Making a FM transmitter

Discussion in 'Radio and Wireless' started by Stoneww, Jun 30, 2019.

1. Stoneww

42
2
Apr 18, 2017
Hey, I've been trying to make a transmitter but I've been struggling to read circuits.

So could someone please explain this circuit to me and then maybe tell me how they go about reading circuits. Thank you
I know little parts of it but others I have no clue why components are there or what they do. E.g: I understand the LC resonator but not the C5 capacitor etc.

2. 73's de Edd

3,063
1,299
Aug 21, 2015
Sir Stoneww . . . . .

And so . . .I checked EVERYwhere, to confirm that this was NOT being ALL FOOLS DAY / APRIL FOOLS DAY, and a quick query of Professor Ulysses
P. Fips
had confirmed that he had not tried to pull a fast one over on on us.
So I just ventured into what was being presented here, for its analysis.

Starting at the initial audio amplifier circuitry of Q1 and its initial audio input, one would expect that R7 top lead to be swung down and connected to the common ground buss.
Then one needs to pull out calculator and impedance chart and spread sheets to determine the auidio output level and frequency range expected to be passed thru C4, at a 10 pf value. Then you look further down the circuitry and see the 0.1 ufd that will be delivering the resultant amplified audio to the base of Q2.
It will respond to that varying audio and in turn, Q2 will squegg its operating frequencty proportionatively to create the required FM frequency shift aspect of FM modulation.
Looking at the C1 path right up directly to the battery and with the battery providing an almost zero attenuated path to ground, . we'll certainly not be seeing any RF on the base of that Q2.
So we must be having ourselves a grounded base oscillator circuit. But in looking at the collector and emitter of Q2 , then one then needs to confirm . . . . . is the collector getting its DC operating power from C2 or the antenna wire Hmmmmmmmm . . . .HMMMMMMMMM ?

You SIR . . . are guilty . . . . . .guilty as sin . . . . . .GUILTY AS CHARGED . . . . . of passing a bogus schematic .

Soooooooooo you go back and "GOGGLE" sic. . . . . . "GOGGLE" sic HARD, REAL HARD . . . until you find

https://circuitdigest.com/electronic-circuits/how-to-build-fm-transmitter-circuit

Then, it is being readily apparent that the reason for the R7 situation /wiring, was to supply operating power for the fet amp being used within an electret design of microphone cartridge.
C1 will now pass the audio spectrum with proper use of a .1 ufd value.
The connector DOT on the schematic at the junction of the resonating inductor and C4 , changes a potential series resonant circuit to a tried and true parallel resonant circuit using "Variable capacitor" / 20 pf and companion .1 uh inductor, with C4 serving as feedback coupler to even initially permit oscillator start up.

READ it all and he tells you all.

Plus . . .at no extra charge , you now have DC operating power getting to the oscillators collector.

AND iffen you can't ever get this critter to work for you . . . . maybe Uncle Scrooge, here, will come on and give you his purportedly tried and true FM xmitter circuit.

Thaaaaaaaaasssssit . . . . .

73's de Edd . . . . . .

Everyone hits a brick wall now and then; but, as you soon learn . . . . the REAL trick is NOT to do it with your head.

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3. Stoneww

42
2
Apr 18, 2017
Thank you for the feedback but even with the new circuit I struggle to see why some components are in place,
From what I understand: (correct me if I'm wrong)
Analogue signal from MIC1 which is then cleaned out by the decoupling capacitor C1, R2 and R6 is a potential divider to raise the minimum voltage to the transistor (Or maybe to make sure only a positive voltage reaches transistor). R4 controls the amount of amplification due to the transistor. Then the second transistor will have its current max current limited by R7 and then will apply the signal to the resonant frequency of the LC network leading to that signal being frequency modulated onto the resonant carrier wave.

So if you have a spare minute I'd appreciate if you could explain why these are needed/What they do:
1) R1
2)R3
3)C2
4)R5
5)C3
6)C4

4. davennModerator

13,782
1,934
Sep 5, 2009

gosh, you want to learn parrot fashion what it too the rest of us years of study !
I will give a couple of them to get you started....

