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 email@example.com. Also, be sure to check out our Wireless Made Simple blog for answers and updates to the latest innovations in the world of wireless.
Yes. The RSSI line is used as the input to the data slicer, which takes the noisy, low amplitude RSSI signal and creates the sharp digital data output. Loading the RSSI line too much can reduce the signal amplitude so that the data slicer cannot distinguish the signal from the system noise, resulting in garbled data output. Significant loading can kill the signal entirely.
The RSSI output cannot source much current. This is a design decision to keep total current consumption low. The result is that it cannot support much of a load before the output is affected. The output is designed to be connected to high impedance circuits, like digital gates or Analog to Digital Converters (ADC). Anything with lower impedance than these kinds of circuits can have an impact on the DATA output.
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.
There are a few different ways to manage a microstrip line that is larger than the pads.
It’s not uncommon for microstrip transmissions lines to be thicker than the connection points. In this case, the trace can be tapered at the module / connector pin. It should not have a large impact on impedance to have the trace tapered ~2mm at the connection points. Especially at the lower frequencies, in the 400MHz band.
The microstrip width can also be narrowed by putting the ground plane on an internal layer, reducing the height to the microstrip, resulting in a smaller trace width.
Another option is a Co-Planar Waveguide (CPWG), which allows for more precise control over impedance and smaller trace width. This adds a ground plane on the top layer, on either side of the antenna trace. The gap between the plane and trace can be varied to make the trace width smaller.
AppCAD is a free program that is useful to help with transmission line calculations.
There is no hard upper limit on the number of nodes in a HumPRO Series system since the module does not store a list of connected nodes. The practical limitations are
- Operating range
- Channel availability
If all nodes are listening to a single master, then the limitation is the time on channel to transmit messages. Since the HumPRO Series only transmits when it has data or an acknowledgement to send, that depends on the system’s transmission activity. Only one transmitter can be active and be heard. If there are multiple systems such that they can be using different hopping patterns (logical channels), then the amount of total traffic can be higher, but less than 6 (number of hopping patterns) times as high because of occasional collisions between hopping patterns.
A system having 100 to 200 nodes all hearing one another is possible, but would only work if the aggregate data rate is low. In general, you probably can’t get over about 50% utilization, due to packet overhead and delays between transmissions. A large system could be implemented using a repeater strategy implemented by the designer.
The modules that contain microcontrollers can become locked up if they are subjected to a brown-out condition. In this case, the supply voltage drops to well below the minimum limit, but not all the way to ground. Some circuits can remain slightly powered while others shut down, resulting in register and memory corruption. This can cause unpredictable operation, including lock-up.
This can happen under several conditions. If there is a lot of capacitance on the supply line, the turn-on and turn-off times can be slow. If it takes more than 50-75ms for the supply line to reach Vcc min during turn on or to reach ground during turn-off, then there could be a problem.
If there are devices in the circuit that pull more current than the power supply can source, then the supply voltage will sag and can cause a brown-out condition. This can happen toward the end of a battery life or if there is a solder short, ESD damage or other damage to the board.
If the module locks up frequently, then it is recommended to monitor the supply line as the device is operated and make sure that it is sourcing the necessary voltage and current to the radio. There are other conditions that can cause a lock-up, but they are much more rare. If the voltage supply looks good, contact Linx Technical Support to check other possibilities.
Yes, the wireless UART modules are designed to be connected to the UART of a microcontroller or similar device. It is not recommended to connect a UART to one of the transparent modules.
Transparent modules do not have any built-in protocol or memory, so all protocol aspects are managed by an external microcontroller. It is very simple to output data on a microcontroller’s UART and so many engineers try this. The issue is that the UART protocol was designed for a wired environment, which does not have much random noise. The RF environment is extremely noisy and UARTs cannot manage that noise well. It is recommended to develop a noise-tolerant protocol and to “bit-bang” an output on the microcontroller to send the data to the transparent modules. See AN-00160: Considerations for Sending Data over a Wireless Link for more information.
There are two primary entry points for ESD, the supply line and the antenna. These lines are commonly protected by Transient Voltage Suppressor (TVS) diodes and voltage dependent resistors called varistors connected between the signal line and ground. These function in different ways, but both become conductive when the voltage on the signal line rises above a threshold. During an ESD event, they shunt the current to ground, preventing it from getting into the radio.
When using an embedded antenna, this is generally not a concern. However, the line should be protected when using an external antenna. The transmitter output power will dictate the working voltage. The diode or varistor should be low capacitance so as to not affect the RF tuning. There are many devices marketed for antenna protection that meet this requirement.
