Help with FM Transmitter

[theimperia]

All righty, got it to work. The problem was dumb, I feel terrible even telling it. I forgot to connect the feedback capacitor (on the oscillator stage) to the emitter. I found it out by connecting almost ANY wire to the emitter and causing much better reception.
Also, the output (using the RF probe circuit, with a diode and 2 caps), was about 10-30mV. But after connecting that cap, it shooted up to 6V on DMM!!!

Heh, somehow, when I'm near the transmitter, the sound is heard in 3 places on the dial (digital FM receiver from the MP3 player). But when I go away, it only appears at 1 place. I guess the harmonics are very powerfull near the xmitter, but fade out fast with the distance.

Thanks ALOT audioguru!!!

TI|CP

 

[audioguru]

Your radio is overloading when it is close to your transmitter, causing it to appear at phantom locations on the dial. A really cheap radio has its entire dial covered by a near transmitter.

 

[mstechca]

...

Hi, Mastertech, I'm your evil twin LOL j/k :twisted:

 

[igrecotrei]

Hey guys, how about a FM TX controlled by a Xtal? An Xtal controlled oscillator let's say on 33MHz with a tripler up to 99MHz. Has anyone such a schematics?

 

[Nigel Goodwin]

Hey guys, how about a FM TX controlled by a Xtal? An Xtal controlled oscillator let's say on 33MHz with a tripler up to 99MHz. Has anyone such a schematics?

The problem is providing wideband FM modulation, 2m amateur radio transmitters commonly used 12MHz crystals multiplied 12 times to give NBFM (Narrow Band FM) - you can't frequency shift a crystal enough for wideband (unless you started at a really silly low frequency).

The crystal method usually uses a PLL, with the modulation applied to the phase lock system - there was a circuit (or link) for such a circuit posted a few months ago?.

 

[igrecotrei]

Nigel, if you use a 12MHz crystal in order to get the 12th harmonics, with a 1kHz shift you could get a 12kHz dynamic transmitter, and this isn't narrow at all. Am I wrong? Anyway I intend to build my TX for the ham bands, since I am a ham. I was interested to use a crystal because a VFO isn't stable at all and PLL would be to large.

 

[audioguru]

A total bandwidth of only 24KHz is much too narrow for hi-fi stereo FM and wouldn't be loud enough, but fine for telephone quality narrow-band.

 

[Nigel Goodwin]

Nigel, if you use a 12MHz crystal in order to get the 12th harmonics, with a 1kHz shift you could get a 12kHz dynamic transmitter, and this isn't narrow at all.

That is the spec for NBFM - and it's very narrow!, I understand 2m FM now uses 12.5KHz channel spacing?, previously the spec was 25KHz channel spacing (also NBFM). WBFM (Wide Band FM) used for commercial FM broadcasting uses something like 150KHz bandwidth?.

You also never mentioned that you were asking about amateur radio transmissions!, the thread is about WBFM on the broadcast band!.

If you're looking to build a 2m crystal controlled FM transmitter there should be plenty of projects on the net, and in the radio ham books - try a google search!.

 

[igrecotrei]

It's true I haven't mentioned that I intend to use my TX on ham bands. It's that I thought, the AF band would be enough. Now I can see that 12kHz are not WBFM. But frankly said I looked many times in the internet, used google as a search engine but found no project regarding a TX driven by a crystal. Would you be so kind Nigel and show me some examples in the internet?

 

[Nigel Goodwin]

It's true I haven't mentioned that I intend to use my TX on ham bands. It's that I thought, the AF band would be enough. Now I can see that 12kHz are not WBFM. But frankly said I looked many times in the internet, used google as a search engine but found no project regarding a TX driven by a crystal. Would you be so kind Nigel and show me some examples in the internet?

You might try

But, as you say, there seems a definate lack of 2m amateur radio projects on the internet - there used to be loads of crystal controlled 2M designs about before the internet days :lol:

If you want a 2m transceiver, I've got a 25W Kenwood one sat at home doing nothing!.

 

[igrecotrei]

Thank you Nigel, you are very kind! I will try to make that tx. Regarding your offer: thanks again, I have on of my own too. But I have the stupid pleasure of homebrewing.