Audioguru's FM Tx Troubleshooting

[ckchew8]

I see, do you have an original post or link to your own final design which i can refer to? i tried googling but the results are all forum discussion...actually my intention for this project is to try and transmit my voice for educational purposes only, so mono is fine, even if it's stereo, i found a solution by you (AGAIN!) which mixes and attenuates the source from an audio output of a computer into the FM transmitter...just out of curiosity, i managed to find a list all the radio stations in my country, i'm pretty sure they have a guard band between stations or channels, is your circuit's transmitting frequency stable enough to use the guard band frequency to transmit (just for educational purposes) as in giving good stability without using any crystal oscillator circuits like you mentioned? Again, thank you so much for replying my messages!

 

[mvs sarma]

Hi audioguru!
it's a great honor to receive a message from the master so thank you for replying my mail! i've been wanting to build one for years and hopes this will work well...just to confirm, the circuit by mvs sarma is the final version you mentioned? Btw, in mvs sarma PCB design it does not have any pour copper to reduce high frequency noise in it.(leaving all unused copper behind and grounding it) Is it advisable to do so? Again, thank you for your reply! have a great day!

i have already told that for those interested to improve, i can forward eagle files and you can improve on it Ckchew8 !

 

[kp2010]

hello all.... i am new to this forum i am interested in electronics ...

i need circuit diagram to build fm transmitter what is mod4 ?

is this fm tx working???

plz help me

 

[audioguru]

do you have an original post or link to your own final design which i can refer to?

I never made it a project because it is illegal. I will post the photo and schematic again for the 250th time in my next post here.

i found a solution by you (AGAIN!) which mixes and attenuates the source from an audio output of a computer into the FM transmitter.

3 resistors mix stereo into mono and attenuates the result.

i managed to find a list all the radio stations in my country, i'm pretty sure they have a guard band between stations or channels, is your circuit's transmitting frequency stable enough to use the guard band frequency to transmit (just for educational purposes) as in giving good stability without using any crystal oscillator circuits like you mentioned?

My FM dial is full of stations from many nearby cities. There is no vacant frequency so I tested my FM transmitter on the same frequency as a low power foreign language station on the other side of my city. If the circuit is mounted in a metal box connected to 0V so that things moving toward or moving away from the circuit board do not change the frequency and use good quality NPO high frequency capacitors then the frequency is very stable.

 

[audioguru]

i need circuit diagram to build fm transmitter what is mod4 ?
I explained the 4 modifications in post #18 here.

is this fm tx working???

My prototype works perfectly. It must be built properly using good high frequency RF practices.
Here is my transmitter again:

 

[ckchew8]

hi audio guru,
Thank you so much for explaining and even showing us the input options, regarding the stereo to mono conversion, why do we need to attenuate after mixing the stereo signal into mono? btw, is the circuit above the final version?Again, many thanx for posting the schematics again!

 

[audioguru]

why do we need to attenuate after mixing the stereo signal into mono?

The mic preamp transistor has a very high sensitivity of about 5mV but the output of an ipod is about 27 times higher.

 

[mvs sarma]

welcome to electro-tech-online.
instead of asking to start. please try to read from posts on these very subject
Audioguru already explained in these very pages, what is Mod4. if it didnot work, perhaps Audioguru would not post it, is it not?

 

[ckchew8]

hi audio guru,
THanx for explaining on the mic preamp transistor part...just one more thing, is the oscillator used (transistor Q2 part) a colpitts oscillator? how did you calculate that part? did you use the formula Fout=1/2*pi*sqrt(LC) ?

 

[audioguru]

Yes, Q2 is one version of a Colpitts oscillator.
I used the same oscillator parts that hundreds of other FM transmitters use.
The frequency is so high that you cannot calculate the tuning inductance and tuning capacitance due to unknown stray inductance and stray capacitance of the wiring.
I use your formula to calculate parts for a speaker crossover filter that works at low audio frequencies, not high radio frequencies.

 

[ckchew8]

Thank you so much for explaning audio guru! will try out the circuit once i get a grasp on the theoratical operation cheers!

 

[kp2010]

I have a question...

can a simple portable fm radio receive the signal which is transmitted using mod4??

sorry my English isn't so good...

 

[mvs sarma]

yes, it can receive

 

[kp2010]

oh thank you Mr. sarma for sharing the pcb design..and for the info i ll definitely try this mod4

 

[kp2010]

sir i feel the pcb is very compact ..

 

[ckchew8]

hi audio guru,
From your original circuit, what does the capacitor C2 does? i can recognise the common emitter configuration but can't seem to figure out the capacitor C2 which shorts base to emitter...btw, what does the whole part do? pre emphasis?how does it work? if yes can i replace it with a simple high pass filter such as LC combination? thanx in advance!

 

[mvs sarma]

try and make one
you may re position these components and if you need I can offer the eagle file.as AudioGURU suggested you can use box caps .

 

[kp2010]

thanks for the reply Mr.sarma if u don't mind can u please share the eagle files..i ll try to reposition it ....and here in Bangalore i am not able to get 5-35 pf cap ...can i replace it with 2-22pf????

 

[sanjubluerock3]

its good to know that a lot of people are here from Bangalore.....yeah kp2010 even i had the same option with var cap it worked well for me but range was very low....i had constructed it a long back... u can use 2-22pf i think

 

[sanjubluerock3]

i think its time for me to again build fm tx mod4 correctly....after seeing this topic going to 4 pages...