World's most problematic FM Transmitter...

[xitronics]

Hi everyone,

i hav attached a 1WATT Mono FM transmitter Schematic with this post. I have tried it on a breadboard and had also ordered for its PCB.

After connecting a 75 cm aerial, the DC power consumed by it is around 80-100 mW. Range is at max. about 10-15 meters. The tall claim of 1km range with 1WATT power o/p seems to like a man counting for no. of coconuts in a huge coconut tree from ground level.


The freq stability tops it all. I switch on the transmitter with the default freq tuned at 102.5 Mhz. After 5 mins i hear the audio in my receiver at 103.6 mhz ..!! I restart me transmitter, and now the freq is around 101.8 mhz and also hear the same reception at frequenies of 97 Mhz, 100 Mhz, 106 Mhz, 89.5 Mhz..............................

Your obvious answer would be try for another FM Tx kit and wish me best of luck for that. But if anyone points out the drawbacks of this ckt would be nice. So guys, here's the rough pic of the schematic.

Thnx.
________________________

[I_Forgot]

What is wrong with that circuit is that it has nothing to give it a stable output frequency. If you move your hand near it, it will shift.

If you want the thing to be tuneable and stable, you need a circuit that uses a PLL IC with a crystal reference to set and maintain a constant frequency.

There are a lot of FM transmitters being sold as accessories to mp3 players these days. I would look at modifying one of those.

I_F

[phase_accurate]

If it is put into a metal case, the ambient temperature is fairly stable and and the supply voltage is constant as well then it should be accurate enough for a TX of this kind.

What I don't understand is the reason why Q2 is connected the way it is. More logical would be an emitter follower or common-base.

Regards

Charles

[Nigel Goodwin]

Originally posted by phase_accurate

What I don't understand is the reason why Q2 is connected the way it is. More logical would be an emitter follower or common-base.

The circuit is drawn incorrectly, Q2 (a 2SC2570) is NPN (not PNP), so the collector should go to the coil at the top, and the emitter is the input from the oscillator stage. The stage is, of course, operating in common base.

Assuming he's not got his PCB yet?, I'm not suprised it's not working well - you can't build VHF circuits on bread-board, the massive stray capacitance will completely ruin the circuit.

[phase_accurate]

Q2 as common-base sounds logical. Maybe it should be biased as also.
For the output stage class C is O.K. but for the buffer I am not sure.

I built more than one quite similar TX using breadboard and they all worked O.K.

Regards

Charles

[moamps]

The circuit is drawn incorrectly, Q2 (a 2SC2570) is NPN (not PNP), so the collector should go to the coil at the top, and the emitter is the input from the oscillator stage. The stage is, of course, operating in common base.

With 22ohms base resistor?
It's CE in class C, IMO.

[phase_accurate]

Definitely common base in class C (if connected like mentioned by Nigel) !

Regards

Charles

[Nigel Goodwin]

Originally posted by moamps


With 22ohms base resistor?
It's CE in class C, IMO.

Yes, it could be drawn completely wrong, and be common emitter instead?.

[xitronics]

Originally posted by I_Forgot
What is wrong with that circuit is that it has nothing to give it a stable output frequency. If you move your hand near it, it will shift.

If you want the thing to be tuneable and stable, you need a circuit that uses a PLL IC with a crystal reference to set and maintain a constant frequency.

I_F

Hey can provide me some links from where, i could find a PLL FM Transmitter tuneable in 88-108 Mhz..?? The power o/p in the range 500mw - 1W, should be ideal for me to build.

Thnx in advance.

[lineup]

Harry's Homebrew
is a nice site with small easy build FM circuits and schematics
It is a wellknow site.

In this page, if you scroll a bit down to
Schematics of Transmitters, Oscillators, RF Amplifiers :
you find many links
Links for FM Transmitter Kits, Circuits, Electronics ...

Here is a nice FM circuit using crystal and transistors
with 250-800 mW output
http://geocities.com/ajpotts19/vhfnbfmtx.html

Even if I guess some (old) members here at diyaudio are good at Radio Freqency Circuits
I am afraid you will not find many topics about it