The power supply line is more general because it does not have the low capacitance requirement. It is important to realize that ESD can travel through voltage regulators and bypass capacitors with enough energy to damage the radio. It is a good idea to put a TVS diode on the power line of the module even if there are other components between the module and the power input.
Yes, but it takes some work. The transparent modules are generally the physical radio only, so a mesh networking protocol can be used in an external microcontroller. The wireless UART modules offer some additional features and can act as the PHY and MAC layers. An external microcontroller can be used for the mesh networking logic.
There are many ways to make a mesh network; some fairly simple and others are very complex. Creating a mesh network can involve significant software and firmware development and require strong experience with wireless communications. For this reason, we generally recommend choosing a product with mesh networking integrated if it is determined that a mesh network is really needed.
Most of the wireless UART modules support the data structures of RS-232 and RS-485. None of our modules support the voltage levels required, so an external driver chip is needed. Some external circuitry may be needed to drive the RS-485 address line.
The QS Series USB module does support RS-485 for conversion to USB.
We do not endorse the use of any potting compound. They have the potential to affect inner component coupling, antenna match and other factors, especially for modules that do not have a lid.
That said, many customers do pot the modules with success. While we do not recommend any specific compounds, be sure to use one that is free of conductive materials or heavy in carbon. If the module does not have a plastic or metal lid, then it is a good idea to cover it and provide an air gap around the RF circuitry. Potting impacts not only the module, but also the antenna. Consideration should be given to the antenna traces and connections, especially if using a PCB or embedded antenna. There is a strong likelihood that the antenna will need to be re-tuned to accommodate the affects of the potting compound.
Regardless of the material, plenty of time should be allotted in the development schedule to test various materials and make sure that the potted radios still meet the product requirements.
Does Linx have sockets for the surface mount modules so that they can be mounted onto a protoboard without being soldered?
We do not recommend the use of the modules on proto boards or solderless breadboards because they result in horrible RF performance. For this reason, we do not offer or recommend the use of sockets. The wires used on proto boards and sockets themselves have inductance and capacitance associated with them and their connections. These ‘parasitics’ have a larger impact as the frequency increases, so at RF frequencies, they are major players in the final performance of the system. Placing the modules on proto boards severely affects their range and performance and potentially gives an impression that the modules are bad. The modules should be soldered to a PCB with a large, solid ground plane either on an inner layer or on the bottom side of the PCB. For prototyping we recommend the purchase of one of our evaluation kits. These kits include a PCB that has been designed to optimize the RF performance of the modules and includes a prototyping area where additional circuitry can be added to aid the integration of our modules into your design. You can find additional information about the evaluation kits on our website and they can be purchased from us directly or from our distributors.
It is a simple task to control relays using Linx RF modules and remote control encoders and decoders. The MS Series, MT Series and HS Series decoders can provide enough current to drive many small relays directly. If a higher voltage or drive current is required, a switching transistor can be used as a buffer.
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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.
Every year, Linx Technologies supports a number of projects and educational institutions with samples of modules, antennas and evaluation kits. These allocations are decided upon in the beginning of the year, and, unfortunately, we cannot support all the requests for donations of products and samples we receive. If you would like to submit a comprehensive outline of your project, a list of companies involved, and the proposed benefit, it will be considered as allocations are available.
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.
We are often asked if one can extend the range of Linx RF Modules. This depends on the radio module. The 300 and 400MHz radios (LR Series, LT Series, KH3 Series) as well as the narrowband 868 and 900MHz radios (NT Series) are capable of achieving the maximum output power allowed for unlicensed operation under government regulations (FCC Part 15, Industry Canada, Europe’s ETSI). In most cases, increasing the output power of the modules would make them illegal. It is possible to add an LNA onto the radios, but that would result in a marginal range increase. A higher gain antenna is also an option on the receiver. The transmitter output power will likely need to be reduced by the same gain added by the antenna, making it pointless on the transmitter.
The 900MHz FHSS radios like the HumPRO Series, HumRC Series, HumPRC Series and TT Series can use an amplifier to increase the transmitter output power. In fact, we added an amplifier to the HumPRO Series to make the Amplified HumPRO Series in order to increase the range. Adding another amplifier to these will run afoul of government regulations. A high gain antenna can also be used with these, but the transmitter output power will need to be reduced.
There are several factors to consider when selecting the best module for your application. They include the usual suspects (price, power consumption, range, size, ease of implementation, inter-operability) as well as legal aspects (which frequency can be used in which country).
Describing all of the consideration would lead to a very long post, so here are some basic items to get started. For applications that need to send data between microcontrollers, look at our Wireless UART RF modules. These have a simple UART serial interface to pass data between the microcontroller and the module. The modules handle all of the aspects of wirelessly sending the data.
