another FM transmitter project

[audioguru]

All FM radio stations have "pre-emphasis" that boosts treble sounds.
All FM radios have "de-emphasis" that cuts the boosted treble sounds back down to normal and cuts hiss at the same time.
A simple FM transmitter does not have pre-emphasis so it sounds muffled (like your stereo with its treble tone control turned all the way down) when heard on an FM radio.

Of course the transmitter must not be designed with high distortion which some simple circuits have.

Don't you want to transmit a stereo signal? Simple FM transmitters are just mono.
There were some excellent FM stereo transmitter ICs made about 10 years ago made by Rohm. I don't think they are available anymore.

 

[Willen]

Hi all.
I'm from Brazil and need some help, i tried to make some fm transmitters, but the sound quality is bad/terrible.
I will use on my friend farm, i need only a 60meters, maybe 100meters range on open grass.
Anyone know a good circuit, some 'easy' to find components, maybe a bc547 transmitter?
Tkz all and sorry for my bad english =/

ps: audio signal from a cd-player, not a eletret mic, and 12v 1A from a 110vac transformer.
Its Possible?

Hi Rodrigo,
Can make Audioguru's FM transmitter. I have made too. Look from Page-5 of this thread. There is a circuit of audioguru. If you want to transmit CD player's sound then you can remove its 1st stage known as pre-emphasis. 2n3904 is easy available transistor. Other components, you can find anywhere in old radios, TVs. You can make other transmitter having 2 stage- oscillator and amplifier, searching from google. It is not hard to transmit 100 meter but you should make RF circuit very very carefully! I am little beginner too and I have made almost 6 simple FM transmitters. And learning basically about RF electronics from experts. Best of luck!

 

[rodrigobad]

Hi Rodrigo,
Can make Audioguru's FM transmitter. I have made too. Look from Page-5 of this thread. There is a circuit of audioguru. If you want to transmit CD player's sound then you can remove its 1st stage known as pre-emphasis. 2n3904 is easy available transistor. Other components, you can find anywhere in old radios, TVs. You can make other transmitter having 2 stage- oscillator and amplifier, searching from google. It is not hard to transmit 100 meter but you should make RF circuit very very carefully! I am little beginner too and I have made almost 6 simple FM transmitters. And learning basically about RF electronics from experts. Best of luck!

Tkz man, here in brazil its realy hard to find 2n3904, but i think, bc547 can replace it. I will make that Audioguru's fm transmitter, tkz for reply.

 

[Nepaliman]

Tkz man, here in brazil its realy hard to find 2n3904, but i think, bc547 can replace it. I will make that Audioguru's fm transmitter, tkz for reply.

Yes! Look continuously.....this thread from page-5, audioguru had taught me a lot. Don't miss each discussion. In one post, audioguru said- BC547 will work almost same as 2N3904. You are lucky because you are from brazil. In town I hope there are lot of semiconductor manufracturers. Normally you cannot find such little unusal components in normal/general electronics shop.
My simple single transistor FM transmitter can transmit up to 100 meter at 9 voltage (in my less sensitive receiver- cell phone). You can do because you are experience.

 

[rodrigobad]

I can do this modification on "audio in" right?
Removed
Yesterday, i try to buy some trimmers, and the prices: 5 to 45pf = R$25,00 or U$12,50 and no one sells LM2931.

 

[Willen]

I can do this modification on "audio in" right?
**broken link removed**
Yesterday, i try to buy some trimmers, and the prices: 5 to 45pf = R$25,00 or U$12,50 and no one sells LM2931.

NO! Remove all the components before C3. Keep C3, it is your Audio Input. If there is no LM2931, buy LM78L05 or 7805. It will work because you have 12 V battrey. You can find trimmers in old Radios, TVs. If you feel hard to make this one, make single transistor FM transmitter. It can transmit 100 meter but friquency won't be stable.

 

[audioguru]

NO! Remove all the components before C3. Keep C3, it is your Audio Input.

Then the pre-emphasis will be removed and it will sound awful like your stereo with its treble tone control turned all the way down or like a telephone or AM radio.
Instead, use the entire circuit but add two or three resistors to mix stereo into mono and attenuate the input signal.

If there is no LM2931, buy LM78L05 or 7805. It will work because you have 12 V battery.

ALL semiconductor manufacturers make many 5V low-dropout voltage regulator ICs.

 

[Willen]

Then the pre-emphasis will be removed and it will sound awful like your stereo with its treble tone control turned all the way down or like a telephone or AM radio.
Instead, use the entire circuit but add two or three resistors to mix stereo into mono and attenuate the input signal.

