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FM Transmitter...Please Help Me!!!

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I have never been to Australia (G'day mates!) so I have never seen a Micromitter kit. I have never made one.
People who made it from scratch reported problems tuning it.

Some cheap no-name-brand MP3 to car radio FM transmitters are absolute garbage. They are mono (not stereo) and their frequency drifts all over the place. My son bought one, tried it then threw it in the garbage.
 
you may reduce the audio input level by managing the Volume control.of your mp3 player.thsignal strength or streo lamp would be an indication of signal tuned. the attached mods have specific ref to siliconchip's micromitter schematic. I thanks the originator of these mods.

Sarma,

Thanks for the document. There is a lot of good info in there. Unfortunatly, for me to take advantage of most of it, I would have to had known about it before I built the PCB. If I have to go back to square one and start over, I'll be sure to make some changes. I attached to this post a zip file with the .sch, .brd, and all the .lbr files in the project. I also included a few datasheets for components used in the circuit. I actually compressed everything with Winrar but I had to rename it zip because rar is not a valid file type for upload on this board. If I forgot a library and you get errors, just let me know and I'll upload it.

Glad your making progress. One thing to consider, going to an air core inductor may reduce your Q of the inductor, this could result in instability in your VCO. You might need a scope to see this on the VCO control voltage TP1. One other thing you might try is to remove one winding on the cored inductor.

Mike,

Well you were certainly right about the air coil instability. If I even sneezed within 5 feet of it TP2 would go off by 300-400 mV. Because of that, I went back to the adjustable coil with the ferrite core. I couldn't modify the existing one but I improvised one. I had some nylon spacers that I drilled out and tapped to make the coil form. I then used 1.5 turns of magnet wire and I was able to get the voltage at TP2 pretty darn close to 2V and it's stable. I haven't seen the voltage at TP2 drift more than 50 mV at any point. I secured everything down with some hot glue so I hope that part of the circuit is good to go now. I did have to use a thinner gauge of magnet wire than what was called out in the article but I hope this won't be a problem. Please tell me if you think otherwise.

So, TP2 is now a stable 2V but I'm still getting the annoying warbling tone. I made a discovery sort of by accident. I don't usually troubleshoot this way but I found out that if I place my finger in the vecinity of R15 and R6 (as shown on my schematic), the warbling went away and I could hear a very faint music coming through the radio. It was a little staticy but mostly OK. I adjusted R12(once again as shown on my schematic) and the music got slightly louder but was still pretty faint. If I removed my finger, the warbling returned and I could no longer hear the music.

I'm not sure if my finger there added capacitance or what but I hope that little clue might help you zero in on what's going on here.

  • Does the stereo receiver pilot light turn on when your testing?

I don't have a radio in the house that has a pilot light so I had to go out to my car to test this out. The short answer is no, the stereo indicator is not present on my car radio. I was able to get it to flicker for a short while by holding my finger near R6 & R15 and adjusting R12. I also noticed if I grabbed the antenna, it flickered on and off a few times. Maybe part of the problem is I need a longer antenna. What do you think? Currently, my antenna is about 2.5 ft of 20 AWG stranded copper wire.


  • Have you tried the unit with no audio in. Should get silence

I have tried it with no audio input but I do not get silence. I get the same repeadative warbling tone.

  • Do you have access to a scope?

Yes, I do. I have an old Tektronix T922 that I bought at the surplus electronics shop for $40. It's certainly seen better days and not all the time and voltage divisions work but it would probably do the trick for some basic troubleshooting. What should I be checking for with my scope?

We keep making slow but steady progress here and it seems the end is in sight. Thank you very much for all the help. It's greatly appreciated.

P.S. This is my first RF project in case you couldn't tell. :)
 

Attachments

  • FM Transmitter Eagle Files.zip
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I have never been to Australia (G'day mates!) so I have never seen a Micromitter kit. I have never made one.
People who made it from scratch reported problems tuning it.

Some cheap no-name-brand MP3 to car radio FM transmitters are absolute garbage. They are mono (not stereo) and their frequency drifts all over the place. My son bought one, tried it then threw it in the garbage.

It would appear I'm having some of the same troubles you mentioned. Mike has been helping me make some decent progress but I'm still not quite there yet. Any thought on my last post? Thanks!
 
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Aren't PLL's fun?Sounds like you still have some loop issues. With the scope you want to look at TP2. Ideally you would have a pure DC component but in the real world there will be some AC component. With the scope you should be able to see how stable or unstable your loop is. You might try increasing the value of C24. For L1 you might try to rewind your coil so it looks like the image below. I also recommend you read up on PLL's and particular read up on loop filters. Here is a few links.
https://www.electro-tech-online.com/custompdfs/2010/11/an95006.pdf
PLL Loop Filter Design :: Radio-Electronics.Com
**broken link removed**

Also ensure that you have a noise free Vcc ie. generous bypassing on the pcb.
 

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To give you an idea of how Q of an inductor affects oscillator performance I have a simulated a PLL osc with a low Q tank inductor and a Hi Q inductor. Notice the phase noise of the low Q tank is much greater. See below. Low Q image is on the left.
 

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  • pllhiQ.gif
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please refer to the word doc i gave last time
the poster was AN920 and here is the link to Micromitter mods, she suggested.
Electronics Lab - Community

Ps: Thanks VNE147 for sharing the files. I am trying to change the audio input to a stereo jack and the DC to a DC jack. As we don't get that filter, perhaps better to realize by discrete components
 
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Hi Sarma,
If you look closely then you will see that AN920 is a female!
 
I am trying to change the audio input to a stereo jack and the DC to a DC jack. As we don't get that filter, perhaps better to realize by discrete components

The audio input jack that I used is a stereo jack. There is a pin for ground and then one each for the left and right channels. What is it you are looking for? I'm also not sure what's wrong with the power input jack either. You can obviously change whatever you'd like but I'm confused because both of the things you mentioned are already that way.

And for the filter, I assume you are talking about the FM bandpass filter at the RF output from the IC. I had a little trouble finding that filter too. I ended up having to buy a lot of 15 of them off ebay. My point is that I have a lot of leftovers and would be willing to send you one for free if you pay for the postage.

Let me know.
 
1. we generally use the components we get in local market. So the Audio and DC jacks change. Please don't confuse.
2. thanks for the offer. the international courier would be too dearer.
thus always choose local persons while you want to give free. I again thank you for that.
 
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