Hi Everyone,

As I moved to 10th grade I was assigned to do a year long project, The Personal Project. For my Personal Project I focus to build a simple, mini, high(good)quality FM transmitter. Part of the Personal Project requirements are research and feedback so I would like you to take a look at the circuit attached below and give me feedback on it. Is it good? Improvements? etc.


Mod Note: Reduced image file size.

 

Hi Everyone,

As I moved to 10th grade I was assigned to do a year long project, The Personal Project. For my Personal Project I focus to build a simple, mini, high(good)quality FM transmitter. Part of the Personal Project requirements are research and feedback so I would like you to take a look at the circuit attached below and give me feedback on it. Is it good? Improvements? etc.

View attachment 93068

Maybe you could start by giving some goals (performance criteria) for your circuit. What do you need it to do when you are done. For example, what are you transmitting to? How far away is it? What kind of signals will you modulat into the FM signal. What frequency will it transmit?...

Also, explain what you want each section of your circuit to do...

 

The first thing the OP needs to do is to learn how to post an image to this forum that is less than ~200Kbytes. I can't wait long enough to download whatever it is you posted...

 

The first thing the OP needs to do is to learn how to post an image to this forum that is less than ~200Kbytes. I can't wait long enough to download whatever it is you posted...

See if this helps.

 

 

First thing first - Good for you doing discrete RF design at ~15 Years of Age. You have a bright future ahead.

Unfortunately I'm 35 and have done very little RF design - forget discrete RF design - so I'll be of little help. Good luck!

 

You need 22n across the power rails. The 47u does nothing.
You will need a fairly large input signal with all that attenuation.

 

Welcome to AAC!
Do you have any way of checking the RF frequency generated? It should be in a licence-free band, or you'll have the authorities paying you a visit .

 

It should be in a licence-free band, or you'll have the authorities paying you a visit .

Although, he probably has more chance of winning the lottery

 

Maybe you could start by giving some goals (performance criteria) for your circuit. What do you need it to do when you are done. For example, what are you transmitting to? How far away is it? What kind of signals will you modulat into the FM signal. What frequency will it transmit?...

Also, explain what you want each section of your circuit to do...

Maybe you could start by giving some goals (performance criteria) for your circuit. What do you need it to do when you are done. For example, what are you transmitting to? How far away is it? What kind of signals will you modulat into the FM signal. What frequency will it transmit?...

Also, explain what you want each section of your circuit to do...

Yup, thats something I have taken into account. However do you have any feedback on a technical perspective?

Welcome to AAC!
Do you have any way of checking the RF frequency generated? It should be in a licence-free band, or you'll have the authorities paying you a visit .

Yes I forgot to mention that, infact I had to explain the law and everything therefore I focus on quality rather than distance, Just enough to cover a house. And you can also adjust the output frequency so I believe I should be good in terms of law.

 

You need 22n across the power rails. The 47u does nothing.
You will need a fairly large input signal with all that attenuation.

By power input you mean the battery or the sound input?

 

22n across the battery

 

Will a 47uF cap interact with a inductor if its placed right next to it?

 

You already have a thread on this subject. Please stay with one thread.
http://forum.allaboutcircuits.com/t...onal-project-mini-fm-radio-transmitter.116625

 

Will a 47uF cap interact with a inductor if its placed right next to it?

 

If placed "right next to it" means a physical placement with no electrical connection then no interaction will take place.
If an electrical connection is a series connection, then you have a series resonant circuit. The resonant frequency will depend on L and 47 uF. It an electrical connection is a parallel connection, then you have a parallel resonant circuit. The resonant frequency will depend on L and 47 uF.

 

If placed "right next to it" means a physical placement with no electrical connection then no interaction will take place.
If an electrical connection is a series connection, then you have a series resonant circuit. The resonant frequency will depend on L and 47 uF. It an electrical connection is a parallel connection, then you have a parallel resonant circuit. The resonant frequency will depend on L and 47 uF.

Thanks, Im working to decrease onboard interference, I suspected some magnetic field interaction between the coil and the significantly large capacitor connected to the powersource. But what If I were to place two capacitors right next to each other in which one of them operates under sound frequency and the other is connected to the power leads will there be some interference due to different frequencies the capacitors operate at?

 

At 100 Mhz- EVERYTHING will interact with your inductor.

These simple FM transmitters have HORRIBLE frequency stability, wave your hand over it and it will change by 200 Khz.
It will work, but expect to be very frustrated by it's overall performance. Making a good stable transmitter at FM radio frequencies requires about 20 X more parts and a good bit if RF design and construction skills.

As a kid, I messed around with these simple FM transmitters until I was blue in the face, learned allot, but they are never quite "Useful" as more than curiosities.

 

At 100 Mhz- EVERYTHING will interact with your inductor.

These simple FM transmitters have HORRIBLE frequency stability, wave your hand over it and it will change by 200 Khz.
It will work, but expect to be very frustrated by it's overall performance. Making a good stable transmitter at FM radio frequencies requires about 20 X more parts and a good bit if RF design and construction skills.

As a kid, I messed around with these simple FM transmitters until I was blue in the face, learned allot, but they are never quite "Useful" as more than curiosities.

Ofcourse I do not expect it to work perfect. what you said is true, If I move it or wave my hand, it will vary a little but luckily it goes back. But this does not mean that I want to leave it as it is, I would prefer to make it even a little better. For example, I live in this university where I met some profesors with whom I fine tuned the components to get better NUMBERED results on a simulation and some spectrum analyzer testing. However, Now its the weekend for us (I currently live in SaudiArabia) and the Professor I managed to work with does not seem to reply during the weekends, This is why Im asking here because Im slowly running out of time. Do you think It would be at least better If I were to place the capacitor which is near the coil, next to the other 47uF capacitor in the pre-emphasis, will this somewhat help decrease interference?

 

MOVE the cap. As stated earlier. EVERYTHING is affected at 100 MHz.

Another question. Will the antenna be inside a plastic container of some sort or just be a live wire extended into space?