FM generation by PIC

[c36041254]

Hi there,
I'm looking for an FM ckt that uses a PIC for doing FM, I mean completely PIC based ckt, no transistor and capacitor etc. (except the amplifier section), receiver and/or transmission module will do ,I just want to understand how they program FM.

 

[Hero999]

What frequency range you you require?

You'll need a pretty fast PIC to get the broadcast range, 88MHz to 108MHz. The only way of doing it I can think of is to use a lower frequency i.e. 22MHz to 27MHz and a band pass filter to get the fourth harmonic.

 

[c36041254]

Hi Hero 999,
As of now I just want to understand that how they do FM in PIC, you know the basic flow chart kind of thing, I thought of using look up table but, it can just provide 14 different values of frequency around the carrier frequency, so what is the logic behind writing code for FM ?

 

[tunedwolf]

Hi there,
I'm looking for an FM ckt that uses a PIC for doing FM, I mean completely PIC based ckt, no transistor and capacitor etc. (except the amplifier section), receiver and/or transmission module will do ,I just want to understand how they program FM.

An interesting idea, although I have never seen anything that uses such a scheme for generating FM much beyond audio frequencies, poor FM wave synthesis in toys being the most common.
If you are planning on using this for RF, I would seriously consider using a crystal based oscillator, PLL, filter and amp stage long before I would use a PIC. A PIC could of course be used to control the PLL etc.

rgds

 

[Hero999]

I'd agree, I don't see who you could use a PIC to directly generate VHF FM. If seems much easier to use a PLL controlled by the PIC.

 

[binzer]

I'd agree, I don't see who you could use a PIC to directly generate VHF FM. If seems much easier to use a PLL controlled by the PIC.

I agree also in theory, but to be FM you do not have to be in the VHF range, you could do HF FM, it's not very common. It was(is ?) used in the recording of data on a floppy, modems,etc so it is used for more than voice (radio) communication which most people associate it with.
Mike

You could probably simulate FM by using PWM at 50% and changing the freq with info from an adc..

 

[c36041254]

Firs thanks for those replies, but if an MCU can not directly generate FM or can not (should not be used ) work as an FM then how come the cell phones and some very tiny gadgets comes with an FM receiver ?

 

[audioguru]

A cell phone doesn't use a general purpose PIC. It uses a special cell phone circuit wirh many inductors and capacitors.
If the cell phone has a camera then its special processor has its circuit built-in or it uses a separate camera IC.
If the cell phone has an FM radio then its circuit is on the special processor IC or it uses a seoarate radio IC.

 

[c36041254]

Hmm..... so there is very less that an MCU can do when it comes to RF except digitizing the signal so as to have better quality, So far I have successfully made a line follower so I was looking to make something wireless as my next project and that's why I was searching for FM but I think an MCU and FM have very distance relationship, o.k. thanks for replies, if any one have a good project to suggest with PIC (wireless, not IR) then I'll be really thankful.

 

[binzer]

Firs thanks for those replies, but if an MCU can not directly generate FM or can not (should not be used ) work as an FM then how come the cell phones and some very tiny gadgets comes with an FM receiver ?

A lot of the circuits are hybrid, analog and digital combined to provide the function needed. The phone part is a radio modem that sends digital voice, freq and channel controlled by a processor as would be all of the other functions in the phone.

You may be able to generate FM with the pic, but not a broadcast type FM (transmitter), the pic could be somewhat of a front end..

 

[tunedwolf]

Get yourself a couple of cheap Radiometrix or similar FM modules, add a PIC at each end to control them and to generate your telemetry information, like left, right, forward etc.
Then you can control your robot using wireless.
These little radio transceiver modules are great for short range radio telemetry and are purposely designed for it. An added bonus is that they are using frequencies that are tightly controlled and license free, so you won't need to worry about killing the old boy next door by messing with his pacemaker
A cheaper alternative would an AM transmitter and receiver pair.
There are a few manufacturers of these modules, again they are on legal frequencies, license free and low enough in power so as not to suck the life out of your batteries.

rgds

 

[c36041254]

Sounds like if I goof up with ckts in FM then can be in BIG trouble right? But then how one can learn things ? I prefer making my own FM or any other wireless module rather than buying one (even if costs little more), Do any one know which band is license free for students in India ? This is what I want to do :
1) Generate PWM from PIC (not decided yet )

2) Vary frequency of PWM output by ADC of PIC, so the FM is done

3) Amplify it (as it is digitally generated I don't think any BPF is needed, or does it ?)

Will this work ?

EDIT: I'm planning to use PIC at, 3V

 

[binzer]

Sounds like if I goof up with ckts in FM then can be in BIG trouble right? But then how one can learn things ? I prefer making my own FM or any other wireless module rather than buying one (even if costs little more), Do any one know which band is license free for students in India ? This is what I want to do :
1) Generate PWM from PIC (not decided yet )

2) Vary frequency of PWM output by ADC of PIC, so the FM is done

3) Amplify it (as it is digitally generated I don't think any BPF is needed, or does it ?)

Will this work ?

EDIT: I'm planning to use PIC at, 3V

It sounds like you wish to make a small radio station, for the band you need to check with the Frequency Allocation section of your government as they sometimes have an experimenters band. Sometimes very low power experiments can be done on regular broadcast bands (here in USA we can build low power FM stations (10 watts I think).
I am not sure if you will be able to do this with just a pic (in the broadcast band)because of the frequency's involved ( this is a lot different from generating FM with the pic).

Mike

 

[c36041254]

Hmmm..., o.k. ,one more thing to ask is this whole radio station thing risky, I mean can I goof it up to a level that can land me in jail or charge a couple of thousands of fine ? As a newbie in RF should I do that ?

 

[binzer]

Hmmm..., o.k. ,one more thing to ask is this whole radio station thing risky, I mean can I goof it up to a level that can land me in jail or charge a couple of thousands of fine ? As a newbie in RF should I do that ?

As with anything new you need to do your homework first before you start.
I am a HAM and we have band areas for experimenting and there are also guidelines to follow so we do not cause interferance to others.
Mike

 

[Hero999]

Hmm..... so there is very less that an MCU can do when it comes to RF except digitizing the signal so as to have better quality,

That's a myth, digitising sound does not improve the sound quality, if anything it makes it worse.

The only good thing about digital is that once the analogue signal is digitised, as long as a lossy compression algorithm isn't used, the signal can pass through many processes without any loss in quality.

 

[c36041254]

That's a myth, digitising sound does not improve the sound quality, if anything it makes it worse.

The only good thing about digital is that once the analogue signal is digitised, as long as a lossy compression algorithm isn't used, the signal can pass through many processes without any loss in quality.

I knew that but we live in a world which leaves impact upon your way of thinking too, so I just wrote that as a "figure of speech".I know that it is just like the mp3 which people believe improves quality of sound but it actually removes the ultrasonic sounds reducing the size of the file
Thank you all for replies

 

[Hero999]

MP3 doesn't just remove the ultrasonic sounds (CDs do that too), it also removes audiable frequencies, it just reject audio information that most people won't notice. Music that's been converted to MP3 and has been edited many times steadily becomes more distorted which is why it shouldn't be used as dynamic format.

 

[atferrari]

Why don't you ask in any radio club near you? At least to know what are the bands where, without license, you could experiment?

 

[tunedwolf]

I would still recommend the radio module approach, however if you wish to make the modules yourself you will need some grounding on radio theory and practical implementation. To that end I would point you towards getting your hands on a copy of the latest A.R.R.L handbook and start reading...
It might already be in your local library, if not ask them to get it for you...

rgds