Two Stage FM Transmitter

[jamin2707]

Hello this is my first time posting on here, but i've run into a problem with my FM transmitter. This is the second one i've built using the assistance of an schematic with my own touches to it, but i'm running into the same problem of it transmitting to a higher frequency then needed.

Project:
Two Stage FM Transmitter using an Variable Inductor

Problem:
Broadcasting between 180-200MHz

Need Solution:
Which component will allow me to bring the frequency between 87.5-108.0 MHz


Here is the report I did on my first transmitter...
https://www.electro-tech-online.com/custompdfs/2010/07/finalreport.pdf


Here are the pictures of my new project...
**broken link removed**
**broken link removed**

the only thing different is an led, 2 AA for a 3v source, and a smaller design.

 

[transistor495]

This schematic looks strange for me.

A transmitter designed to be work on 1.5v may go out of range with a 3v supply.

No input coupling cap for audio.

After all I don't get this circuit

 

[Nigel Goodwin]

I agree, it's a completely bizarre circuit, I don't see how it can work if it's built anything like the circuit?.

However, he does claim to have tested it on a breadboard first, and you can't build VHF bugs on breadboards successfully, I'm even a bit dubious about the RadioShack board in the pictures?.

I'm even more confused over the way the project is labelled? 'Final Project' - seems rather feeble for a 'final project'?, a first year project perhaps?, but more suitable as a school project for 13-14 year olds.

On a more 'positive' note, the ciruit needs redesigning properly in order for the frequency to be in the correct range, currently I imagine it's simply unstable?.

 

[JimB]

I agree that the circuit is bizarre and cannot possibly work in any defined manner.

As for trying it on a "breadboard", just have a look at my recent thread "Breadboards - how bad are they at RF?"

Here:
https://www.electro-tech-online.com/threads/breadboards-how-bad-are-they-at-rf.109193/

JimB

 

[jamin2707]

lol thanks for the calling me crazy comments, but the first one i built i was able to transmit clearly and i mean clearly up to 60ft away on 104-108MHZ (changes with the twisting of the variable inductor). Yes i know breadboards are horrible for RF circuits, but that was in the way early testing phase when building it for the professor as a demo, and the main problem it gave was it would transmit on about 5 different radio stations at one time. Also on the breadboard it easily operated from 80-108MHZ. Then i moved to a protoboard and it shifted up higher. So i'm running into the same problem with this second design. I can believe some of it can be due to leads being to close, but i'd like to see if anyone can suggest a single component that will allow for it to drop.

Through experimenting with a power supply with the same circuit, I was able to see that the increase of voltage increases the peak value of the FFT, so the signal is just being amplified more in the second stage.

This was a final project that was for a second Microelectronics class that has a writing element to it. So most of the projects we did dealt with different set up of transistors and focused mainly on the writing of the report. Also we were able to choose the project and why choose a project that is gonna be ridiculously hard when i could get an A on this one haha.

I know this isn't like many other FM transmitters, because most use a variable capacitor. There was one group in my class trying to build a transmitter with one of those but they couldn't get it to work plus it costed them $7 for that piece. Where mine is homemade coil with 6 turns of insulated wire with a ferrite core inside. Also another assumption i have is that the frequency is being chosen by the modulation stage, but it's hard to test once it's soldered in.

 

[Nigel Goodwin]

lol thanks for the calling me crazy comments, but the first one i built i was able to transmit clearly and i mean clearly up to 60ft away on 104-108MHZ (changes with the twisting of the variable inductor). Yes i know breadboards are horrible for RF circuits, but that was in the way early testing phase when building it for the professor as a demo, and the main problem it gave was it would transmit on about 5 different radio stations at one time. Also on the breadboard it easily operated from 80-108MHZ. Then i moved to a protoboard and it shifted up higher. So i'm running into the same problem with this second design. I can believe some of it can be due to leads being to close, but i'd like to see if anyone can suggest a single component that will allow for it to drop.

It doesn't work because it's not a proper circuit, there's no oscillator stage - if it worked at all it was because it was only because it was unstable - you need to use a real working circuit.

I imagine I'm like everyone else, completely bemused as to where you invented that from?, it just seems a random collection of components? - at best I would suggest it's a circuit drawn purely from memory, from someone with no electronics background or knowledge, who once saw a real circuit five years ago.

You can't get circuits like this (even proper ones) to work on a breadboard - and any professor who knows what he's on about, would mark you down for attempting such a thing.

If I was called up on to mark your original PDF, the only marks I could offer would be for the quality of your writing, the content wouldn't get any.

