of operation, transmitter output power, receiver sensitivity, transmitter and receiver antenna gains, and the path loss of the operating environment. There are also sources of loss internal to the system. For more information on this, please see the Considerations for Wireless Range post in our blog.
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Since 1996, the Federal Communications Commission (FCC) has set regulations and guidelines on all wireless communications devices sold in the United States. The wireless products must meet minimum guidelines for safe human exposure to radio frequency energy. The limits established in the guidelines are designed to protect the public health with a very large margin of safety.
The Specific Absorption Rate (SAR) is a measure of the rate at which energy is absorbed by the human body when exposed to radio frequency (RF) energy. The general population exposure limits set by the FCC for the frequency range that is utilized by smart meters and other devices like cordless phones and baby monitors, is 0.6 milliWatts per centimeter squared (mW/cm2) at 902 MHz and 1.0 mW/cm2 at 2.4 GHz. Products used within 20cm of the human body must undergo SAR testing to ensure that the rate is below the limits
The FCC has outlined exclusions where the power is low enough that SAR testing is not warranted. In Knowledge Database (KDB) article 447498 this is outlined as:
[(max power of channel, including tune-up tolerance, mW)/(min test separation distance, mm)]*[√f(GHz)] ≤ 3.0 for 1-g SAR and ≤ 7.5 for 10-g extremity SAR.
As an example, use the LR Series at 433.92MHz. The maximum output power that the module is capable of is a little under 10mW. Assume a separation of 5mm as a reasonable separation distance through an enclosure. The calculation is:
(10mW / 5mm) * (√0.43392GHz) = 1.3
This is less than 3.0 and would be excluded from SAR testing.
Electromagnetic fields, radio waves, microwaves and wireless signals are referred to as radio frequency (RF) energy. RF energy is all around us. RF is used in various electronics and appliances, which include radio and television broadcasting, cellular telephones, satellite communications, microwave ovens, radars, and industrial heaters and sealers. These are just a few applications.
Electromagnetic waves are measured by wavelength and frequency. Wavelength is the distance covered by one complete cycle of the electromagnetic wave. Frequency is the number of electromagnetic waves in one second, also known as a Hertz or Hz. One Hz equals one cycle per second. One megahertz (MHz) equals one million cycles per second. Generally, microwaves are radio frequencies measuring more than 1 GHz.
An RF signal is transmitted in all directions from the transmitting antenna much like a water ripple travels in all directions from the point where a stone hits the surface of a pond. When that ripple hits a rock or the bank of the pond, the wave will bounce off and travel in a new direction across the pond.
The same thing happens with radio waves. They bounce off of trees, mountains, buildings and other objects. Since straight and reflected waves take different paths some arrive at the receiving antenna more quickly than others, causing cancellation due to differing phase relationships. This concept is called multipath. It results in lowered signal levels at the receiver and thus shorter useful distances for the link. Multipath is particularly a factor in interior environments where objects provide many different reflection paths.
There are many ways to combat multipath ranging from careful frequency and antenna selection, error correction, transmission redundancy, antenna diversity and the use of spread spectrum and frequency hopping.
Many times designers assume that because a transmitter has a unique address code or protocol that it can transmit at the same time as other units with different codes or protocols. It is important to remember that even though the original signal may be digitally distinct, it enters airspace as an analog electrical signal. This means only one unit can operate at a time without contention. If two people are screaming at you, it does not matter what they are saying, you will not understand either one. The same idea applies to an RF receiver. While protocol or encoding is useful once a signal has been successfully received, it will not be of any use if the signal has been corrupted in the analog domain of free space. A system’s modulation method can also have an impact on its proximity. For example, in most simple AM/OOK systems, everything will be corrupt during overlapping high bit times. In an FM/FSK system, the receiver will lock onto the strongest signal and still provide usable output (assuming a reasonable differential between the two signals).
In some applications, where transmissions are infrequent and not of a critical nature, simply sending data redundantly with randomized breaks can allow the successful operation of multiple units. For applications requiring more reliable transfer, contention must be eliminated through either a sequenced network or through channelization. Either of these methods adds to system cost and complexity, but, when properly implemented, make it possible for the successful operation of multiple units without contention in the same environment.