Thursday, September 03, 2009

The good and the bad news on the airband receiver

The bad news first.... my Si570 kit is broken... not completely but can't see problem with it, probably the chip is dead.

The good news... I made an ne602 vhf vfo to temporary replace the kit with an air coil and and some more components and can finally have acceptable reception on "Volmet" frequency also changed the mixer to an ne602 and the AM demodulator to an TCA440 that I had bough some 6 years ago and never had used it :)

Will post some pictures the next days since right now you will only see a bunch of wires connecting the boards and the vfo is not as stable as I want.

I am quite sure my previous unsuccessful attempts were some how related to poor construction technic and bad IF selection.... the frequency I was interested was 126.400Mhz, if you put an IF of 10.7 one of the products will be 113.7 and the other will be 105Mhz... right on top of a local FM broadcast frequency... now the IF is 455Khz.

Design is based on this one at: http://hem.passagen.se/communication/pic/118sch.gif
except my rf preanp and vfo is diferent and there is a resistor and capacitor from ne602 to 455khz filter as per ne602 datasheet.

Here is the video. First image is the audio amp/speaker in the box then moving the camera do the left the rf preamp and vfo/mixer and more to the left board the tca440 demodulator, the big blue cap between pcb boards gets the IF output from the ne602. In the middle of the video you can see my finger de-tuning the vfo coil.


video

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