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# Very low power FM transmitter discussion

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I'm asking the entire world what I learn from this picture...

The picture provided her is from: http://www.rf.fcpages.com/Fm-spy.htm
If you go to this page you will find only this picture without any reliable comment. You find similar pages in more than 90% of the web. They only told you that they are super men and can do what you can't, they challenge you if you can build this circuit, they know you can't because they didn't provide you with the minimum limit of information you need to build it and how they choose the components. If you lucky you may succeed in making this circuit work in any level, what you gain, what you can tell others, I built this circuit and the only thing I know about is that it is FM transmitter, is it enough? Is this the only thing for which you spent your time?
If I design a circuit and want to put it for others to help them understanding something I shall provide them with the secrets of my design, how I make it, how I calculate these values...
So I want to complete the missing info by correct and incorrect statements, I want to put what I know or think though some is not true.
(1) MIC circuit: consists of power supply (1.5VDC battery) - R 22k - MIC - Ground - R 100 ohm - C 1uF.
The R 100 ohm is not necessary be part of ordinary MIC circuits but I think that he put it to attenuate the level of MIC signal so that not to get a distorted o/p sig of the VCO. Its value is obtained by experiments, but the thing which I don't know is how he discover that he needs to put this 100 ohm resistor, may he has an oscilloscope.
R 22K ohm used to limit current feeding MIC, the higher this R the lower current pass, this current is modulated by the voice sig. striking MIC diaphragm, you can calculate this current by dividing 1.5VDC/ R (22K). I'm not sure if we can neglect MIC impedance, but the important question her is: how they decide this current value? And the more important question is that: are there some standards to the peak voltages out of MIC circuits that help the designer by consider them in his design.
Another point I want to refer to is that if the 100 ohm resistor is to attenuate the o/p sig. from MIC, he can do this by increasing the 22kohm and reduce the number or elements used! May be this incorrect!
C1 (1uF) is for DC blocking, it is her to prevent dc currents from flowing to where it must not, if it does it then alter the transistor bias. This C has a value different from design to design. I see many many FM trans. circuits that uses different values of C1 for the same circuits, and when I ask about this they tell me this is not critical, it affect only the tone of the incoming voice, is it true? The human voice frequency is approximately 300-3400 Hz and a capacitor of 1uF has a reactance of about 530 ohms at 300 Hz and 47 ohms at 3400 Hz. These are the two limits of audio frequencies for communications purposes. These values are considered low enough to use this capacitor here. Of course a higher value can be used. Reactance of C denoted Xc = 1/(2 pi f C), where f is the frequency.
(2) Voltage Control Oscillator (VCO) consists of the transistor and related components. VCO is an oscillator that changes its output frequency According to transistor's base voltage. What signals are at this base?
There are two signals, the first signal is the dc voltage bias from 1.5v battery through the 39kohm resistor to base, which make this oscillator to oscillate at a constant frequency, say 100MHz, the carrier signal. The second signal is the MIC signal which when applied shift the DC voltage bias up and down result in changing the previous mentioned frequency also up and down, thus Frequency modulate the carrier signal according to voice signal. All this is ok I think, now, this oscillator is basically colpitts oscillator. How I know? From the two 10pF capacitors connected from BATT to collector and from collector to emitter.
As mentioned before, if no MIC signal applied to base, this oscillator will o/p a constant frequency calculated as: f = 1/[2 pi sqr(LC)].
L is the inductance of the coil and I can't calculate it due to the existence of the ferrite core. I have an expression to calculate inductance for air coils from: http://www.electronics-tutorials.com/basics/inductance.htm
C is the equivalent capacitance of the two 10pF capacitors which are in series => C1. C2/(C1+C2) = 5pF. If we know L we can find the main (carrier) frequency. You may get approximate value of this frequency if you operate the circuit and looking to receiver pointer at FM range. If you get it, you can use the frequency expression above to find L.
An alternative approach to make this LC portion of colpitts oscillator is to use air coil, calculate its inductance L using the expressions in  http://www.electronics-tutorials.com/basics/inductance.htm . You can wound say 5 to 8 turns of insulated copper wire (from a bad transformer or anything else) on a former like a pen. Record the radius of the former {r} and the length of the coil {len} (not the length of the wire) and number of turns you choose {n}.
Then choose your carrier frequency say, 88MHz , and finally calculate the capacitor values. You may change the frequency by changing L value, if you squeeze the coil you have more inductance and if you stretched it you have lower inductance since L is inversely proportional to {len}.
It is easy to do so if the two caps are equal, but I see many circuits using different values for these caps. In fact most of FM transmitters using colpitts oscillator uses a trimmer upper cap and a fixed lower cap. The upper cap with the coil makes a tuning circuit that permit only one frequency to pass from it.
I don't know if we want to calculate the center main frequency whether or not to take the lower cap into account. Someone may tell me! I leave this portion of discussion about oscillator to an expert.
Now I go to rest, but when I return I shall tell you what I know about the feedback 1nF cap, the emitter 100 ohm resistor, the filter 100nF cap and finally the antenna 22pF cap.
Goodbye.

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Hi Walid,
Did the FM transmitter work?
I think the value of the 22k supply/load resistor for the electret mic is much too high with only a 1.5V supply. Most electret mics operate with a 9V supply, a 10k supply/load resistor and draw about 500uA. I think it would work a lot better with a 2.2k resistor in this circuit.

I don't think that the 100 ohm resistor between the mic and the transistor does anything, and can be eliminated. The input impedance of the transistor is its beta times its emitter resistor. The beta for a typical 2N3904 transistor at 1V is about 150. Therefore its input impedance to the mic is 15k. The 39k base bias resistor is in parallel with it so the combination is a 10.8k load to the mic. Therefore the voltage divider of a 100 ohm resistor feeding a 10.8k ohm impedance doesn't do anything.

The 1nF capacitor at the base of the transistor is not for feedback. It is a very low impedance at the RF frequency to ensure that the transistor operates common-base. The feedback capacitor is the parallel combination of the 10pF capacitor between the collector and emitter of the transistor and the transistor's internal voltage-controlled capacitance. ;D

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Hi audioguru
you confused me, is the transistor operate in common emitter or common base?
and, if L = 0.2uH what is the carrier frequency? thanks.

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Hi Walid,
The transistor is a common-base RF oscillator. Its output is the collector and its input is its emitter.
You could say that the transistor operates as common-collector (emitter-follower) for audio frequencies because changing its base voltage changes the voltage between its collector and emitter which changes its capacitance.

You can't calculate inductor and capacitor values in tuned circuits easily at VHF. You must include all the inductive and capacitive strays from the pcb and all parts, or your calculation and the oscillator's frequency will be way off.  ;D

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