FM transmitter, how does it work??

[$$broke_deuce]

**broken link removed**

I'm doing a little project on frequency modulation and I just can't figure out how it works. From the image above I can see the input, the amplifier, and the oscillator; but how does it modulate? I am a EE major near my senior year so I do know my basics; not enought but... Also tried to run a similar circuit in circuitmaker but it wouldn't run.

My question is how does the oscillator modulate the input signal? I am some how just blind to that. Read a few books/article on frequency modulation but none explaining how the circuit actually works. If you know please explain and you're welcome to be as technical and theoretical as possible.

Also this is what my project will consist of..
Going from stage one
-input from audio device
-colpitts oscillator
-tuned amplifier
-class ab amplifier
-antenna

I guess modulation is supposed to occur between the oscillator and input, BUT HOW???

Thankyou for you time

 

[Roff]

The collector-base (C-B) capacitance of a transistor is nonlinear, i.e., the capacitance varies as the voltage across the junction varies. The audio modulates the C-B capacitance of Q2, which is part of the oscillator tank circuit, so the oscillation frequency is modulated by the audio. Very crude and low fidelity, but it works well enough to transmit voice.

 

[audioguru]

It doesn't have a tuned amplifier.
It doesn't have a class ab amplifier.
The 1st transistor is a class-A audio preamplifier and the 2nd transistor is the high frequency oscillator. The amplified audio signal from the audio preamplifier modulates the oscillator.

 

[$$broke_deuce]

It doesn't have a tuned amplifier.
It doesn't have a class ab amplifier.
The 1st transistor is a class-A audio preamplifier and the 2nd transistor is the high frequency oscillator. The amplified audio signal from the audio preamplifier modulates the oscillator.

I know the design posted doesn't have any of those, but the listed component is what I'm planing to have in "my" fm transmitter. Sorry for the confusion. I'm just trying to understand how the posted fm transmitter works.

 

[audioguru]

I have a tuned high frequency amplifier in my FM transmitter circuit. It isolates the antenna from the oscillator so getting near the antenna doesn't change its tuned frequency like simple FM transmitters, and it boosts the power output for better range.

What are you going to use a class ab amplifier for? It is usually used to driver a low impedance speaker.

 

[$$broke_deuce]

The collector-base (C-B) capacitance of a transistor is nonlinear, i.e., the capacitance varies as the voltage across the junction varies. The audio modulates the C-B capacitance of Q2, which is part of the oscillator tank circuit, so the oscillation frequency is modulated by the audio. Very crude and low fidelity, but it works well enough to transmit voice.

So if I were to applied a higher voltage to the base of the transister I'll see higher oscillation compared to a lower voltage applied to the base.

 

[Overclocked]

I have a tuned high frequency amplifier in my FM transmitter circuit. It isolates the antenna from the oscillator so getting near the antenna doesn't change its tuned frequency like simple FM transmitters, and it boosts the power output for better range.

What are you going to use a class ab amplifier for? It is usually used to driver a low impedance speaker.

Im guessing you used a buffer to seperate the Osc. and the antenna right?

 

[$$broke_deuce]

I have a tuned high frequency amplifier in my FM transmitter circuit. It isolates the antenna from the oscillator so getting near the antenna doesn't change its tuned frequency like simple FM transmitters, and it boosts the power output for better range.

What are you going to use a class ab amplifier for? It is usually used to driver a low impedance speaker.

I really don't know, my instructor told me that I needed one to drive it though the antenna. From my previous experience with AM transmitter I didn't need one, but somehow I was told to include one.

 

[$$broke_deuce]

Also, is anyone in here good on circuitmaker? I'm trying this experiment on circuitmaker first, but it hasn't work for me yet. Hardware is coming last because my school doesn't have high frequency oscilloscopes.

 

[Sig239]

In an FM transmitter, all the "information" is contained in the frequency deviation. You do not need to maintain any linearity, so using a class ab amplifier would be a waste. You should use a class c amplifier.

 

[StopGo]

Ive built that circuit, and it's simpler version with 1 transistor. The first transistor is a single stage audio amp, the 2nd transistor and LC circuit forms a simple voltage controlled oscilator.

The audio signal modulates the LC resonant frequency.

I could easily pick up the signal on my FM radio clock, quality was so good It sounded the same as real stations, but it did tend to drift out of tune over time.

 

[HiTech]

That circuit you show is the standard 2-xistor FM bug that's all over the internet. I built one of those in my college days and attached it to a 40 meter dipole (only outdoor antenna I had at the time) and it yielded a usable signal to 2 miles!! BTW, that diagram you show is really an AM modulated oscillator that's tuned to the FM broadcast band. It's not true freq. modulation. Raise the audio level input and you get distortion. Do that on a real FM xmttr and you get frequency deviation.

 

[Roff]

So if I were to applied a higher voltage to the base of the transister I'll see higher oscillation compared to a lower voltage applied to the base.

You will probably see some incidental AM (which is what you're referring to), but the FM is what you're after.

 

[Hero999]

The circuit in the link below looks good but I don't think it has any pre-emphasis.
**broken link removed**

 

[$$broke_deuce]

In order for Q2 to modulate I must first bias the transistor to be in saturation mode first; correct?? So itl'll be operating in the non-linear region?

To do so I should bias Vce to about .2 volts??

 

[audioguru]

Q2 is the oscillator. Q1 amplitude modulates it by changing its input bias current. The amplitude modulation changes the transistor's capacitance which changes its frequency producing FM.

A saturated transistor can't oscillate and can't amplify. Most transistors must be biased to operate in a linear region so the input signal can cause them to conduct more and less. The oscillating transistor saturates on half a cycle then is cutoff on the next cycle and it repeats due to the positive feedback and gain.

 

[naveenon90]

long range walkie talkie

hi guys........i am intrested in doing electric projects.i want to do a long range (minimum 10KM)walkie talkie project.........it's very urgent PLEASE if anybody kows the procedure to construct it or the circuit diagram please reply me..........

thanx in advance

 

[evoke]

is there any fm transmitter that work off the power of the mp3 player. trying on the net for one but the lowest i could find was 1.5Vjust need a schematic

 

[audioguru]

What is the power of your MP3 player? They are all different.

Do you want the schematic of a lousy FM transmitter that drifts its radio frequency, is mono not stereo, is distorted and doesn't have any treble audio frequencies? Then build a simple toy FM bug transmitter.

These are the good FM transmitters:
https://electronics-diy.com/schematics/BH1417_fm_transmitter.htm
https://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php

 

[Hero999]

just need a schematic

You'll need more than just the schematic if you want to build something you know. Often the board layout is important, you'll also find a bill of materials very handy too.

I could post a simple schematic of a switching regulator it it will be of no use if you don't know what value each resistor is supposted to be or the part numbers for the IC and transistors.