Have anybody Build that "4 Watt FM Transmitter " In projects section

[Shahriar]

Hi
I want to build an FM Transmitter to control two Realys. the range of the
transmitter should be about 700 meters. I selected 4 watt FM Tx from project section
I used PT2262 encoder to creat DATA. and schematic is as below. but some questions:"If you do not see any thing wrong"

1- Can I Omit those Inductors(L1,L2,...) and replace them with those Inductors which are similar to Resistors,
instead of building them myself? If yes/ how to calculate their values?

2- What does RFC exactly do?

3-Can we use IF as a RFC?

4- there is an Inductor in schematic which is created on PCB. I am building my own PCB. How to creat that?
Just draw it as it is?

Your help is appreciated in advance.
Shahriar

[Shahriar]

Here My schematic

[audioguru]

Hi Shahriar,
A 4W FM band transmitter is very illegal in my country and might be even more illegal in yours.

That transmitter doesn't have pre-emphasis. Are you using an ordinary radio to receive it? An ordinary radio has de-emphasis that might corrupt your data. What frequency is your data?

It is difficult to calculate inductor values at very high RF frequencies because stray capacitance makes up about half of the actual. You might have to purchase many different values and try them to find one that works. It is simple to make your own. The inductors that I made for my FM transmitter worked fine the 1st time.

RFC is Radio Frequency Choke. A value of 1 to 2 uH might work if its self-resonant frequency is high enough. An IF transformer might have too much inductance and capacitance.

If you make your own pcb that is different from the project, it might not work because yours will have different stray inductance, capacitance and interfering proximity between parts.

[Shahriar]

Dear Audiguru

Would you please give an explantion that what are Pre-emphasis & de-emphasis? The DATA frequency coming out of PT2262 is about 7-10Khz and its Amplitude is too big(nearly 10V) so I decreased it with a 50K volume.

2-what is the diference between RFC and an ordinary Inductor?
3-Is that PCB Inductor (L4 in main circuit) so important to cause a fail in the project?

ThanX - Shahriar

[audioguru]

Hi Shahriar,
It ain't easy to transmit with a simple transmitter.
1) Pre-emphasis is the treble boost that all radio and TV stations use. De-emphasis is the corresponding treble cut that radios and TVs use to reduce hiss and return the audio to normal sound. The audio corner frequency depends on where in the world, since the USA invented FM and chose a treble boost starting frequency of 2133Hz, and later the Europeans decided it was too much boost so selected 3.2KHz instead for them. Therefore if your radio is normal, it will cut the treble and any data pulses that are narrower than others.
The very high resistance of your volume control and data circuit will combine with the high value input capacitors of the circuit and additionally cut treble and your data.
2) An RFC is an ordinary inductor that has its resistance chosen to be low, its inductance chosen to be high and its self-resonant frequency to be much higher than its operating frequency.
3) L4 is very important to the circuit since it must be combined with the capacitor feeding it to make a series-tuned resonant circuit to pass the RF frequency. If they don't tune to the RF frequency then the circuit will have low output or no output. Its location and direction is chosen to prevent induced feedback from the other coils.