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AM to FM convertor


Andbor

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Hi Andbor,
Welcome to our forum.
Why would anyone want to listen to lousy-sounding AM on a good FM radio?

I think this project would work fine, but I would double the number of turns on the coils and swap the values of C1 and C3 so it is more like the FM transmitter projects here and all over the web.
It will also work with many transistors like BC547, 2N2222 and 2N3904.

The inductors are about 0.07uH to 1.5uH, but don't buy them, they are simple to make.

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Why would anyone want to listen to lousy-sounding AM on a good FM radio?


There are many reasons here in Canada. FM stations focus on cities and doesn't travel as far so you cannot get those stations in remote areas. Plus in the mountains, it is virtually impossible to get an FM station while driving. If your car radio doesn't have AM (or a casette/CD player), you are kind of screwed.
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HI audio guru
I tried as you told me but still nothing happened. As the description says, it is supposed that when you switch it on, you will hear am stations on the fm band, right? Do you have any suggestion of what i can do? to make the inductors I tookk a 5mm diameter philips screwdriver and wraped the 24 swg for 3 turns and 7 turns

Thanks
Andbor

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Hi GG,
I am also in Canada, eh.
My FM dial is jammed full of good stations, a few I listen to wherever I go. I listen to AM only to hear where there is a traffic accident that I might be able to go around. AM sounds awful.

Hi Andbor,
It is too bad that your project doesn't work. I think that its parts values make it operate at an RF frequency away from the FM band.

Yesterday I made an FM transmitter, very similar to your project and it worked fine the moment I powered it. It was difficult to tune to my radio's frequency since its tuning capacitor (C1 in your circuit) has a wide range. Slowly tuning it, I heard a pop sound from my FM radio whenever I tuned past it. Then I narrowed it down to be close, and fine-tuned with my radio.

Even without an antenna, your project should transmit a quiet carrier on the FM band to a nearby radio. Tune your FM radio to noise between stations (its muting turned off) and you will hear the project 's transmission quiet it as you tune the project with C1.

You can change your tuning parts to be like mine if you want:
Q1= 2N3904
C1= 5pF to 35pF trimmer capacitor
C2= 470pF ceramic disc
C3= 5pF ceramic disc
L1= 8 turns of 1mm enamelled wire close-wound on a 3mm screwdriver
I didn't have R1 since the collector of my transistor connected to the positive of my 9V battery with a 2000pF ceramic disc to ground.
I also didn't have L2, R3 and C4. Instead I had a 220 ohm resistor from the transistor's emitter to ground.
When you get it working my way, add R1 and a very long antenna. Leave the 2000pF cap at the positive of the battery. ;D

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Hi audioguru,
Where abouts do you live? When I drove from Vancouver to Calgary, there were no FM stations once I left the two major cities. So I was forced to listen to AM because I forgot to put songs on my mp3 player. It was kind of funny because there were some religious talk shows and funny old songs (from the 40s and 50s). And the announcers didn't seem to care if anyone was listening because they said whatever they wanted. As for the drive from Regina to Calgary, it is just about as bad.

As for Andbor's problem. It is also possible that if he is using a digital FM tuner, it won't let him select the actual frequency his FM circuit signal is transmitted on. Or am I wrong?

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Hi GG,
I'm near Toronto. I was raised in Vancouver and nearly drowned in its "liquid sunshine" but never drove the trip through the mountains.
I left Vancouver a long time ago when Toronto's hippies went there.

I think the AM to FM converter will just play many AM stations together, power line buzz included. I don't think it will be able to tune separate AM stations because it doesn't have any AM tuning and with a spacing between stations of only 10KHz, 20 AM stations fit into the bandwidth of only one FM channel. I suppose that each group of 20 stations will appear twice on the FM dial (sum and difference frequencies).
It also won't be very sensitive, so maybe picks-up only the powerful AM station a short distance down the street.

My simple FM transmitter is very difficult to tune for clear sound on my digital FM tuner. It is fine with my analog Walkman radio that has a wide AFC.

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  • 2 weeks later...

Hi Andbor,
VHF doesn't work on a breadboard. The capacitance between tracks and inductance is 10 times higher than some of its most important capacitors.
I made mine on Veroboard with its strips, but cut the extra lengths with a drill-bit and planned its outputs away from its inputs. Also I used a very tight layout for short wiring between parts. With extra space for mounting screws, my transmitter board is 2.0" by 1.3". A pcb would be a little smaller.

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Ok thanks, just one more quesiton (sorry to bother you). Before I got your reply I built it on a breadboard, and obviously nothing happened in the fm band.however, when i set the radio mode to am, whenever I switch the circuit mode I hear like a small "
put" on the radio, like someone is tapping a microphone. Do you know what can it be?

