Go Down

Topic: how to calculate the exact frequency of this FM transmitter? (Read 17553 times) previous topic - next topic

firashelou



I don't think they will allow you to make a transmitter to do this. Also see "type approved in Lebanon".


but why what is the reason, if it is just like the FPV used on RC airplanes ?
Aiming for the unreachable !

polymorph

Because that radio is carefully designed by trained engineers, extensively tested, and approved.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

firashelou


Because that radio is carefully designed by trained engineers, extensively tested, and approved.


so is it possible to build a small talkie walkie similar a bit to the one used in the racing world ?
and what about the toys i guess it's very low frequency that doesn't cover much more than 20meters or something ?
Aiming for the unreachable !

Grumpy_Mike


but why what is the reason, if it is just like the FPV used on RC airplanes ?

Because some one who does not know what they are doing can cause havoc interfering with vital communications of the Police and Military. Governments do not need that extra hassle.
If you do know what you are doing you can apply for type approval which means your design is independently tested to make sure there is no interference outside the band you are using and that it is being manufactured in a professional way. Of course you pay for that.
If you want to learn then there is always the Amateur or Ham license but there are restrictions on the use you can put to that.

RC airplanes use a specific band and the equipment has to be type approved. Imagine you were flying your plane and it was brought down by someone messing about with an FM transmitter. You would not be pleased and it could kill someone.

Grumpy_Mike

Quote
so is it possible to build a small talkie walkie similar a bit to the one used in the racing world ?

No in your country you are not permitted.

Quote
what about the toys i guess it's very low frequency

You guess wrong.

Quote
doesn't cover much more than 20meters or something

Range is not a consideration for most countries, as yours and mine, it is the act of making and using an unlicensed transmitter. Again there are bands for type approved equipment and that includes toys.

firashelou

#20
Jan 16, 2014, 10:46 pm Last Edit: Jan 16, 2014, 10:49 pm by firashelou Reason: 1

Because some one who does not know what they are doing can cause havoc interfering with vital communications of the Police and Military. Governments do not need that extra hassle.
If you do know what you are doing you can apply for type approval which means your design is independently tested to make sure there is no interference outside the band you are using and that it is being manufactured in a professional way. Of course you pay for that.
If you want to learn then there is always the Amateur or Ham license but there are restrictions on the use you can put to that.

RC airplanes use a specific band and the equipment has to be type approved. Imagine you were flying your plane and it was brought down by someone messing about with an FM transmitter. You would not be pleased and it could kill someone.


aha ok your right
ok i have a question which i didn't get the answer i am looking for which is :
how to define the frequency i want to work on ? is it by making calculations and defining what cap, resistors and inductors i must use or .. ?
and still didn't get how a walkie talkie can't interfere with another walkie talkie or even other devices ? well so far i know there is some sort of encryption, so how is that done ? and what components ?

i remember when i was a kid i got a walkie talkie for about 10$ but extremely powerful according to a toy, it was very similar to the old big real ones and can cover a very big distance, + we used to hear people talking sometimes but very few moments that this happened !
Aiming for the unreachable !

Grumpy_Mike

Quote
how to define the frequency i want to work on ?

It depends on the frequency range you are working in.
Quote
is it by making calculations and defining what cap, resistors and inductors

For low frequencies say less than 30MHz you can do this although the value of all these components will change with temperature with careful design you can keep it down to manageable proportions.
For higher frequencies it is more complex and a simple free running oscillator will not be stable enough. There are different techniques like heterodyning, crystal oscillators and mixers, and the use of phase locked loops to name but three techniques.


firashelou


Quote
how to define the frequency i want to work on ?

It depends on the frequency range you are working in.
Quote
is it by making calculations and defining what cap, resistors and inductors

For low frequencies say less than 30MHz you can do this although the value of all these components will change with temperature with careful design you can keep it down to manageable proportions.
For higher frequencies it is more complex and a simple free running oscillator will not be stable enough. There are different techniques like heterodyning, crystal oscillators and mixers, and the use of phase locked loops to name but three techniques.




alright instead of a simple oscillator made of conductor and 60pf, it is more efficient to use the crystal oscillators for example
Aiming for the unreachable !

retrolefty

#23
Jan 16, 2014, 10:58 pm Last Edit: Jan 16, 2014, 11:03 pm by retrolefty Reason: 1
Well one saving grace is that most FM receivers utilize AFC (automatic frequency control) to lock the receiver's frequency to the transmitters signal actual frequency over a given (small) capture range, so the frequency transmitted doesn't have to be exactly on the standardized channel frequency.

You don't see many building these simple FM transmitter circuits much anymore. They are of very questionable quality, drift bad, distortion is pretty high and most beginners never get them to work. If you need the functionality rather then the project experience you might consider something like this pre-made with digital frequency control.

http://www.ebay.com/itm/Naztech-N3030-Hi-Fi-Digital-Universal-FM-Transmitter-/251416114395?pt=US_FM_Transmitters&hash=item3a899174db

Grumpy_Mike

Quote
it is more efficient to use the crystal oscillators for example

Not efficient but stable.
You have to modulate it so one way with FM is to start with an 8MHz crystal, frequency modulate it and multiply it up to the FM band.

firashelou


You have to modulate it so one way with FM is to start with an 8MHz crystal, frequency modulate it and multiply it up to the FM band.


well i don't really understand what you mean by multiply it up to the FM band, seems a bit advanced thing
Aiming for the unreachable !

firashelou


If you need the functionality rather then the project experience you might consider something like this pre-made with digital frequency control.

http://www.ebay.com/itm/Naztech-N3030-Hi-Fi-Digital-Universal-FM-Transmitter-/251416114395?pt=US_FM_Transmitters&hash=item3a899174db


it's about the project and learning, if i wanted to just buy then i won't be on this forum :S
Aiming for the unreachable !


retrolefty

#28
Jan 16, 2014, 11:29 pm Last Edit: Jan 16, 2014, 11:31 pm by retrolefty Reason: 1


If you need the functionality rather then the project experience you might consider something like this pre-made with digital frequency control.

http://www.ebay.com/itm/Naztech-N3030-Hi-Fi-Digital-Universal-FM-Transmitter-/251416114395?pt=US_FM_Transmitters&hash=item3a899174db


it's about the project and learning, if i wanted to just buy then i won't be on this forum :S


Understood, it's a valid learning experiance. Just be aware that very few people successfully get those simple analog FM transmitter circuits to function at all, let alone to function well. There are  ICs that include phase lock loop to generate the FM carrier frequency from a single low frequency crystal.

You might want to look at another persons analog FM transmitter circuit design. The Elliott site has many great projects ideas:

http://sound.westhost.com/project54.htm

SirNickity

i don't really understand what you mean by multiply it up to the FM band, seems a bit advanced thing

it's about the project and learning, if i wanted to just buy then i won't be on this forum :S


I'm not trying to be condescending, I genuinely have the best of intentions here, but I think this project is well over your head.  RF isn't beginner material -- even folks that understand it think it's half science and half black magic.  If you want to learn something new, you should probably weigh your expectations against your experience level and pick a project that is beyond your current understanding, but only a by a little -- a gap you can realistically close.  OTOH, if RF is really something you're passionate about, you probably need to pick up a book on theory, and read that (and related material) until you feel like you have a solid grasp on the concept.

I see a lot of people come through here asking for advice on topics that are obviously leagues beyond their capabilities.  It's a bitter pill to swallow sometimes, but providing answers to those questions is about as helpful as speaking in a foreign tongue.  If the poor chap were close enough to understanding that a forum post would clear things up, they probably would've figured it out on their own.

Just my two cents...

Go Up