Why do the FX604 and FX614 apparently receive data when there is no input signal?
A. The FSK receiver of the FX604 and FX614 has a very high sensitivity and in the absence of a valid FSK signal will decode noise as data. This is not a fault but is due to the wideband properties of noise where sufficient energy may be present within the FSK decoder’s bandwidth. To prevent spurious reception a baseband carrier detect output (DET) is used to determine the validity of the received signal. The detector block is a simple energy detector that receives predominantly in-band energy from a band-pass filter in the device input. The DET should be monitored during reception and used to ‘intelligently’ decide whether the data is likely to valid, suspect or invalid. During periods when there is no data reception the DET will be predominantly low but is likely to false so the receiver should be powersaved where practical. DET does not internally gate the FSK receiver because this would prevent data reception during short duration events such as fades when data recovery may still be