And a last question. i am building this as a project to achieve my national diploma in telecomms, so you can understand it's importance. Some people have doubted if it will work. Can you tell me if it will work or not?
Thanks a lot for your patience ::)
Andbor

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Hi Andbor,
The AM to FM converter circuit is extremely simple to do such a difficult task of converting an entire band of frequencies to a completely different band of frequencies without having an RF mixer stage. It must also spread-out the crammed-together AM stations. It must also convert amplitude modulations to frequency modulations which this simple circuit does anyway, even when it is trying to produce only FM.

It is basically a simple FM transmitter toy. Its FM RF frequency will drift with temperature change, battery voltage change and anything that gets close to it. But it should transmit an RF carrier frequency on the FM band.

All fairly strong amplitude modulated signals from its antenna will AM and FM modulate its RF carrier, but the signals will be power-line buzz and all local AM stations combined.
Maybe you can add an AM-band bandpass filter to the antenna or just a high-pass filter so that it rejects power-line buzz, but try it first.
With all the AM stations combined, you can reduce the length of the antenna so it gets just the closest station, or add tuning to the antenna to select a single station.

I don't see how it could select different AM stations by tuning its RF carrier frequency. ???


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  • 2 weeks later...

Hi audioguru

finally i managed to get the 1mm enameled wire as shops didn't have it and I finally got it from a transformer manufacturer(as you said).I am using the 1mm instead of the 24swg. Can you give me an idea if it will work? Now I am ready to build it on a pcb but when I showed it to my friends they said that there was something  weird about only having 1 aerial. As you helped me a lot in this project, what is your point of view about this?

Question2 : I aready asked you about if it works on a breadboard, and you said that vhf doesn't work on breadboard. Sure enough, when i tried it on a breadboard nothing happened, and my teacher gave me the same answer. However, when I contacted the author of this circuit, he said that it worked on a breadboard, and another technician said that it wil work on a breadboard. ??? ???I am really confused. Anyways, i now have got the pcb ready and will try to solder it tomorrow.

                                  10x a lot

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Hi Andbor,
I think the coil doesn't have enough turns and the value of the 22pF cap is too high, but maybe they balance each other.
If the circuit needs 5pF from the trimcap to tune to 100MHz, what frequency would it tune if you add 50pF track-to-track capacitance of a breadboard? The 22pF cap would actually be about 72pF and it just wouldn't work.

Can you ask the author to post his explanation of how the circuit separates the AM stations?

I understand that a mixer stage would take a 1000KHz AM station and mix it with a 100MHz FM carrier from the circuit's oscillator. The result would be signals at 99MHz and 101MHz. These new signals would be frequency modulated because the long aerial picks up amplutude fluctuations from mains wiring and AM radio stations which causes the collector voltage of the transistor to swing up and down. The flucuations of the transistor's voltage causes its capacitance to change, producing FM. If the aerial picks up other AM stations at 1010KHz, 1020KHz etc, they would all end up on the same FM channel bandwidth which is about 150KHz wide (15 adjacent AM stations). 

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Why not just build a simple crystal or even an MK484 set and attach it's output to the fm transmitter discused here: http://www.electronics-lab.com/forum/index.php?topic=2814.0

Thats how I would do it anyway and it's most likely to yeild the best results too.

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Hi Andbor,
We know exactly what a simple AM receiver will pickup and how a simple FM transmitter works. We don't see how the AM to FM Converter can possibly do what its author says it will do, "The capacitor C1 can be varied to receive different AM stations".

I'm not going to build the thing to find out what happens because I hate the lousy sound of AM, and if this circuit works will be doubly lousy-sounding because it doesn't have pre-emphasis. We were hoping that you will build it and let us know.

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Hi audioguru
as you have helped me a lot in this project ( i thank you a lot for this) I immediatly decided to follow your suggestment. Can you tell me where can I find a simple am circuit and a simple fm transmitter circuit and how can I get them working together. 1 thing though, the MK484 is hard to find here in Malta, and you have to order it from abroad, that passes the limit of time to complete the project. Is there an Am receiver without an mk484?
                                              Regards
                                            Andbor

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Hi Andbor,
Aw, c'mon make the darn thingy and let us know what it does. Maybe it will work fine. It only has a few parts!

I don't know what a suggestment or an MK484 is since I hate AM radio.
There are lots of simple FM transmitter circuits that can be found here but our "Search Site" button is broken. A Google search will find them and many others.

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HI audioguru
well to be honest today I tried to operate it and nothing happened. I tuned the radio into noise and tried to hear if there was an am like station broadcasting, but there was nothing. Also, I tried to replace the parts with those you suggested, but still nothing happened. I am now going to double the number of turns of the inductors
Thanks

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