I have built a 7- transistor FM transmitter.
http://forum.allaboutcircuits.com/blog.php?b=517
Original schematics: http://cappels.org/dproj/FMXMTR/fmxmtr.htm
This schematic does NOT work- but I guessed that the 330R and 1K resistors need to be adjustable. Indeed the operational point is only within a very small range!
Also I can only get it working at the very end of the FM scale. The coil is not so much a problem- it is easily modifyable by pulling/stretching it.
There is still a lot of noise. As well the gain is a problem (oversaturation).
But it works now, even if at first, there seemed to be little hope. It is even stable!
The antenna also did not work, until I had the idea to insert a piece of wire inside the antenna loop, and feed it back into the 1K adjuster for the output transistor (weird isn't it?).
If you have any suggestions, or ideas for a better FM transmitter?
Also the 2n3904 varactors don't seem to work very well (but I can use the intended frequency adjust to improve the signal).
Maybe the FM band is very crowded here (large city)? I had to compress the coil a lot, and even so, it only works at 108 MHz. I get a weaker signal within the FM spectrum, but that's not useable.
Pictures + demo video see the blog entry!! OK here two more.
Recorded the microphonic noise as well: http://youtu.be/psnYl2Y9T6g
http://forum.allaboutcircuits.com/blog.php?b=517
Original schematics: http://cappels.org/dproj/FMXMTR/fmxmtr.htm
This schematic does NOT work- but I guessed that the 330R and 1K resistors need to be adjustable. Indeed the operational point is only within a very small range!
Also I can only get it working at the very end of the FM scale. The coil is not so much a problem- it is easily modifyable by pulling/stretching it.
There is still a lot of noise. As well the gain is a problem (oversaturation).
But it works now, even if at first, there seemed to be little hope. It is even stable!
The antenna also did not work, until I had the idea to insert a piece of wire inside the antenna loop, and feed it back into the 1K adjuster for the output transistor (weird isn't it?).
If you have any suggestions, or ideas for a better FM transmitter?
Also the 2n3904 varactors don't seem to work very well (but I can use the intended frequency adjust to improve the signal).
Maybe the FM band is very crowded here (large city)? I had to compress the coil a lot, and even so, it only works at 108 MHz. I get a weaker signal within the FM spectrum, but that's not useable.
Pictures + demo video see the blog entry!! OK here two more.
Recorded the microphonic noise as well: http://youtu.be/psnYl2Y9T6g
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