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.