Why does the CMX469A generate interrupts for received data when there is no data present?
A. The FFSK receiver of the CMX469A 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 output (CARRIER DETECT O/P) 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 CARRIER DETECT O/P is set high. To qualify the presence of FFSK, the CARRIER DETECT O/P is not set high immediately. The comparator output triggers a re-triggerable monostable whose time constant is determined by an external RC
