315MHz is primarily used for remote keyless entry (RKE) systems and garage door openers. As a result, this frequency is somewhat crowded, increasing the chances for interference. The FCC allowed power is lower than 418MHz or 433MHz and the selection and efficiency of antennas is limited.
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.
433.92MHz is primarily used for RKE applications in Europe. It is also a popular frequency for active RFID tags which can have a range of up to 1000 feet. It is not good for use in the US because of the chance of interference from amateur radio and the nearby pager band.
868 – 870MHz is an unlicensed band in Europe. The band is subdivided for different applications, but there are not many restrictions on the type or duration of data. Unlike the 902MHz-928MHz band in the US, there are only 2MHz to support many applications, so the band 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.
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 and FCC Title 47 for more information. A resource document containing a hard copy of the application notes ships with every Linx evaluation kit. You may obtain a hard copy of FCC Title 47 from your local government bookstore or from the Government Printing Office in Washington.
Putting a 220-ohm resistor between the 5 volt power supply and the module will drop the supply voltage and protect the module.
Why does the DATA line of the LR Series receiver seem to switch randomly when the transmitter is not on?
This is a result of the increased sensitivity of the LR receiver. The sensitivity is below the noise floor of the board, so it is picking up thermal noise and other random signals and outputting it as data. 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 will allow 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 will output a voltage that is relative to the strength of the received signal. Since the LR Series is On-Off-Keyed, this output will follow the data and look like a square wave, so it will be at a lower voltage when receiving a ‘0’ and at a higher voltage when receiving a ‘1’. D1, C1, and R1 form a peak detector that will follow the peak voltage of the ones.
This voltage is then 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 will release the output line. When the signal level falls below the reference voltage, the comparator will pull 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 used to control an analog switch that will pass the received data to whatever external circuitry the application requires. When the control line is high, the data gets passed, otherwise it is not connected. This means that when the received signal is greater than the threshold, the switch is closed and the data is passed. When laying out the board, it is a good idea to place the output of the analog switch close to the device that will be using the data. A long trace or wire here has the potential to couple AC noise onto the data line while it is squelched.
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.
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.