I'm setting out to design a module for integration into a larger [wired] building management system that will listen on a fixed frequency (or better yet, cycle through a pre-defined batch of frequencies) in the NOAA weather radio band (162.400 MHz - 162.550 MHz in 25 kHz increments). The intention is to monitor for severe weather events and alert when a Specific Area Message Encoding (SAME) broadcast (i.e. emergency alert system) transmission is received, and is assessed to contain an alert containing a predefined location code. The message decoding and processing is to occur further down the signal path and is outside the scope of this question.
After scanning through the product offerings, it appears to me that the ADF7020-1 would be a great application for this, since it includes a built-in FSK decoder and can actually offload some processing power by pre-processing the header and raising an interrupt when an alert is incoming. However, I have a few questions pertaining to this device:
- Is this the most appropriate device? I know that's a philosophical question, as I could always design my own superhet receiver and use a separate FSK decoder, but that's a lot of circuitry that this device all handles internally, and for ~$5, it's hard to pass up the simplicity.
- Obviously, I wouldn't want to transmit in this band at all, so I would be completely disabling the transmit power amplifier via software and will just be linking up the antenna to the RFIN/RFINB pins. Do I still need to provide a matching impedance on the PA_OUT pin to avoid cooking the power amp (or providing spurious emissions), or is the amp truly "powered down" when the appropriate software bits (PE1 in register 2, and/or TR1 in register 0) are configured? Can I just provide an arbitrary low-impedance path to ground on this pin, or does it need to have complex impedance to match the internal circuitry at the center frequency?
- I've tried to analyze this design in ADIsimSRD Design Studio, it crashes at the "frequency wizard" page. Every. Single. Time. I've tried numerous combinations of XTAL frequencies, bitrates, etc., all to no avail. Crashes every time. No error message. The data sheet suggests you can use ADIsimPLL to design the loop filters, but I think this is a typo and they probably meant ADIsimSRD.
Anyone have any suggestions? Also, some pertinent information:
Data bit rate: 520.83 bps
Logic zero: 1562.5 Hz
Logic one: 2083.3 Hz
Channel Frequencies: 162.550 MHz, 162.400 MHz, 162.475 MHz, 162.425 MHz, 162.450 MHz, 162.500 MHz, 162.525 MHz
Signals are transmitted on a single frequency, but a single area may receive signals from several in-range stations; environmental conditions or a service outage may make the preferred station unavailable, so having the ability to switch to a backup station programmatically would be a nice feature.
Thanks in advance for any advice. I tried leveraging the evaluation boards for this chip for inspiration, but the suggested values in AN-859 stop at 150 MHz and don't pick up again until 310 MHz, so they don't cover the band I'm interested in, and with ADIsimSRD not behaving for me, I'm worried I won't get the matching network or loop filter tuned right.