audioguru Posted February 23, 2005 Report Share Posted February 23, 2005 Hi Tazmania,1) 0.22uH is too high when you add about 10pF worth of wiring capacitance to its tuned circuit. Reduce the number of turns in its coil or spread-out the turns. Most toy FM transmitters use 0.1uH.2) The transmitter has harmonics because it has only one tuned circuit. "Real" transmitters have many tuned circuits to attenuate harmonics.3) Many people complain about picking-up their transmitter at many point on the dial of a cheap clock radio. I think that the radio's input stage is overloaded by the transmitter. Cheap radios don't have a tuned circuit ahead of its input RF amplifier like "real" radios have.4) The inductor in series with its antenna is supposed to be part of an impedance matching circuit when you have coaxial cable to a low impedance antenna. If you have only a piece of wire for an antenna, the transmitter will probably work better without the extra coil. Quote Link to comment Share on other sites More sharing options...
tazmania_kid Posted February 23, 2005 Author Report Share Posted February 23, 2005 I didn't use an air core coil, instead I used a normal inductor which I bought from supplier. Is this possible? Quote Link to comment Share on other sites More sharing options...
audioguru Posted February 23, 2005 Report Share Posted February 23, 2005 Hi Tazmania,The project instructs you how to simply make a suitable coil. Why did you buy an inductor with the wrong value? Quote Link to comment Share on other sites More sharing options...
tazmania_kid Posted February 24, 2005 Author Report Share Posted February 24, 2005 As am new to circuit design, I thought, with coil value at 0.22uH, a 3-18pF capacitance change would in theory give you a frequency range of 80-196MHz. Too bad, it's not as easy as I think :( Quote Link to comment Share on other sites More sharing options...
audioguru Posted February 24, 2005 Report Share Posted February 24, 2005 Too bad, it's not as easy as I think :(Yeah, you must think of everything at these very high frequencies. Since stray capacitance has doubled the "theory" capacitance, buy or make a smaller inductor to match. Quote Link to comment Share on other sites More sharing options...
tazmania_kid Posted February 25, 2005 Author Report Share Posted February 25, 2005 ok, I will try to make a coil myself and see what's the output :) Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted February 26, 2005 Report Share Posted February 26, 2005 The coil in series cancels out the capacitance of an antenna that's shorter than 1/4 wavelength.You could reduce the harmonic output by adding a LC tuned circuit in parallel and series with the antenna this will help to filter out the harmonics. Quote Link to comment Share on other sites More sharing options...
tazmania_kid Posted February 27, 2005 Author Report Share Posted February 27, 2005 Hi, anyone can explain to me the differences between the attached circuits in terms of construction and their performance?The reason is: understanding more on these circuits will help me proceed with better and more complicated tx circuits. Circuit 1:http://www.geocities.com/tomzi.geo/4w_fm/4W_transmitter.htmCircuit 2:http://www.boondog.com//tutorials/rfTransmitter/rfTransmitter.htmCircuit 3:http://www.aaroncake.net/circuits/fmtrans.htm Quote Link to comment Share on other sites More sharing options...
audioguru Posted February 27, 2005 Report Share Posted February 27, 2005 Hi Tas,It's obvious that the 1st complicated 4W transmitter (same as the one on our site) will flatten a battery in no time and probably put you in jail! It has a strong signal by using two RF amplifier stages, and proper LC impedance matching circuits.The 2nd circuit is a cheap little toy with a range of a whopping 25 feet!The 3rd circuit isn't much better than the 2nd but has a mic preamp for greater sensitivity. Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted February 27, 2005 Report Share Posted February 27, 2005 Circuit 1:Looks the same as one of the circuits on this siteTr4 is an audio amplifier.Tr1 is a hartly oscillator, coupled to the audio amp, the varying voltage on the base alters it's capacitance like a varicap tuning diode, thus altering the circuit's resonant frequency.Tr2, 1st class C RF amplifier.Tr3, 2nd class C RF amplifier.Circuit 2:Looks like a good tutorial but half the pictures and schematics are missingCircuir 3:A crappy 2 transistor transmitter drawn upside down, with no RF amplifier.Here's a good transmitter circuit: http://sound.westhost.com/project54.htmI know it looks like a simple 2 transistor transmitter it's actually a lot better.Q1 is the oscillator and Q2 is an RF amplifier, there's no audio pre-amp - for that look at the bottom of the page there's also an option for pre-emphasis too. And I would recomend a 2N2222A for Q2 instead of the BC547 as it has a higher Ft. Quote Link to comment Share on other sites More sharing options...
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