FM transmitter

Discussion in 'The Projects Forum' started by takao21203, Nov 26, 2012.

  1. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    I have built a 7- transistor FM transmitter.
    http://forum.allaboutcircuits.com/blog.php?b=517

    Original schematics: http://cappels.org/dproj/FMXMTR/fmxmtr.htm
    This schematic does NOT work- but I guessed that the 330R and 1K resistors need to be adjustable. Indeed the operational point is only within a very small range!

    Also I can only get it working at the very end of the FM scale. The coil is not so much a problem- it is easily modifyable by pulling/stretching it.

    There is still a lot of noise. As well the gain is a problem (oversaturation).

    But it works now, even if at first, there seemed to be little hope. It is even stable!

    The antenna also did not work, until I had the idea to insert a piece of wire inside the antenna loop, and feed it back into the 1K adjuster for the output transistor (weird isn't it?).

    If you have any suggestions, or ideas for a better FM transmitter?

    Also the 2n3904 varactors don't seem to work very well (but I can use the intended frequency adjust to improve the signal).

    Maybe the FM band is very crowded here (large city)? I had to compress the coil a lot, and even so, it only works at 108 MHz. I get a weaker signal within the FM spectrum, but that's not useable.

    Pictures + demo video see the blog entry!! OK here two more.

    Recorded the microphonic noise as well: http://youtu.be/psnYl2Y9T6g
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here is a quote from the article.

    "Build this on a ground plane board. Layout is important."
     
  3. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    I don't think it makes any difference.

    Also it has not much to do with the outlined problems.
    There is strong microphonic noise, so the oscillation exists.

    What should improve by a ground plane?
    Have you seen commercial radio PCBs (for FM) with ground plane?

    All the connections are very short. And some are air wires.

    When I build another transmitter, I want to use a total new, additional circuit element!
     
  4. takao21203

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    Apr 28, 2012
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    I was thinking the difficulties are caused by the fact I use 2n3904 only.

    They have not so much hFE, and they maybe also perform different when used as varactors. This is why the operational point is more critical, and adjustable resistor is required.

    Also I don't know properly about antenna/grounding.
    I think I built something resonant, which works well, but just accidentally.

    Just having a piece of wire does not work (even if the microphonic noise can transmit).

    I hoped someone can analyze the circuit, and point out one or two component values to change.

    I don't think the author has calculated or optimized the component values (39K and 220pF), rather this was stuff he had at hand. So these may work with the original transistor model, but with the 2n3904, it is actually difficult to make it working.

    But using 2n3904 as varactors is interesting.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    You should notice that the purpose of coupling to the output buffer with a gimmick capacitor is to keep the output of the transmitter very low so as to not be picked up by the user's neighbors. In many places such transmitters are illegal and unnecessarily maximizing the range is in some opinions, an unwise practice.

    The 39k and 220 pf values are not critical to the operation of the circuit. Having to adjust the oscillator emitter resistor (330 ohm) suggests that the transistor needed more gain or a higher Ft in order to oscillate, either because of that particular transistor or the layout.

    It is hard to make an air core inductor that doesn't have microphonics. Harry Lythall shows some printed circuit inductors in an FM transmitter that probably has very low sensitivity to vibration and a printed circuit antenna is a better choice if you are going to be moving the transmitter around while using it -have a look: (http://www.sm0vpo.com/tx/bug.htm ).


     
  6. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Hmm I like this circuit, searched the web, and then I selected that circuit.

    This is also what I believe to be the cause of the difficulties.
    2n3904 has relatively low hFE, and I did not select/test them for hFE.

    I searched the web again and found the information 2n3904 as variable capacitor gives 2pF to 4pF.

    I have tested the range (through walls), connected the receiver to the grid, and at 50m distance, it can only be received a very little, when I touch the telescope antenna with my hand.

    Microphonics are not a problem!

    I want to build a simple frequency counter using a 74hc164 register (100 MHz possible), but I don't know properly how to couple in the HF, or how to amplify it to the right level. Would be interesting to see the frequency.
     