For applications that have non-standard data structures or critical timing constraints, look at our Transparent RF Modules. These are typically the physical radio only, with no microcontrollers or memory. There is a 1-wire interface that directly controls the RF carrier. This allows for the creation of custom protocols and has a very consistent latency from transmitter input to receiver output. The consistent latency means the module does not get in the way for timing critical systems.
For applications that need to activate a device remotely, look at our Remote Control RF Modules. These are fully integrated remote control solutions. Take a line high on one side (generally with a button) and a corresponding line goes high on the remote side to activate whatever circuits are required by the application.
This provides a starting point on selecting the best solution for the application. Contact Linx Technical Support for a more detailed discussion of your application. We will guide you to the best product and answer any questions you may have.
This is a concern when using Linx transparent radio modules with OOK modulation in the 260 to 470MHz band.
Part 15 Section 231 governs operation in the 260 – 470MHz band. In these frequencies the FCC has decided to restrict the type and duration of data that can be transmitted but has allowed for exceptions as well. Keyed carrier signals such as those produced by Linx OOK products can be averaged over a 100ms time, allowing a significantly higher potential output power. The high-bit-to-low-bit ratio of the data will thus have a direct impact on allowed power and thus range.
In the case of the 902 – 928MHz band, the FCC has only specified the output power and harmonic levels. There are no restrictions on the type or duration of data that can be sent. This gives the design engineer a great deal of freedom in the possible applications, but also results in the band being more crowded. Additionally, the band legally allows for the operation of high power spread spectrum devices. This means that a good protocol and a more robust modulation method, such as FM/FSK, is needed to try and ensure data integrity.
For more information, review the FCC Resource Document. A hard copy of FCC Title 47 can be obtained from your local government bookstore or from the Government Printing Office in Washington.
In some cases, yes. Some Linx modules (particularly the 300MHz and 400MHz products) are capable of achieving the maximum transmitter output power per FCC Part 15 and other regulatory agencies worldwide. Concentrating the power in a particular direction would, in most cases, result in illegal power levels and require the transmitter to be attenuated. This would negate the advantage of the high gain antenna to get more range. Using a high gain antenna on the receiver is a way to improve range with much less concern about regulatory issues.
For most of the 900MHz FHSS products, a higher gain antenna can be used to get more range. However, none of the modules have been certified with high gain antennas, so the final system will need to get tested and certified.
For questions on a specific product, please contact us.
Range is determined by a number of factors, including transmitter output power, receiver sensitivity, antenna gain and the operating environment. The ranges listed here are what we have been able to achieve using our development kits in open areas with straight line-of-sight between the antennas. It is possible to get more or less in specific environments.
The Amplified HumPRO Series can get around 8 miles (12.9km) with a good antenna (like the HWR Series) and line-of-sight. This module is great for general data transmissions.
The TT Series is designed for remote control applications and can get over 2 miles (3.2km).
The LR Series and LT Series can get around 3,000 feet (900m) and are primarily used for basic remote control but also for shorter range and lower power data transfer. Also in this same range, the KH3 Series is for remote control and the NT Series is for data transfer.
The 2.4GHz version of the HumRC Series is about 300 feet (90m) for remote control applications.
Why does the DATA line of the LR Series receiver seem to switch randomly when the transmitter is not on?
This happens because the module does not have any squelch built in. A squelch circuit stops the data output when the received signal strength falls below a certain level. While this is beneficial from a data standpoint, it does reduce the sensitivity of the receiver and the range of the system. Without the circuit, the receiver loses lock in a few milliseconds and outputs random bits based on noise in the system.
This is generally not a problem for off-the-shelf decoders and can be resolved in software for custom microcontrollers (see application note AN-00160 for protocol recommendations), but an external squelch circuit can also be used. Using a squelch circuit allows the designer to only allow data when the received signal is above a certain threshold, but it will sacrifice the range. This allows the user to make the tradeoff between random noise and range.
The RSSI line outputs a voltage that is relative to the strength of the received signal. Since the LR Series is On-Off-Keyed, this output follows the data and looks like a series of pulses. The output is at a lower voltage when receiving a ‘0’ and at a higher voltage when receiving a ‘1’. The amplitude of the ones determines the received signal strength.
An example squelch circuit for use with the LR Series receiver is shown below.
