Colpitts oscillator FM transmitter

Thread Starter

Sparkboat

Joined Jun 27, 2018
4
Hey,
I'm fairly new at this and a real beginner when it comes to rf design. So, here is my problem, I wanted to design a FM low power transmitter, using 2 transistors. I wanted to use the colpitts oscillator for this and have one that I found online that works great on multisim. Please see attached. The problem that I have now is, how do i turn this into a transmitter ? Where would I put the Mic in the circuit and do i need extra components for modulation ?

Any help would be much appreciated
 

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Thread Starter

Sparkboat

Joined Jun 27, 2018
4
Thanks for your response Dana,

If I am honest I am not 100 percent sure what deviation is. I don't want the fm transmitter to be the best thing since sliced bread. However, I would like it to be a functional transmitter of a reasonable quality. I would love to find a book that could help, I've tried looking but couldn't really find much online.

That fm transmitter schematic you've posted seems like a design that could be something I am after, so that's brilliant and thank you for that. Do you know if it's of reasonable quality ?

Any guidance on your behalf is much appreciated.

Tom
 

simozz

Joined Jul 23, 2017
104
A good textbook for these concepts is the one by Clarke & Hess.
Otherwise the ARRL Handbook has a lot of information and schemes, it's more practical.
 

Dodgydave

Joined Jun 22, 2012
8,600
Thanks for your response Dana,

If I am honest I am not 100 percent sure what deviation is. I don't want the fm transmitter to be the best thing since sliced bread. However, I would like it to be a functional transmitter of a reasonable quality. I would love to find a book that could help, I've tried looking but couldn't really find much online.

That fm transmitter schematic you've posted seems like a design that could be something I am after, so that's brilliant and thank you for that. Do you know if it's of reasonable quality ?

Any guidance on your behalf is much appreciated.

Tom
Deviation is the amount of Volume your Audio signal has, most FM transmitters are limited to 10Khz max, as a general rule use 2.5Khz and increase it until the sound isn't distorted.

The more Deviation you have, the bigger the sweeping frequency of your oscillator has to change.
 

Thread Starter

Sparkboat

Joined Jun 27, 2018
4
Thanks for the help guys it is much appreciated.

The reason I've selected to use 2 transistors (as a minimum) is I am currently doing a HNC course. Year 2 starts next year. For the second year I have to create a project to overcome a work problem. My company isn't the best at attending these project days and do not really get involved. The lecture knows this and has said "do something you would like to learn about".

So, I have selected a fm transmitter project. That I would research, build and test functionally. Along the way I will learn as much as I can (probably only scratch the surface). So I'm trying to get a little ahead of the game and try out some circuits on multisim, research what elements I need to include and how to then incorporate them all into a working design.

That why I started with the colpitts oscillator, I know there are various others but I thought that one would be good. I have just purchased a book from eBay called rf design basics and I'm hoping it will help. I had a look at the ones recommended earlier and they are a bit too pricey for me at the moment. Anyway, all of you so far have helped me with little snippets of knowledge, so I thank you for that.

Maybe, if you guys don't mind, I could respond to this message over the months of me researching and testing and ask for some expertise and assistance.

Please all feel free to have an Input to anymore information that you think will help me along the way.

Kind regards

Tom
 

Audioguru

Joined Dec 20, 2007
11,251
Many people have built simple FM transmitters that have problems that I solved when I fixed them:
1) The radio frequency changes when the battery voltage runs down as it is used. So I added a voltage regulator.
2) The radio frequency changes when something moves towards or away from the antenna. So I added an output buffer amplifier to isolate the oscillator from the antenna.
3) The sound is muffled with no high audio frequencies when heard on a normal FM radio. So I added pre-emphasis like all FM radio stations have and it matches the de-emphasis in all FM radios.

Here is the FM transmitter I designed and built and it works well and sounds great:
 

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Parkera

Joined May 3, 2016
69
Welcome Sparkboat - It has been about 50 years since I played with FM transmitters. The one I liked the best was a small module Radio Shack used to carry. As I recall, it was about the same circuit you are working with. I used to use a small dynamic mic they used to sell and a 9V battery. The audio quality was reasonable with VERY good sensitivity. Transmitting range varied from module to module, but generally was around 350 feet or so and actually worked best without an antenna wire hanging off of it. The battery would last for about 4-weeks non-stop.

Getting more to questions and comments raised so far -
  • Frequency Deviation is the amount the oscillator frequency shifts from the carrier frequency when a modulating signal (audio) is applied. It is a function of the amplitude of the audio signal, not the audio frequency.
  • The amount of frequency deviation you need depends on the receiver you intend to use. Commercial Broadcast FM (88-108 MHz) require a 75 kHz deviation. Most communications receivers (i.e. police and fire band) radios required 25 kHz. Radios intended for amateur use or telemetry will use even smaller deviations. When less deviation is used, more channels can be fit into a given band. The drawback is the available audio signal is less, so it requires more amplification with a degradation in signal to noise ratio.
  • Audioguru brings up some good points and adding voltage regulators and RF buffers will go a long way toward making a stable transmitter.
  • Also brought up by audioguru, each receiver "system" will require a different 'pre-emphisis' to be added to the transmitter. Commercial FM radios need 75us pre-emphisis. I'm not sure what the other systems require.
The one thing I always found with the really simple transmitters, like those mentioned above, is they distort very easily - way before the deviation spec is reached. This is caused by the non-linearity of the junction capacitance's of the oscillator transistor. A very significant improvement in audio linearity can be obtained by putting a varactor diode across the total tank circuit capacitance and modulating it (not the base). You still need to have a split capacitance to obtain the correct feedback ratio of the oscillator, but the values of the capacitors (and probably the inductor) will be less because of the addition of the varactor capacitance.
 

DickCappels

Joined Aug 21, 2008
6,013
I think you will have a good start if you just use danadek's circuit in post #2.

Don't worry about range, mic sensitivity, or fidelity at the start -just get a signal that you cane hear on an FM receiver. An antenna would only complicate things. After you get that far you will be able to easily try refinements.

Below is a brief but useful discussion of construction techniques.
https://forums.qrz.com/index.php?threads/vhf-uhf-construction-techniques.162985/
 
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