Working on a little FM radio module, help me out with the details?

Early this year I mentioned i got a few Aliexpress kits based on the RDA5807FP - a full FM radio tuner on a chip. The kits cost 2 euros per and sound great.

I was thinking of turning the circuit into a module, and while researching, found out that they have a lot more features if you talk to them through i2c: people managed to get RDS data out of them, to display the name of the radio on a LCD. (In fact, it’s not clear how you can wire buttons directly to the pins and get default behavior out of it, the datasheet doesn’t mention it… but clearly my kit can do that).

One thing that leaves me stumped, especially since i have no experience with radio circuits: what’s the exact purpose of the inductors in my kit? Besides “they make the signal better possibly probably maybe”.

Pin 12 and 13 are the stereo output. Each goes through a 1uH inductor then a 100uF electrolytic in series, and the more i look it up, the less i understand what it’s supposed to achieve when it’s wired in series. The datasheet uses ferrites instead.
Pin 4 is “FM In”, i think think that means it uses the ground of the jack as an antenna, and the datasheet recommends using a LC choke.

My kit:

The datasheet:

So far, my thinking how to go about the project, stop me if i’m about to do something very stupid:

  • See if i can use a simpler variant of the RDA5807 chip depending on the features i want
  • Create the 3.3V rail my chip needs, using a switching regulator (i think a linear from 12V would get too BBQ with it if i ran the numbers correctly)
  • Add a common display module and a MCU that can run at 3.3V (ATtiny402 maybe?)
  • Use the existing RDA5807 Arduino library
  • See if i can program a behavior to deliberately get noise rather than a radio station, in addition to expected features
  • Tuning input that accepts 0V~5V so you can LFO your radio? (Would need a MCU with more pins probably, not a given the RDA5807 can do that at audible rates anyway)
  • Save last station to EEPROM
  • Resist the temptation to try to design a PCB trace antenna, try anyway, watch it fail embarrassingly forcing me into a plan B Update: Science says I’m gonna have to rule out this one unless I want a gigantic PCB
  • Merge the stereo to mono, amplify the output voltage to Eurorack levels, and protect the output
  • Add Blinkenlichten (important)
  • Add a S&H/T&H bonus circuit as the module is a good source of randomness
  • Infringe international copyright from the privacy of my own rack
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Coils (inductors) can serve multiple purposes. Here, they could form part of an LC filter or be used as a signal amplifier perhaps.
I’m looking forward to seeing what you do with this. Crack on!

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