    Last edited: Nov 27, 2012
  7. Audioguru

    New Member

    Dec 20, 2007
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    My FM transmitter is built on stripboard neat and tidy, not a tangled mess like yours.
    My coils are tightly wound of thick 1mm enamelled wire, not flimsy wire that "boings" like yours. My antenna is a proper 80cm long.

    My transmitter has a range of across the street to a cheapo "radio" from the dollar store, 300m to a cheap clock radio or my cheap Sony Walkman radio set to "local" or more than 2km across a large river valley to my high quality home FM stereo or high quality car radio.

    It is your cheapo radio that has trouble receiving in the big city, not the transmitter.
     
  8. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Thank you. I could build your circuit too, and see if it has better behaviour.
    Is it OK to use 5V also for the output stage? It comes from USB. I can add ferrites for EMI as needed.

    And I would like to tap the headphone output from USB sound. Is the sensitivity too high?

    I have varcap's as well available.

    Yes it is a mess, but it works fine. Frequency is off, somehow, I guess the 2n3904 have different capacity, and too much different gain, than the original transistors. I only get response at the very end of FM (108 MHz), if I compress the coil really as much as possible. So I guess the 2n3904 don't give enough capacity.

    Made another video: https://www.youtube.com/watch?v=YeD4TH0vS0k
    This time, distance is about 7m.
    The radio is cheap, but has varcap tuning, not pushbutton tuning.
    It is a CD/tape/radio combo.

    Actually I have some read-made coils here.
    They have a ferrite slug. Could I use them for your transmitter?
    I don't know the nH value.
     
    Last edited: Nov 27, 2012
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Please describe the coils: length, and ID. Thanks.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Then remove my 5V regulator. With only 5V for the output stage then the range will be 1/4 of mine.

    Use this attenuator:

    My oscillator transistor is the varicap.

    Why didn't you use common and cheap 2N4401 transistors like the original?

    It sounds awful like a cheap, distorted muffled AM radio. the pre-emphasis is completely wrong for FM, mine is almost perfect.

    Maybe not. At such a high frequency you do not want a core in the coils.
     
  11. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Because I don't have any of these here. I have some 2sc1740 sitting on an old Mitsubishi VCR PCB, if this helps.

    Isn't as bad in real world as in the video. But yes all this stuff is cheap. The original recording isn't very good as well, guess from VHS.

    These cores can be partially or even completely removed.

    I don't fully understand your circuit. What is the purpose of the second coil (on the right side)? Is the value critical?

    I think I will try to build your circuit, but I will make the emitter resistors adjustable.

    Adjustable resistors I have many here, including some on that old PCB. Not that I needed to save a few cents for components :)
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Then go ahead and use the WRONG parts.

    It is a tuned RF amplifier. It is tuned to the radio frequency and it removes harmonics that might cause interference to aircraft, ambulance, police communications and TV. It is tuned so its output swing is doubled then the output power is 4 times as much as if the tuned LC is a simple resistor.
    The LC values are critical but it has broad tuning so if it is tuned for a peak in the middle of the FM broadcast band at 98MHz then it still works fairly well at 88MHz or 108mHz.

    If you build it like I did then the resistor values are fine. It might not work if you replace the emitter resistors with variable resistors.
     
  13. KCHARROIS

    Member

    Jun 29, 2012
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    Just out of curioaity what does C2 and C5 do...

    C2 is feedback from emitter to base does this increse your gain or gives you better sound quality?

    C5 is acting as a resistor because of UHF?

    This is for audioguru's circuit, Thanks.
     
    Last edited: Nov 28, 2012
  14. Audioguru

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    Dec 20, 2007
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  15. takao21203

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    Apr 28, 2012
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    I don't fully get this one. What's the point having a fixed 330R resistor, or a 1K adjustable one? In the circuit I built it allows me to use it at a point with minimal distortion.

    I also want to try these coils which have a (removeable) ferrite slug.

    The purpose is not to use some specific component, or to follow a schematic 1:1 like a kit. Or to meet a specification.

    The only purposes are to have fun, and to learn new things. I deal with radio circuits only for hobby purpose :)

    It is OK if the sound is crap, even if I'd like to improve my circuits.

    I'd like to change your circuit to PNP (because I have some good PNPs here), but that's a big effort. I could try maybe to simulate it...

