Audioguru FM Tx

[audioguru2]

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.

 

[walid1]

hi guru

The battery voltage quickly drops to 7.2V

quickly =15, 30, 45, 60 .... hour!

 

[audioguru2]

A couple of years ago, Energizer's datasheets for their batteries had more detail and higher currents than now. I got this graph then:

 

[walid1]

Hi guru

Make the received sound no louder than a radio station.

How to do this?

 

[audioguru2]

"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.

 

[AN920]

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. "

View attachment 39627

 

[audioguru2]

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.

 

[AN920]

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.View attachment 39632

View attachment 39633

View attachment 39634

 

[sssaaa]

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?

 

[audioguru2]

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.

 

[audioguru2]

They built and modified the mod 3 version of my FM transmitter. The mod 4 version works much better and has the 5V regulator.

 

[AN920]

"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."View attachment 39635

View attachment 39636

 

[audioguru2]

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.

 

[audioguru2]

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.

 

[sssaaa]

Hi Sir Audioguru
what do you think about this transmitter it is good transmitter or not.
its out put is written on its web is 250mW is it right?
thanks in advance.

 

[sssaaa]

circuit

View attachment 39669

 

[audioguru2]

Hi Sssaaa,
The circuit you posted uses a crystal in an odd oscillator circuit. Since a crystal cannot be frequency modulated very much then the transmitter must be narrow-band for a ham radio receiver. It also doesn't have pre-emphasis like radio station FM transmitters use.

250mW is probably its input power, not its output power.

 

[sssaaa]

250mW is probably its input power, not its output power.

input power??
this is written on its web
"This Narrow Band FM VHF transmitter will output approximately 250mW of RF power using a 2N2222 and can operate between 75 and 146MHz"

 

[sssaaa]

Link

http://www.ajpotts.fsnet.co.uk/Circuits.html

 

[audioguru2]

This Andy guy says his FM transmitter has an RF output of 250mW. The other guy says that my FM transmitter that has nearly the same output circuit has only 10mW output. I think it is much more.

I suspect that the output stage is operating in class-B or class-C and therefore has a pretty high output power which is about half the power supplied by the battery. The coupling capacitor charges by the base-emitter junction in the output transistor rectifying the signal from the oscillator. Therefore the base voltage is a little bit negative and the transistor swings widely between saturation and well into cutoff. The tuned circuit at the output helps the wide swing and filters out some of the harmonics.

Who are we going to believe?