 

[JimB]

lol thanks for the calling me crazy comments,

I don't think that any one has called you "crazy".
Your circuit is bizarre and cannot possibly work without the effects of stray inductance and capacitance on the breadboard and stripboard, which is why the frequency changes greatly when the circuit is moved from one to the other.

but the first one i built i was able to transmit clearly and i mean clearly up to 60ft away on 104-108MHZ

Provided that there is RF energy being radiated at the receiver frequency, you will hear it, but that is not a guarantee that it is working correctly.

Yes i know breadboards are horrible for RF circuits, ... ...the main problem it gave was it would transmit on about 5 different radio stations at one time.

Which probably means that as well as the carrier frequency, there was an spurious oscillation at a lower frequency (a few hundred khz) modulating the carrier and producing a comb of frequencies all carrying your ipod signal which you could hear.

Also on the breadboard it easily operated from 80-108MHZ. Then i moved to a protoboard and it shifted up higher. So i'm running into the same problem with this second design. I can believe some of it can be due to leads being to close, but i'd like to see if anyone can suggest a single component that will allow for it to drop.

The first "component" to change would be the circuit board.
Use a piece of blank, un-etched copper clad board and build the circuit using "ugly construction" or "manhatten construction", (Google it). It may be ugly but it will give reliable operation at VHF.
The second "component" is the whole of the rest of the circuit, at the risk of repeating myself, it is a complete non-starter.

This was a final project that was for a second Microelectronics class that has a writing element to it. So most of the projects we did dealt with different set up of transistors and focused mainly on the writing of the report. Also we were able to choose the project and why choose a project that is gonna be ridiculously hard when i could get an A on this one haha.

There should be no way that you should get a mark higher than F for this work base upon that circuit.

I know this isn't like many other FM transmitters, because most use a variable capacitor. There was one group in my class trying to build a transmitter with one of those but they couldn't get it to work plus it costed them $7 for that piece. Where mine is homemade coil with 6 turns of insulated wire with a ferrite core inside.

A good idea, permeability tuned circuits are not new, but in this case it is a good solution.
$7 for a variable capacitor? I am assuming you mean a small trimmer type capacitor, wow that is expensive!

Also another assumption i have is that the frequency is being chosen by the modulation stage, but it's hard to test once it's soldered in.

Not sure what you mean here, but the modulator should not be able to "chose" the carrier frequency.

JimB

 

[Nigel Goodwin]

IA good idea, permeability tuned circuits are not new, but in this case it is a good solution.

I would disagree, the ferrite core in the photos looks totally unsuitable for VHF - ferrite is generally not suitable for higher frquencies, and requires specific types to be used. The number of turns on the core is also more than enough to be too low, even without the core, but as it's not doing anything (as it's not in an oscillator) it doesn't really matter.

Air cored is by far the most common, with mechanical movement of the coil spacing for adjustment (followed by wax to fix it) - but a brass core would be suitable.

But a cheap trimmer capacitor would be quite suitable, and usually used in 'bugs' - presumably the $7 was for an expensive air spaced variable, and not a trimmer?.

 

[jamin2707]

maybe i just have the wrong idea how this circuit works and i just got lucky. here is what i think things do...
**broken link removed**

I know without the core the frequency is shifted higher. I might be wrong with calling it a ferrite core, it was just a core i found in the stock room that looked like what i should be using. I know it has metallic properties, because it's attracted to a magnet; but i guess the person that told me what it was could have been wrong.

Idk about the breadboard, i've heard they don't work well with high frequency; but i had to put it on there first to demonstrate that it actually produces a frequency. Lets forget about the breadboard. What is wrong with a protoboard though?

I believe in the stray inductance and capacitance because i was able on my first design to lower it but not allowing for a capacitors leads to be side by side and stretching them horizontally to different positions.

I have a question about the ugly contruction thing. Now would i be able to attach a copper plate at the ground to get the low impedance?

now is it possible to add in a trimming capacitor to help achieve a lower frequency?
so i should try to stretch out the inductor and allow for it to be an air coil?

I still have no idea what is allowing the carrier frequency to be chosen then. :/

 

[transistor495]

But a cheap trimmer capacitor would be quite suitable, and usually used in 'bugs' - presumably the $7 was for an expensive air spaced variable, and not a trimmer?.

Should be high quality air variable as the price is that high, and those cheap plastic trimmers won't cost more than 1/5th of a $?

I think you'll get a bag of 10-20 cheap trimmers for just one or two USD.

And if you make your own variable cap, then the cost is almost nothing. Google