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walid

Audioguru FM Tx

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Hi Walid,
Since your circuit transmits hum then maybe your wires are too long and are picking up mains hum.

It is important to have very short wires in circuits for VHF frequencies. Side by side wires have capacitance between them. Wires have inductance caused by their length. Extra capacitance and inductance messes up VHF radio circuits.

My FM transmitter is made on stripboard with each track shortened by cutting with a drill bit. My transmitter is 5cm long by 3cm wide. The wires are about 2mm long and the wires on the electret microphone are about 5mm long.
The pF capacitors are ceramic disc with very short wires, the nF capacitors are film and the uF capacitors are electrolytic.

My FM transmitter sounds perfect, can transmit all sounds heard in a room and has a range to a good FM radio with nothing in between of more than 2km.

Good luck with stopping the war in your country.

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The only 2903 that I know is the LM2903 dual comparator IC. It is not a low-dropout voltage regulator.
All semiconductor manufacturers make a 5V low-dropout regulator in a small plastic transistor case.


audioguru am building ur fm project but still i coundt  get LM2903 so i think it use to make the supply voltage stable when the battry getting down  ? so can i try with out LM2903 but with the new battry for instant ? and what should i do with that capacitors they are in parrallal to use LM2903 . please tell me the correction to get full working of  ur Fm with out LM2903 . thanks again

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audioguru am building ur fm project but still i coundt  get LM2903 so i think it use to make the supply voltage stable when the battry getting down  ? so can i try with out LM2903 but with the new battry for instant ? and what should i do with that capacitors they are in parrallal to use LM2903 . please tell me the correction to get full working of  ur Fm with out LM2903 . thanks again




ohh ...my bad ..
Sorry audioguru i i thoght i typed  LM2931 . ( then the above post corrected instady of  LM2903 just placed LM2931 ) .
still i dont think i can get what u told me 78L05 or 7805  i will try to get one .
but is there any way to use ur Fm with out voltage regulator  ?

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What did u mean by:"has a range to a good FM radio with nothing in between of more than 2km."

My FM transmitter was in the open window on the 2nd floor of my house. My house is near the top of a hill. It transmitted a few hundred meters to my cheap Sony FM Walkman radio and more than 2km to my good car radio. Other houses in between blocked the signal which went the farthest with nothing in between the transmitter and the radio.

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Hi guru

I HOPE U READ IT ALL

I understand u but i intend to use this inside a building of 4 floors, i want to talk while move from place to place, may be inside the lifter ........

when i first test it with that problem mentioned above my son talk near mic inside a room while i'm going away from the building, i hear him at about 150 meter and still hear the transmitter noise at more than 300 meter, there are many things in between.


I want to spend any time to succeed, but i'm afraid from that after spending this time and still have those problems of stability; its freq moves away
and the human body affect and the clearness of the received voice

In my work I have pair of wireless (about 200 meter) with LCD and 8 channels and some functions like outoscan..... i remove its cover to take a look, one second and i quickly close it, it contains many SMD ICs and tiny parts, i feel not OK, disappointed, in fact it is one of my big dreams to build one pair with one function TX and Rx voice with good fidelity and range.

Now  I ask u some questions:
1) you said the antenna must be about 80 cm long, is it important the diameter of this wire and if it have a plastic cover or not; that is can i use a 1.5 mm wire like that used in the electrical power connections

2) to reduce human body effect why not to enclose the oscillator stage inside a metal can like tunner used in TVs

thank you audioguru

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i intend to use this inside a building of 4 floors, i want to talk while move from place to place, may be inside the lifter .

My circuit has a low power and might be blocked by concrete and the metal in a lifer.

still have those problems of stability; its freq moves away

It is better than most other simple transmitters but it doesn't have a crystal like good transceivers.

the clearness of the received voice

My transmitter is extremely clear. But it is very sensitive. Maybe you are talking too loud or are too close to it. Make the received sound no louder than a radio station.

1) you said the antenna must be about 80 cm long, is it important the diameter of this wire and if it have a plastic cover or not; that is can i use a 1.5 mm wire like that used in the electrical power connections

Antenna wire diameter doesn't matter. Mine has a plastic cover.

2) to reduce human body effect why not to enclose the oscillator stage inside a metal can like tunner used in TVs

I didn't try a metal box but it will probably be better.

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With a brand new 9V alkaline battery, I guess my FM transmitter has an RF output of about 250mW. I didn't make a meter to measure it. The battery current is 53mA max.
The battery voltage quickly drops to 7.2V and the output power also quickly drops. Then the battery voltage slowly drops lower and the output power also slowly drops lower.

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"Make the received sound no louder than a radio station."

How to do this?

I designed the mic preamp of my FM transmitter to be very sensitive, so it picks up all sounds in a room. If you speak too loud or are to close to the mic then the transmitter and radio will be over-modulated and will sound very loud and bad. Radio stations are never too loud. Speak to the mic on the transmitter so that its volume sounds the same as a radio station.