D1, C1, and R1 form a peak detector that follows the peak voltage of the ones. This voltage is fed into the non-inverting input of a comparator where it is compared to a reference level set by a potentiometer. When the signal level becomes greater than the reference voltage set by the potentiometer, the comparator releases the output line. When the signal level falls below the reference voltage, the comparator pulls the output line to ground. Most comparators have open collector outputs, meaning that they can only pull the line to ground or release it. They cannot pull the line high, so a weak external pull-up resistor (R3) is needed to pull the line to Vcc when the comparator releases it. The feedback resistor (R4) is used to stabilize the output.
The output of the comparator is fed into the non-inverting input of a second comparator. The DATA output of the receiver is also connected to this input through resistor R4. The inverting input is connected to a voltage divider that sets the reference voltage at half of the supply voltage. As with the first circuit, R8 provides a weak pull-up and R7 provides feedback.
When the first comparator pulls its output low, the non-inverting input of the second comparator is low and the output of the second comparator is pulled low. When the first comparator releases its output, the DATA line drives the non-inverting input on the second comparator. The output of the second comparator then follows the DATA line.
A discreet voltage divider or a voltage reference IC can be used in place of the potentiometer, and the values for C1 and R1 can be adjusted to tune the response as needed.
Linx offers radio modules in several frequency bands targeted for different applications and different geographical regions.
315MHz is a common frequency for RKE and garage door systems. As a result, this frequency is somewhat crowded, increasing the chances for interference. Also, the antenna size gets larger, or the efficiency decreases for the same size antenna.
418MHz is a good frequency to use in the US as it is not very crowded. This gives the least likely chance for interference and therefore the best performance. This is the standard recommended frequency for North America.
433.92MHz is almost a global frequency, primarily due to automotive key fobs. This makes it a good frequency for products that will be marketed in multiple countries since only one product needs to be built. However, this makes the frequency popular and more crowded than 418MHz, increasing the chance for interference.
863 – 870MHz is an unlicensed band in Europe. The band is subdivided for different applications, and has restrictions on the amount of transmitter on time per hour based on which sub-band is being used. The band has only 2MHz to support many applications, so it has become somewhat crowded.
902 – 928MHz is more versatile than the 260 – 470MHz band in the US because the FCC has only specified the output power and harmonic levels. There are no restrictions on the type or duration of data that can be sent. This gives the design engineer a great deal of freedom in the possible applications, but also results in the band being more crowded. A disadvantage for cost-sensitive applications is that 900MHz modules are typically more expensive due to the more complex filtering and modulation required for link reliability at these higher frequencies.
2400 – 2483.5MHz is a true global frequency. Almost all developed nations have adopted the same regulations and requirements for this band. This was driven by applications such as Wi-Fi and Bluetooth but been adopted by many more applications. This means that it is possible to make one product and sell it into many countries, though the product does still need to get certified in each country or region. This has made the band extremely popular and extremely crowded. In addition, the range of these systems is much shorter and the signal penetration through walls and obstructions is much worse at these frequencies than at lower ones.
5725 – 5875MHz has not been adopted by many applications outside of Wi-Fi. This band is generally covered under the same rules as the 2400MHz band. There are not many products in this band. Currently, Linx does not offer any radios in these frequencies.
Review AN-00125: Considerations for Operation within the 260 – 470MHz Band, AN-00126: Considerations for Operation within the 902 – 928MHz Band, AN-00128: Data and bidirectional Transmissions under Part 15.231, FCC Resource Document and FCC Title 47 for more information. You may obtain a hard copy of FCC Title 47 from your local government bookstore or from the Government Printing Office in Washington.
Most of the FM transmitters and receivers have a constant current consumption when in use, so placing a resistor between the between the 5-volt power supply and the module drops the supply voltage and protects the module. This can also be done with our OOK receivers, such as the LR Series. However, the current consumption of the OOK transmitters varies with the data bit, so they cannot be used with a dropping resistor.
This does have potential risks. The current consumption of the modules is normally steady, but it can change depending on how the modules are connected. It is best to measure the current consumption in the final design and size the dropping resistor appropriately.
The receivers output data at the lower voltage. The microcontroller or device that is reading the data needs to be able to recognize the lower voltage or there needs to be some sort of voltage level translation between the module and microcontroller.
While this does work, it is always safest to operate the modules with a regulated voltage supply at the appropriate level.
What you really want is a better receiver, not a transmitter. The FCC limits the output power of all transmitters to about 0dBm (varies with frequency and modulation) in the ISM band, so you can’t get a more powerful transmitter and still be legal. The only place for improvement is on the receive side. This is exactly what gives the LR Series its exceptional range: a receiver that is potentially up to 20dB more sensitive than our previous line, the LC Series. With a doubling of range with every 6dB of sensitivity gained, the LR provides a significant improvement in range over the LC Series.