We are not able to expain about this circuit, so we request you to explain this circuit and send me explanation in my respective address.

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sudhan shrestha
sudhan_shrestha@yahoo.com

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T1 and the components around it make a basic FM transmitter. R4 and R5 make a voltage divider, but C2 can allow R4 to be electrically bypassed if it is the correct value. Any components up to the coupling capacitor C9 appears to be a weird setup in my opinion because a standard FM transmitter setup will have the input at the NPN's base and the output at the collector.

L1 and L2 make transformer coupling. I don't use transformer coupling personally.

I think VC1 and VC2, L3, L4 and C7 make one or more filters to prevent interference from happening at the wrong stations.

The 100K resistor limits the signal, but I think 100K is too high.

I don't use a varicap.

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[quote="mstechca"]T1 and the components around it make a basic FM transmitter.[quote]
T1 is just a VHF Colpitts oscillator.

The varicap diode's capacitance plus C9 in series with it determines the RF frequency. Since the varicap diode's capacitance changes with voltage changes, then VC1 tunes the RF frequency and the input audio modulates the varicap diode's voltage providing pure FM.

L2 has fewer turns than L1 so the transformer matches the high impedance at the collector of T1 to the low impedance of the base of T2.

VC1 adjusts the RF frequency of the oscillator by changing the capacitance of the varicap diode.
VC2 adjusts the RF power output level. Lower resistance equals higher power.
L3, VC1 and VC2 match the impedance of the collector of T2 to the antenna.
L4 is just an RF choke with C7 a bypass cap to keep high power RF away from the supply for T1.

R2 doesm't limit or reduce the audio signal since the varicap diode is reverse-biased and therefore is a very high impedance, and the tuned circuit at the collector of T1 is also a very high impedance.
If R2 was less than 100k then it would reduce the sharp tuning.

Try it. It allows an FM transmitter to make pure FM without the AM added by simpler circuits. :lol:

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Uncle $crooge

 

man that thing looks dodgy (crap). I doubt you'll get any range from that. The first stage should be a pre-amp for the mic but it will work...just not as effectively as say compared to this circuit.

http://www.electro-tech-online.com/v...ic.php?t=15609

some pretty good FM designs here

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Actually the design is far better than the ones you suggest!, for a start it generates proper FM (which the others don't), and has a properly coupled RF power amplifier stage. It's range will probably be far greater than the previously mentioned designs - it's output power is probably around 1W, making a range of 50-60 miles easily possible!.

The actual audio gain is probably fairly low, as the mike feeds the varicap directly, but it's quite possible that it's intended for NBFM, and NOT WMFM. In any case, a simple audio preamplifier could be added to increase it's deviation for WBFM.

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I designed my FM transmitter with enough audio gain (too much, actually) to pickup voices from across a room. I had to keep telling my wifey to "watch what you're saying".
If I speak to it up close then it has an extremely high FM deviation (and probably AM over-modulation) causing severe distortion in radios. :cry:
I had to attenuate the signal from my audio generator way down in order to test its pre-emphasis and low distortion at normal deviation. :lol:

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Uncle $crooge