    For the time being, I will maybe just build it from your schematic, and maybe use classical through-hole method, not dead-bug style.

    I have capacitors kit here, and also a few adjustable capacitors. One day maybe I want to construct a frequency synthesizer (should not be too difficult since I know how to use microcontrollers).

    I have downloaded the datasheet for these 2sc1740, there are at least 20 of them on the PCB, and they have a transition ranging from 180 to 500 MHz, based on current. Could this be a reason why 2n3904 did not work so well? Because maybe a certain current is required to obtain the required gain at that frequency. But I guess a couple of reasons play together, why it is only marginally possible to get it working.
     
  16. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    I have built the 3-transistor transmitter.
    But I have not yet found the time to wire it up and to test it.

    I used 2SC1740, as well two adjustable coils. I hope that it will be possible to tune the circuit within the FM range. The coils have 8 turns but less diameter than 5mm. With the ferrite slugs partially moved in, inductivity will increase. I do not know if this will be enough.

    It is also built regular through-hole style, not so much tangled mess here.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    My FM transmitter?

    Then the mic preamp transistor might be saturated and not work because it depends on the narrow range of hFE of a 2N3904 transistor. The hFE of a 2SC1740 has a wider range and is higher.
    Also the transition frequency is fairly low for the 2SC1740 so the output power will be low.

    Get rid of the ferrite cores because they cause the tuned frequency to be much too low. Your radio will be tuned to a weak harmonic of the low frequency.

    It looks better.

    How are you powering it? The preamp is designed for a 5V supply.
     
  18. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Yes, basically.
    I also want to work out a stereo chopper. Maybe not needed but I have one weird 19KHz crystal here. I could generate some kind of 19 KHz sine using a PIC, and also use it to alternatively blank one of the channels at 38 KHz.

    The input has an attentuator. The bias (160k/30k) is adjustable. I don't need much output, 20m range is OK.

    They are easy to remove/adjust. The thing is I have many of these coils here.

    It is supplied from USB 5V. I will try later to power up the circuit. If it does not work at all, maybe I will have to change the coils/transistors. 2n3904 is not a problem- many of these available here. If need be, I can order anything I want from Farnell, and it will be here within a few days.
     
  19. Audioguru

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    Dec 20, 2007
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    My FM transmitter circuit will not work for stereo because C5 (required for the RF oscillator) causes a lot of phase shift for the stereo subcarrier sidebands from 23kHz to 53kHz. Another problem it has is that the audio response extends well above 15kHz so higher audio harmonics will beat against the 19kHz pilot tone and against the stereo subcarrier sidebands causing odd sounds.
    You must add a "brickwall filter" at the output of the preamp to cut frequencies above 15kHz so there is almost nothing at 19kHz and higher. The filter will have a very low output impedance to drive C5 without a phase shift.

    You should make a "Micromitter" that was published by Silicon Chip magazine. It uses a modern BH1417 FM stereo transmitter IC that has a crystal controlled frequency synthesizer (with selectable output frequencies) with phase-locked-loop and a very good stereo multiplex circuit. It replaces the lousy old BA1404 analog stereo transmitter IC. The Micromitter has an attenuator on its RF output for legal short range. The BA1404 is obsolete and is not made anymore. The BH1417 also might no longer be available.

    Here is an article, "Stereo Multiplexing for Dummies":
    http://transmitters.tripod.com/stereo.htm
    It talks about problems with phase shift, why a "brickwall filter" is needed and using "oversampling" to smooth the switching edges without much phase shift.
     
    Last edited: Dec 1, 2012
  20. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Maybe I could duplicate the input stage, and alternatively blank out one. Then somehow filter it, and mix with the pilot tone.

    Yes I could use an IC. Or I could buy a ready-made transmitter.

    I do this mainly for experience, and to learn about the technology.

    This means I want to build various transmitters, and see how they behave.

    Today I replaced 14 LEDs which did not work for my big 768 LEDs matrix. 2 of the 4 display sections fully work now. And all the 4 small 16f1503 PICs will be controlled by a PIC32. I have not even downloaded the C32 compiler...

    So maybe I should wire the new transmitter now, and hope for the best.
     
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