Hi,

I am trying to understand a circuit about a FM-radio. It is copied from: http://www.instructables.com/id/Simple-FM-Radio-sure-Works/
My drawing is attached (or : http://imgur.com/HkqR1dd ).

So far I figured out, that we have an LC-circuit, which oszillates at a specific frequency. The antenna catches a frequency from a radio station and if the two frequencys match, they add up. The first Transistor combined with the LC-circuit is an oscillator.
The germanium-diode is used to demodulate the signal.
The other transistors are used to amplify the signal.

I dont understand, (1) why the inductance is seperated in two parts and
(2) what the small circuit after the germanium-diode is used for. One capacity would decouple the two transistors, but why is there a resistance in parallel?

Thanks in advance,
Lukas

 

Your circuit is drawn wrong,!!! your antenna has a feed resistor, the germanium diode is on the Collector of Tr1, and is used as the AM demodulator feeding Tr2,3 audio amplifier. The inductor is a center tapped fed antenna with dc bias from the led across the variable resistor for tuning.

 

Also, The LED in the instructables article is backwards. As Dodgydave pointed out, you did not copy the original schematic faithfully.

To address your questions directly,

1) Per dodgy Dave, the two inductors are really a center-tapped coil.

2) Beats me. It doesn't really look like a conventional AM detector at all. R5 and C3 would go from the diode cathode to ground.

Personally, I would look around for a more conventional circuit.

 

From the looks of it that is a super regenerative receiver. When tuned (by C1) slightly above or below the carrier frequency it generates a modulated RF current in Q1. This is detected by the diode and coupled to Q2 by C3. I think there is a missing resistor in the emitter of Q1 (otherwise your detector diode is grounded) and R6 is used to bias Q2 into conduction.