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Some More Simple FM Transmitters

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#2
I have some comments about FM Transmitter #3.

I can tell that it came from Mr. Antoon's site (from University of Guelph).
Do a search for ~antoon with google and you will find all his circuits.

BUT, He has a simpler one. The simpler version has everything removed from the left side of the coupling capacitor (except for the microphone).

The funny thing is that 99% of the circuits I see on the internet do not include an equation that determines all their features.

All the manuals say that the inductor and capacitor in parallel change the frequency. but to me, that is of the least importance. My question is why is the value of C3 0.01uF and the value of R6 10K?

I seem to think that these values make up an equation for something else. I'm not sure if they make the equation for bandwidth or what? but if people must use specified values all the time, then whats the point of experimenting when the specified values do not work? Get my point?
 

Nigel Goodwin

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#3
mstechca said:
The funny thing is that 99% of the circuits I see on the internet do not include an equation that determines all their features.

All the manuals say that the inductor and capacitor in parallel change the frequency. but to me, that is of the least importance. My question is why is the value of C3 0.01uF and the value of R6 10K?

I seem to think that these values make up an equation for something else. I'm not sure if they make the equation for bandwidth or what? but if people must use specified values all the time, then whats the point of experimenting when the specified values do not work? Get my point?
R6 is the base bias resistor for the transistor, used to bias the transistor at a suitable point. C3 is to decouple the base at RF frequencies (so it's operating in common base as an RF oscillator), but to allow audio frequencies to pass - it's value isn't at all critical, the higher the value the more it will reduce the higher audio frequencies.

There are many values in a design which don't require an equation to work them out, and would be pointless to do so - very often the starting point for most equations requires you to make an initial assumption, and things work out from that - but you often can't calculate the initial assumption.
 

audioguru

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#6
The RF oscillator in most simple FM transmitters is a Colpitts type. Its positive feedback through a capacitor from collector to emitter is described in tutorials in Google.
The audio causes the transistor to swing its average collector voltage up and down. Since the capacitance of the transistor's collector to emitter and collector to base varies with voltage (see transistor's datasheet), then the frequency of the tank is changed and the result is FM.
 
#7
What does C4 do for the 4th circuit? It looks it makes the collector voltage on Q2 increase as the audio frequency goes up maybe? :?

And do a get the center frequency? Would I factor in C5, L1 and C6; or some other combination?
 

audioguru

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#8
C4 bypasses the transistor's base to ground at RF frequencies so that the transistor operates in a common-base mode.

The center frequency is adjusted with the values of C5, L1 and C6 as you say, plus the capacitance of the antenna and anything that gets near it.
The capacitance of the pcb wiring and of the transistor adjusts the RF frequency and since the capacitance of the transistor changes with supply voltage changes then the frequency is also adjusted by the supply voltage. Temperature change affects the value of most parts listed above so also would adjust the RF frequency.

That is why I say that the RF frequency of such a simple "toy" FM transmitter varies all over the place!

Also that is why my FM transmitter uses a voltage regulator. It also uses an RF amplifier to isolate its tuning parts from the antenna. :lol:
 
#9
im a pretty new guy in electronics with only an associates in electronics under my belt, 2 years in car audio, video and security and a new aviation tech so bear my questions. What in these circuits determines output power of the transmitter? What can i do to make it more powerful? Several inputs can be appreciated. Thanks.
 

audioguru

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#10
The simple transmitters that operate from a 9V battery have a good range, 3V ones don't.
Reducing the value of the oscillator's emitter resistor increases range if its base bias resistor value is correct. Most of these circuits have their base bias resistor value too low for the transistor.
Reducing the value of the emitter resistor might stop the oscillator requiring the value of the collector to emitter positive feedback capacitor to be increased, which lowers the frequency requiring the tuning capacitor's value or inductor's value to be decreased.
 

audioguru

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#14
I don't know your radio.
My FM transmitter goes more than 2km line-of-sight to my very sensitive car radio sharing an FM frequency because my dial is full. The interfering station was low power on the other side of my city.
My FM transmitter has pre-emphasis for great sound.
 

panic mode

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#15
that other low power station must be 2watt transmitter (fmtransmitter[2watt].gif) from top of this page :D
 

audioguru

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#16
panic mode said:
that other low power station must be 2watt transmitter (fmtransmitter[2watt].gif) from top of this page :D
I think the low power station has at least a few thousand watts. It is a real radio station that covers the largest city in my country.
 

panic mode

Well-Known Member
#17
i guess i meesed this one up, your circuit is fine,
i was kidding about 2 WATT output from the circuit above :)
 
#18
audioguru,
Your circuit is also better because it has an amplifier on the output to isolate the oscillator from changes on the load like someone walking in front of it and reflecting some of the radiation back to it.
 

audioguru

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#19
In my electronics career, I hardly worked with RF things.
Recently, somebody on a forum posted a simple FM transmitter circuit that didn't work so I fixed and improved it in stages. My FM transmitter is just a bunch of circuits of other people that I put together.
 
#20
Question about "fmtransmitter3.gif"

yes i have a question about the circuit "fmtransmitter3.gif", i wanted to know could i put a headphone jack input in place of the mic? because i was going to use this for my ipod inplace of buying an itrip so any help would be good thanks

dan
 
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