1) R1 --- DC supply to the electric microphone

3) C2 -- signal coupling between the 2 transistors

4) R5 -- DC bias supply to the second transistor

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5. hevans1944Hop - AC8NS

4,581
2,142
Jun 21, 2012
Back in the early 1950s there was a guy named Earl "Madman" Muntz who made a fortune by pruning parts from TVs, one-by-one, until the last part removed made the TV stop working. Muntz then had the part soldered back into place and sold the "newly designed, simplified TV" at a much larger profit than his competitors could because they included those unneeded parts.

I strongly suggest that you try the Earl Muntz approach, now that Edd has so painstakingly done the homework you should have done, and provided you with a working, but no-doubt over-designed and over-engineered FM transmitter circuit. Just start clipping parts out after you build the FM transmitter and are satisfied that it is working.

If the FM transmitter stops working after you remove a part from a working FM transmitter, put that part back in and then try removing another part, again verifying the FM transmitter is still working afterwards. Wash, rinse, and repeat until all unnecessary and over-engineered parts have been removed.

Or perhaps you could study circuit theory for a few years, build a few successful projects like most of us here have done, and figure out which parts are unnecessary and which parts represent over-engineering and over-design using your own powers of analytical and critical thinking developed by years of all that hard study. Or maybe not. Some folks just don't get it, and probably won't ever get it, because they lack "The Knack" for electronics. "
Really? Cleaned out, huh? Decoupling capacitor C1, when it is clearly coupling signal to next stage? Whatta FWOT this thread is!

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6. Stoneww

42
2
Apr 18, 2017
Just finished my first year at university and I wanted to try to learn something new but I think I went into a project that is too advanced for my knowledge, but I will try the approach of ripping things out.

7. duke37

5,364
769
Jan 9, 2011
I do not see any components that can be eliminated.
Split the circuit into an audio amplifier and an RF oscillator.

Audio. The three resistors set the operating voltages and the gain The base current will be low so the gain will be approx R4/R3 or 100. The collector voltage should be about half the supply so the signal can rise and fall equally before clipping.

Oscillator. L1and C2 form a tuned circuit fed by the collector of Q2. A small amount of the energy is fed to the emitter via C4 and the current is output at a higher impedance at the collector. Note that there is a connection missing in your first diagram.
The oscillator current is modulated by the audio frequency. This will affect the output amplitude and since the transistor capacitances are voltage dependant will also affect the frequency

Aerial. The aerial will add an unknown impedance to the tuned circuit and stongly affect the operation of the oscillator.

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8. hevans1944Hop - AC8NS

4,581
2,142
Jun 21, 2012
Perhaps too advanced for your current understanding, but keep at it. Things become much clearly after a few years of study. Lab work (hands-on experimentation) is important too... you learn a lot about what doesn't work with lab work, and maybe pick up a few pointers on what does work. As for ripping things out... make sure the original circuit with the original parts installed is working first to your satisfaction before trying to "simplify" anything. And why did you substitute a 2N3904 in place of the 2N2222 used in the original design that @73's de Edd found for you? The two are similar, but not identical. Read this note from a ham website.

As @duke37 says, this is a simple FM transmitter circuit, about as simple as it gets except for when a condenser microphone directly FM modulates the RF oscillator, instead of using the more modern electret microphone (which has all but replaced the condenser microphone in most applications, including professional audio recording) but which does require some amplification as well as a bias supply. Circuits of this type have been sold for many years via mail-order ads in the back of pulp magazines. Most are touted as "secret" ways to covertly listen to conversations from a remote location using an ordinary FM receiver. Problem with that is anyone with an FM radio can also "tune in" to the "secret" microphone.

Good luck in your university studies. Never be afraid to bite off a little more than you can chew. Just carve it into smaller chunks and learn how to swallow those. Either that or change to a liberal arts path. Engineering is supposed to be FUN. Remember that.

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