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A 70cm FM receiver for the radio
Hardware Features:
High dynamic range RSSI (received signal strength indicator) of -136dBm to -54dBm, calibrated directly in dBm with look-up table in software.
Very good large-signal performance due to mechanical inter-digital filter, high level diode ring mixer and due to two crystal filters at the first IF frequency (image rejection is >100dB for both IFs)
Intelligent squelch circuit using active audio frequency band pass filters to evaluate the noise level at frequencies above the voice band.
Software Features:
Controller features an especially developed boot loader for remote updates of the software
Supply voltage, temperature, squelch-level and RSSI can be read out via RS232 interface
Software-Implemented real-time-clock using on of the AVR 8bit-counters in clear-on-compare-mode
Software-implemented 1750Hz tone recognition
The RSSI value can be requested by a DTMF tone ( *-key = code 11 ) and will be reported as CW code
Relay identification is send in CW code every full hour
Supply voltage, internal temperature, received signal strength and squelch level can be reported via RS232
Squelch functionality is implemented partly as software algorithm to prevent short interference spikes form opening the squelch. The squelch trigger level can be remotely adjusted for maximum sensitivity.
Relay can be remotely disabled via a secret DTMF tone sequence
Frequency Plot of the BFP420 low-noise amplifier
Frequency plot of the inter digital band pass filter
Demodulator RSSI dynamic range, read out by a 10bit ADC from the NE614 demodulator