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Guys, found this on another board. Looks like some analysis and mod of Audioguru transmitter. I attach the file posted there.

"Here is a little simulation I did for a friend on the RF section of this transmitter after he complained of some TV interference after building this.

Basically I modified the circuit slightly by taking the feed for the final RF amp from inside the tuned circuit, as this helps filtering out harmonics. The coupling cap was reduced 10 times to minimize loading the tank circuit. The osc feedback cap was increased slightly for reliable oscillations. The result is still the same output level around 10mW but the 2nd harmonic is reduced another 10dB. Also there's about a 6dB improvement in phase noise.

The simulated results proofed to be close when measured on the actual circuit after modification.

The last variation has another extra emitter choke added, resulting in about 50% more power at the wanted frequency and even more harmonic attenuation.

Also lowering the value of the 47k to around 33k it is possible to get up to 20mW while still keeping harmonics low. That is a 100% increase in RF power with much lower noise and harmonics as the original circuit.

Just thought it may be helpful to others. "

post-9230-14279143064836_thumb.gif

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Guys, found this on another board. Looks like some analysis and mod of Audioguru transmitter.

Hi AN,
Welcome to our forum. ;D
Which website is talking about my FM transmitter circuit?
I would like to find out why my 5V regulator wasn't used to keep the oscillator frequency more stable and how the low power of only 10mW was calculated.

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edit: There is another posting on the same topic:-

Also it will draw around 53mA at 10mW and about 55mA at 20mW (black trace) so one of these small 9V batteries won't last very long. If you plan to use it on a power supply make sure it is well regulated.

And for those interested, the final modification we played with adding some DC feedback to reduce flicker noise in the oscillation device gives 13dBm (20mW) and another 6dB lower phase noise. Overall improvement 100% RF power, -10dB TV band harmonic and 12dB reduction in phase noise. It should be possible to improve the phase noise some more by lowering the 100k feedback resistor but then 2nd harmonic starts to rise. With 82k feedback simulation results give 2nd harmonic at 2dB higher and lower PN(at)10k (-112dBc), PN(at)100k (-132dBc). So you win some and loose some.

It is amazing how well the measured results compared. I have uploaded images to show original to final improvement. Average carrier noise (related phase-noise)was measured on a mod-meter for easy comparison. Close-in noise can also be observed on the analyzer display.

post-9230-14279143066712_thumb.gif

post-9230-14279143067009_thumb.jpg

post-9230-1427914306724_thumb.jpg

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Also lowering the value of the 47k to around 33k it is possible to get up to 20mW while still keeping harmonics low. That is a 100% increase in RF power with much lower noise and harmonics as the original circuit.

20mW or 250mW?

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Hi Sssaaa,
They changed a lot of things in my circuit, starting with using 9V for the RF oscillator instead of the regulated 5V that I used.
They measured 3V p-p at the output which I think is very low. Maybe they terminated the output with 50 ohms which results in a power of only 22.5mW. I didn't use a 50 ohm antenna, just a piece of wire.

My FM transmitter circuit is similar to many others that claim a power of from 150mW to 500mW.

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"If you need more power, use my last circuit but with 2N4401's instead of 2N3904's. It can handle a bit more power. Current will go up to around 84mA and RF power just over 100mW. Just remember that this power can get you into trouble in most countries and the 2nd harmonic, although still 30dB below the carrier is now at about -10dBm enough to cause TV interference. You will need some extra LPF to get the harmonics down more. Picture shows results of the 2N4401 version we tested before and after adding the filter. 29pF indicates variable cap set value."

post-9230-14279143067479_thumb.jpg

post-9230-14279143067553_thumb.jpg

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Can we believe the improvements they claim? See also they claim 100mW when use another transistor 2n4401. What do you think?

Maybe they are getting 10 times more power into a 50 ohm resistor than my circuit, but I get excellent range with just a half-wave long antenna wire. They tested all their changes, I didn't.

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Can and how will it be possible to get more power >100mW from this? Can another booster be added?

The modified circuit's power supply current is 84mA. Minus about 5mA for the oscillator and mic preamp leaves 79mA for the 2N4401 output transistor. Therefore the 2N4401 dissipates 100mW in RF and 611mW in heat. Its absolute max heat dissipation rating is 625mW so it is ready to smoke.

To increase the power then an RF power transistor would be needed as the output transistor with a heatsink. It would need to be biased for class-B and driven harder for a full 18V swing into a 50 ohm antenna. The output power would be about 810mW. Maybe the oscillator cannot drive the output transistor hard enough, requiring a buffer transistor in between.

To increase the power even more, an RF power transistor with a heatsink could be used as the output transistor and operating from a higher supply voltage.

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