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Need help with my RF design

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what's the purpose of these links? taking ideas on how to improve my FM transmitter?

i got an LM2940-5 i plan to use it so i can see if the power supply gets more steady without any oscillations.

The links were so can see similarities between example designs and to get ideas.

I presume the 'un-regulated supply voltage' transmitter is working better on the freq meter? The 5V reg ones might possibly not have enough RF signal to register, especially if the reg voltage is much lower.

Just a thought about the regulated voltage... AG says on another thread that the circuit can be improved by adding a 1nF across the supply rails at the oscillator. Maybe the regulator is being adversely affected by the RF? It's easy to try out.
 
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My FM transmitter works perfectly with a 5V National Semi low dropout regulator and proper supply bypass capacitors.

Its output is so strong that my Fluke multimeter cannot read many of its voltages because there is strong RF all over the place.
 
The links were so can see similarities between example designs and to get ideas.

I presume the 'un-regulated supply voltage' transmitter is working better on the freq meter? The 5V reg ones might possibly not have enough RF signal to register, especially if the reg voltage is much lower.

yes you presume right.. the unregulated version of the FM transmitter works fine on the frequency meter but the frequency is either lower or higher than the 88-108MHz range.


Probably stray capacitance messes my LC tank and i can't know what inductance value i need for the coil. I'm saying that because i tried to wind
my own coils and i also used 100nH inductors with no results, of course the coils were kinda random because i don't have handy my LC meter to check them.


you told me that the 5pF feedback adds to the capacitance tough you aren't so sure..

as far as i know too little capacitance needs much higher inductance than 100nH... so the LC can tune inside the range we want.. and too little inductance needs too much capacitance. Since the fixed inductors are as close as possible to the 100nH value then what's the case here?

Just a thought about the regulated voltage... AG says on another thread that the circuit can be improved by adding a 1nF across the supply rails at the oscillator. Maybe the regulator is being adversely affected by the RF? It's easy to try out.

first of all let's make sure my resistor values are correct..
can you recomend values for my circuit using 3.3V power supply?
 
I didn't measure the inductance of my coils. I simply copied them from Rod Elliot's site. I used very thick (1mm) emamelled wire and wound them tightly on a drill bit. Then I spaced them away from the pcb and away from other parts.
 

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I didn't measure the inductance of my coils. I simply copied them from Rod Elliot's site. I used very thick (1mm) emamelled wire and wound them tightly on a drill bit. Then I spaced them away from the pcb and away from other parts.

is it really that simple?!

concerning the unregulated version..if the tunning problem of the LC isn't the stray capacitance then i don't know what else could be...do you?!
 
Stray capacitance of the tuned circuit to other parts or to something near the pcb could change the RF frequency a lot.
You must use a plastic screwdriver to adjust the tuning trimmer capacitor.

Your nearby radio might be overloaded if it is cheap. My Sony Walkman radio was overloaded if its antenna attenuator was set to "distant" but worked OK with it set to "local". When the radio was overloaded the signal was all over the dial.
My high quality home stereo tuner and car radio were not overloaded.
 
Stray capacitance of the tuned circuit to other parts or to something near the pcb could change the RF frequency a lot.
You must use a plastic screwdriver to adjust the tuning trimmer capacitor.

i do! though i still observe frequency change at the slightest attempt to approach or get away from the capacitor while i'm regulating it.

i'll try few more things and then i'll leave it alone...i think i spent too much time on it, after all i think the theoritical part became quite understandable with all this effort i spent.
 
is it really that simple?!

Even simpler, those coils have always looked much more complicated than the bugs I made years ago.

I just used to strip the inner solid core out of TV coaxial cable, and wind something like 4 or 6 turns round a pencil, leaving a gap between each winding about the thickness of the wire.

Open and close the windings to adjust the frequency.
 
Open and close the windings to adjust the frequency.

A suitably-sized snipping of sponge pushed into the inside the coil, and then a dripping of candle-wax after tuning, makes the coil very robust. A slight shift in frequency at first, but I think it's worth it.
 
My coils are made with 1mm wire.
They are so strong that they are not going away.
 
Yes, quite right 1 mm is already stiff enough, adding a 'hot-melt former' not neccesary.

What I had in mind was a Minaturised coil using fine wire. I used to make 'bugs' built on the connector itself of the MN1604 9 V battery, but the game was given away by a few inches of antenna wire, Z-matched using one of these hot-melt coils.
 
Last week, I simply added a self quenching oscillator with a BF494 based transmitter and it simply worked as an fm band receiver. The sound is bad.
 
Last week, I simply added a self quenching oscillator with a BF494 based transmitter and it simply worked as an fm band receiver. The sound is bad.
The internet has a few "super-regenerative" radio projects.
One is called the Radio Shack Special because all its parts came from a RadioShack store. You can find the project in Google.

A super-regen "radio" uses a single transistor as a receiver. It detects AM but can detect distorted FM by using slope-detection (tune to one side of an FM station). The super-regen radio is easily overloaded by local stations.
It causes interference to other radios.
 
the Q of an axial inductor (the ones that look like a resistor..) can be lower than the Q of an air coil?! if yes.. is it important for the LC tank?
 
updating...

looks like Audioguru was right... changing regulator almost everything went back to normal.. i got the same regulator as the one he used for his design only difference mine is the SMD version.

probably no need for ferrite beads...so i'll replace them with zero ohm resistors so i can confirm it.

but still i couldn't manage to bring it inside the 88-108MHz range.. i quited my idea of using axial inductors and i did the same coil as RFguru :D instructed but nothing.. so i went back to the axial inductors and i was keep replacing until i got an 180nH inductor and managed to bring the transmitter to the expected frequency range.

i need to understand which part of the circuit is responsible for that result...

i went into the conclussion that there is too little capacitance so that's why i needed higher value inductor, but why?? my trimmer capacitor has the right value...so maybe the capacitance of the LC is substracted because of the feedback capacitor?
 
My circuit might have much more stray capacitance as your circuit.
My oscillator transistor might have much more capacitance than your transistor.
I did not measure the inductance of my coil, I simply made it instead.
I copied my oscillator circuit from an FM transmitter at Elliot Sound Products and many people built it without any problems with frequency.

Maybe the strong nearby signal overloads and blocks your radio and changing the transmitter's frequency to the much weaker image frequency of the radio allows the radio to work. My cheap Sony Walkman radio overloaded very badly when its antenna attenuator was set to "distant" and the radio was near the transmitter. Maybe every cheap FM radio in my neighbourhood was overloaded and blocked.
 
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