FM or AM transmitter, choosing the right one.

[Jarod_C]

I was looking to build a good FM or AM transmitter. I would think AM would be easier, but I've only made one transmitter and it was an FM transmitter that of course didn't work. But if you look at the bright side the LED did come on. Sense then I've got some better equipment, and a little bit more knowledge. Ok the second part might not be true. I found the transmitter in the picture HERE online, and should be in the range of 100MHz, but I don't know anything about modulation. So, what I really wanted to know is, If I spend the time building this, and doing perfect will it be worth my time? Or does anyone know any other transmitters that work good, that are somewhat simple. By simple I mean, no microcontrollers, lots of gates etc. Range doesn't matter that much, but I would like good Q. I wan't to use it for an Ipod trasmitter, and other things like that for my car.

Thanks for any help.

 

[blueroomelectronics]

You do know a Stereo iPod FM transmitters are common and cheaper than you could build it for. An AM one would be mono and poor quality.
FM Stereo Transmitter For Car

$7.99

 

[Jarod_C]

Yeah, they're pretty cheap. I'd like to make my own for many reasons. Experience, Multiple inputs, etc. I never made one(that works) and just want to try it out. Might as well do a really nice one while I'm at it. I'm going to make three input types with a nice audio filter, also mutiple power supply types, 12V, 9V, and 120VAC. I could do all that with a store bought one, but where's the fun, and gratitude.

I always see websites with lots of transmitter designs that are pretty similar. Most the cheap premade ones aren't any better then a 2 dollar homemade one. I don't really want one of these that bad, it comes down to just wanting to have a better understanding of how, and why it works. That might give me a jump for next term in school too, if not, it would still be neat.

Edit: is Blue Room Electronics your site? I've been eyeballing the mongoose kit for my son when he gets older.

 

[blueroomelectronics]

Yep, blueroomelectronics is my site

If you want Stereo FM you'll need an IC, not my field but audioguru might be able to help.

 

[audioguru]

Hi Jarod,
The FM transmitter you built is missing some very important parts.
The output transistor is missing an emitter resistor and the tuning and modulating diode is missing an adjustable bias voltage that tunes the frequency.

If it is fixed then it would sound lousy anyway.
Its frequency would drift if anything got near it or moved away, its frequency would drift as the supply voltage changes, it is mono not stereo and it is missing the pre-emphasis (treble boost) that all FM stations have, so it would sound like your stereo with its treble tone control turned all the way down, like an AM radio.

 

[blueroomelectronics]

It's only a mono design isn't it. How would you make a stereo FM transmitter?

 

[Jarod_C]

I don't have any idea how to transmit stereo. The input schematic I designed is for mono. If I could do stereo even better. Do you know what kind of IC you would need for that? I have three old radios, a CB, and a TV I can salvage from, or I guess I can buy something pfff.

I said earlier that I made a FM transmitter that didn't work. I just remembered I made a small amp for my guitar, and I listened to an AG Report on an AM channel. So I guess I've made a receiver

 

[Jarod_C]

Hi Jarod,
The FM transmitter you built is missing some very important parts.
The output transistor is missing an emitter resistor and the tuning and modulating diode is missing an adjustable bias voltage that tunes the frequency.

If it is fixed then it would sound lousy anyway.
Its frequency would drift if anything got near it or moved away, its frequency would drift as the supply voltage changes, it is mono not stereo and it is missing the pre-emphasis (treble boost) that all FM stations have, so it would sound like your stereo with its treble tone control turned all the way down, like an AM radio.

I didn't design the transmitter, only the parts not shown in the schematic. Just the input and voltage supplys(not shown) If I designed it would have been bought by NASA by now. Well if they like amachure transmitters that don't work.

I knew that it was mono, and that's ok, but I would like it to be stereo if it wouldn't be real difficult. I'm still new to the subject, what's the best way to pervent drift?, and where does the pre-emphasis go? Also can you recommend any schematics that work well.

Thanks.

 

[crutschow]

DC Electronics - DC KITS Home sells several FM transmitter kits. They use an NJM2035 chip to generate the stereo signal for modulating the FM carrier.

I build their $29.95 Stereocaster kit some time back. The only problem with it was that it overmodulated the carrier which gave a distorted signal. I had to increase the R9 modulator drive resistor from 4.7k to 220k to get proper modulation. With that mod it worked fine. I used it with a pocket radio as a wireless headphone.

 

[Jarod_C]

Thanks, that site is in the favorites now. Did you notice that a few of their pictures were mixed around from the description.

 

[audioguru]

The NJM2035 stereo generator IC is pretty old. It was used with some pretty old FM transmitter ICs.
Now Rohm has made a line of FM stereo transmitter ICs that use a crystal with a frequency synthesizer circuit that works very well. Most little MP3 to car radio FM transmitters use them. A few years ago Silicon Chip magazine made a project with one called The Micromitter.