I am trying to use RDA5807M to build a radio. My circuit is basically the same as the reference design in data sheet. enter image description here

I know the design uses the GND of earphone as antenna. But I don't understand the function of F1, F2, C1 and L1. I chooses to remove them from my circuit and the circuit works well. What if I put them back? Will the antenna work more efficiently?

Another question, why the LC tank consists of C1 and L1 is 107MHZ?


1 Answer 1


F1 and F2 are inductors that are designed to have relatively high impedance at VHF .This is to stop valuable RF signal being wasted .It also stops RF getting into the AF output pins of the chip .If you went close to a transmitter tower strong RF could cause distortion ,Instability ,Malfunction of the chip.L1 and C1 form a tuned circuit that broadly accepts signals in the FM band and attenuates out of band RF energy .This is a simple receiver and one low Q tuned circuit is better than nothing .If it was not present the chip would have to deal with pretty much all RF signals and overload would be more likely. L1 C1 is normally tuned midband when you have a fixed tuned circuit .You might want to make C1 variable and peak it on your favourite station .The surrounding circuit does have some detuning effect .The overallparasitic capacitance tunes it slightly lower .


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