Oh yes!!
I was just thinking about De-emphasis friquency without ''Pre-emphasis''. Now got simple solution! I will inject mono audio just removing a 27K resistor.

 

[Willen]

Is this colpit oscillator? And how this can give 200mW only at 9V? I think these kind of transistor has high gain.

 

[audioguru]

Is this colpit oscillator?

Yes, Google shows it as one form of a Colpitts oscillator.

How this can give 200mW only at 9V? I think these kind of transistor has high gain.

The BFR96s is a microwave transistor so its power gain at 100mHz is high. But its maximum allowed current is only 100mA so when it drives a 75 ohm antenna its maximum allowed output power is only about 200mW.

200mW into 75 ohms is an output voltage swing of 11V peak-to-peak. The maximum current is 5.5V/75 ohms= 73mA peak.

 

[Willen]

Yes, Google shows it as one form of a Colpitts oscillator.


The BFR96s is a microwave transistor so its power gain at 100mHz is high. But its maximum allowed current is only 100mA so when it drives a 75 ohm antenna its maximum allowed output power is only about 200mW.

200mW into 75 ohms is an output voltage swing of 11V peak-to-peak. The maximum current is 5.5V/75 ohms= 73mA peak.

In this type schematics, Is it possible to replace Varactor Diode with Trimmer? Or which type of modification is necessary? Any schematic?

 

[audioguru]

In this type schematics, Is it possible to replace Varactor Diode with Trimmer? Or which type of modification is necessary? Any schematic?

The varactor diode is used to modulate the frequency of the oscillator (for FM modulation) because the audio preamp varies the DC at the varactor diode which varies its capacitance. There is already a trimmer capacitor for fine tuning.
Usually the DC feeding the varactor diode is used for tuning the frequency but here it is fixed by 5.1V divided by R6 and R7.

 

[Willen]

Hi audioguru,
I made your Tx design on PCB its sound quality is amazing!!! 100%clear without any background noise at night. At night time it is 101% stable Wow!! But during day time it catches other stations with its friquency. I mean- 90 MHz is blank in my area so I tuned my FM receiver at 90MHz, it is really blank sound in receiver BUT when I turn ON my Tx at 90Mhz I received other stations in background as a noise with my Tx's friquency. And it's not stable during day time. Friquency is changing 0.1 to 0.2 MHz up-down. Why day makes such? My Tx circuit is in open without metal case. And I am using 75 ohm dipole with coax feed line.

 

[audioguru]

I think your radio circuit might be cheap and is overloaded by the nearby transmitter signal. Then other stations also overload the radio and they all cross-modulate and are heard in the background. A cheap FM radio circuit also has poor adjacent channel rejection and a poor capture ratio.

Clock radios and the FM scanning "radio" from The Dollar Store have very cheap circuits.
My home stereo and car radio have very good circuits that are never overloaded. They also have excellent sensitivity.

Anything that comes near the tuned LC circuit in the transmitter or moves away changes the stray capacitance which changes the frequency a little. A metal box connected to 0V will eliminate this problem.

 

[Willen]

I asked to many experts and at last I am asking to 'Audioguru' for confirmation, it really is or not:- Can this RF Amp produces 5 watt by feeding ONLY 100 or 200mW?? Is it good?
www.electronics-diy.com/electronic_schematic.php?id=1068

 

[audioguru]

The Japanese transistor operates in Class-C so it produces harmonic interference that might interrupt airplanes communications, police, ambulance, fire department or TV. The project says to filter the output (then the output power will be less).
Its datasheet shows it only at 175MHz but its power gain will be a little better at 100MHz.
It might need 500mW input for a filtered 5W output.

 

[Willen]

The Japanese transistor operates in Class-C so it produces harmonic interference that might interrupt airplanes communications, police, ambulance, fire department or TV. The project says to filter the output (then the output power will be less).
Its datasheet shows it only at 175MHz but its power gain will be a little better at 100MHz.
It might need 500mW input for a filtered 5W output.

I think I should have to connect some inductor and capacitor network as a filter. But I don't have accurate idea about it. Do you have any schematic? Or can I use this filter used on 30watt amp?

 

[audioguru]

The filter on the amplifier you found should work but it reduces the output power.

 

[Willen]

The filter on the amplifier you found should work but it reduces the output power.

Among 5 watt, how much power (almost) I can get through filter after power reduction?

 

[audioguru]

Among 5 watt, how much power (almost) I can get through filter after power reduction?

The resistance of the inductors reduces the output power. The resistance is reduced if the wire in the inductors is plated with pure silver.

The filter uses extremely simple LC circuits that also filter away some of the output frequency that you want.