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FM transmitter

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soundman

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Hello guys,

I am trying to analyze the bellow transmitter.
I want to build a RF transformer for the Antenna section to have the MAX of power to be transferred to the antenna; can somebody direct me how to find the impedance of the output of the transistor and my antenna which is a small aluminum rod?

By the way how to calculate the values of those 2 caps on Base-ground and on Collector-Emitter?
Sorry, I know that the values are clear in the pic but want to know how to calculate them by myself?

I don't want it as "a Make and go" project but want to learn how to analyze it.
 

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That transmitter circuit is just a simple oscillator, not a very good one at that, with a random piece of wire tapped onto the frequency determining tuned circuit.
The modulation applied to the base of the transistor is not well defined.
(I will leave it to Audioguru to explain how bad the audio frequency response will be).

The output impedance of the transistor will be quite low, however any load you apply to this circuit will damp the tuned circuit, eventually killing the RF oscillations.

The impedance of your antenna cannot be considered in isolation, if it is just a "small aluminium rod" it needs some kind of groundplane to work against, you are probably just using the mass of the rest of the circuit, the antenna feed impedance will be totally undefined.

Accurate analysis of this circuit is just about impossible, its operation is very depandant on stray capacitance and inductance which cannot be defined without knowing the exact physical construction.

JimB
 
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The circuit is too simple so its radio frequency will change all over the place, its range will be very short and it will sound awful.
 
Ok thanks, Can you give me an idea how to improve it?
Besides How to calculate those caps at a certain freq?

What do you mean by "groundplane"?
 
The extremely simple FM transmitter can be improved:
1) Use a frequency synthesizer circuit using a quartz crystal so the RF frequency is rock solid.
2) Use an RF amplifier.
3) Use pre-emphasis (treble frequencies boost) like an FM radio station.
4) Add a stereo circuit.

Rohm make a few FM stereo transmitter ICs that are used in most MP3 to car radio transmitters that have these features.
Silicon Chip magazine in Australia had a Micromitter project about 5 years ago that used one of these ICs and a kit is available.
 
Why do you show a garbage circuit that is missing pre-emphasis, a voltage regulator, a crystal oscillator and an RF amplifier?
In most countries, 0.1W is illegal and this one might have an output from its obsolete video transistor (2N2219) of 1W if its supply voltage is high enough and you can find a heatsink for its old case.
 
Why do you show a garbage circuit that is missing pre-emphasis, a voltage regulator, a crystal oscillator and an RF amplifier?
In most countries, 0.1W is illegal and this one might have an output from its obsolete video transistor (2N2219) of 1W if its supply voltage is high enough and you can find a heatsink for its old case.

I did not understand your first sentence! What do you mean by "Pre-emphasis"? What was my fault?
 
FM radio stations have pre-emphasis which is a boost to high audio frequencies. All FM radios have de-emphasis which is a cut to high audio frequencies which is exactly the opposite of pre-emphasis so the audio is perfect but the hiss caused by radio transmissions is reduced.

The extremely simple FM transmitters do not have pre-emphasis so the sound received on an FM radio will sound muffled like your stereo with its treble tone control turned all the way down. Real bad sound. FM is supposed to be high fidelity.
 
Thanks audioguru, I seen a FM transmitter designed by you in this forum, I want to know if it suffers the pre-emphasis problem too?

I made the above circuit (at post No 7)and used 2N2219 not 2N2219A, the circuit works just ok by voltages lower than 10V but it stops working at voltages above that, I want to use it at 18V so why it does not work at higher voltages?
 
My FM transmitter has pre-emphasis so it sounds perfect.
Pre-emphasis has different amounts in North America (75us) than in Europe and Australia/New Zealand (50us). My circuit shows parts values for both.
My FM transmitter is mono, not stereo.

The simple FM transmitter you found will probably melt its output transistor if a supply higher than 12V is used.
 
Ok thanks to you guys,

Just another question regarding to the first picture, I want to know how the transistor and The Tank circuit do cooperate to generate a continued freq? I do Know that the feedback capacitor (10pF) is very important but how it does work in the circuit? Does it communicate with the 1nF cap to do something in the circuit?
 
The 1nF capacitor is only 1.6 ohms at 98MHz so it "grounds" the base of the transistor at the radio frequency and then the transistor is a common-base amplifier with its input at the emitter (from the 10pf feedback capacitor) and its output is at the collector.
The LC tuned circuit (tank) is a high impedance at resonance so the circuit oscillates at the tuned frequency. Stray capacitance even from things near the antenna changes the frequency.

The audio modulates the base-emitter of the transistor causing AM. But the AM modulation changes the voltage of the transistor which changes its capacitance so the frequency of the oscillator is modulated making FM.

Unfortunately the 1nF capacitor also reduces high audio frequencies. FM transmitters are supposed to boost high audio frequencies (pre-emphasis) because all FM radios have de-emphasis (high audio frequencies cut) to reduce hiss. This FM transmitter circuit will sound muffled (no high audio frequencies).
 
Thanks audiugru for your good explanation, Can you just tell me how the 10pF cap does work as a feedback cap and does it is put there to shut the transistor down somehow too, or there is another reason for it?
 
Since the transistor has its base "grounded" by the 1nF cap, it is a common-base amplifier with its input at the emitter and its output at the collector.
The 10pF capacitor feeds the signal from the output (collector) to the emitter (input) so that the signal goes around and around like an oscillator.

The transistor oscillates all the time and is never shut down unless the battery goes dead.
 
No it does not make sense,
I have read somewhere in the net that the feedback cap causes the transistor to be shut down for parts of a cycle so that it lets the tank circuit to oscillate normally, and the tank circuit will cause a very big voltage at times and this voltage goes to the emitter via the feedback cap and causes it to be turned off?
 
The oscillator transistor does not get cutoff while it oscillates. The low impedance of the emitter of the transistor and its low value emitter resistor attenuate the signal from the very low value 10pf feedback capacitor so there is not enough energy for the transistor emitter to be driven into cutoff.
 
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