Why does the FX429 generate interrupts for received data when there is no data present?
A. The FFSK receiver of the FX429 has a very high sensitivity and in the absence of valid signals 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 FFSK decoder’s bandwidth. To prevent spurious reception a baseband carrier detect flag (Rx CARRIER DETECT) in the Status Register is used to determine the validity of the received signal. The detector block comprises a comparator fed from two sources, one sourcing predominantly in-band (FFSK) energy through the decoders band-pass filter and the other sourcing wideband energy through a wider band-pass (noise) filter. The two levels are continuously compared so that when the in-band energy is significantly above the background noise energy, the Rx CARRIER DETECT flag is set high. To qualify the presence of FFSK, the Rx CARRIER DETECT flag is not set high immediately. The comparator output triggers a re-triggerable monostable whose time constant is de
