The FET is a mixer down-converter that mixes the input RF with the crystal frequency to make a lower IF frequency.
The transformer matches the high impedance from the FET to the low input impedance of the IC. It might be tuned but it doesn't appear to have capacitors. It also provides DC isolation.
The IC isn't an opamp. Opamps have two inputs. Look at its datasheet- it is a complete AM radio. It amplifies the IF frequency and AM detects its modulation.
To be more exact, it's a complete TRF AM receiver, in this case being used as an IF amplifier, in conjunction with an external mixer/oscillator to make a superhet.
Isn't the ZN414 also long obselete?, but I seem to remember a new version - perhaps the ZN424?.
i search through the article that introduces this circuit, but i cant find the specs of the IFT. they just mentioned that it is a 470KHZ IFT. thats all... so i am stuck here..
any idea what is that? or is there anything i can find that can replace the transformer?
i search through the article that introduces this circuit, but i cant find the specs of the IFT. they just mentioned that it is a 470KHZ IFT. thats all... so i am stuck here..
any idea what is that? or is there anything i can find that can replace the transformer?
Hi pouchito
the issues are
1. IF transformer -tuned --yes-- there is an internal capacitor-the schematic did not show it. it is parallel to primary (full einding)
2. IFT frequency-- generally 455KHz-- but you can tune it to 470also by the ferrite screw over there. -- the core has to come out, say few degrees anticlockwise.-- if not the tone decoder can perphaps be worked at 455KHz.
3. ZN414-- availability-- already HERO999 suggested later version . MK484--even if we find -how you get and at what cost? Perhaps with little more effort you can locate a piece or two ,sometime even from scrap. if bulk is needed definitly some like digikey,farnel etc will supply