Audioguru's FM Transmitter Questions

Discussion in 'General Electronics Chat' started by tracecom, Nov 28, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I have attempted to redraw the schematic, which is attached.

    In addition, I am thinking of experimenting with the circuit, and have some questions.

    1. Is 1/4 watt, 5% suitable for all resistors?
    2. Which three capacitors are film caps, C1, C3, and C4?
    3. Why are there two options for C4? What is different in Europe and Australia, deviation?
    4. Is the value of C12 critical? Would 33 pF work as well?
    5. The antenna is shown as 30". Is that for the center of the FM band?
    6. Would the circuit work on 12-15 V instead of 9 V, and would that increase the output power?
    7. I am licensed to transmit on 147 MHz. What would have to change to move the tuning range to include that frequency?
    8. If I used a separate antenna connected with coax, would 52 Ω be appropriate?
    9. Do you see any errors in the attached schematic?

    Thanks.
     
    Last edited: Nov 28, 2012
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You missed saying that R7 is 220 ohms.

    Yes.

    Yes.

    C4 makes the pre-emphasis (treble audio frequencies boost).
    America began broacasting FM first and decided to use a lot of pre-emphasis because in those days music sources did not produce much high frequencies. Later, Europe and Australia began broadcasting FM and noticed that their newer music sources had an abundance of high frequencies and the pre-emphasis over-drove the transmitters so they decided to use less pre-emphasis.
    FM radios for America and for Europe and Australia are also different.

    33pF will work almost the same.

    Yes but the length is not critical. If it is cut for 108MHz then it still works fine at 98MHz.

    The output transistor will produce many harmonics causing interference and it will melt. It gets noticeably warm with a 9V supply.

    The circuit produces wideband hifi FM. Isn't 147MHz used for narrow-band low fi? Reduce the number of turns on the coils and run away when you see the RF cops.

    The RF cops caught me when I was 17 years old. I found an electric motor and made a fan with it. Its brushes arc'd and caused radio and TV interference over most of my city. The cops took away my fan but my neighbour bought me a new one.

    The output does not "match" a 50 ohm antenna but it is loaded down but it still works.
    The capacitance of coax cable might destroy the tuning of the output LC tuned circuit.
     
    PackratKing and tracecom like this.
  3. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I made the correction and updated the attachment in post 1. Thanks for your help.
     
  4. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I would like to have the option to connect either an MP3 player or an electret mic. If the MP3 player and the mic were never attached at the same time, would the arrangement shown cause any problems?

    Thanks.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You must disconnect your R1 when the signal comes to the 27k resistors.
    So connect R1 only when an electret mic is used.

    I don't know how much level comes from your music sources so the levels might be too high or too low.
     
    tracecom likes this.
  6. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I think the change shown in the attachment will work, but it's too late in the day for me to be sure. :)

    I may have to adjust R9 and R10, but the MP3 player has its own volume control, so that may be sufficient.

    Thanks again.
     
  7. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Here is the preliminary PCB layout.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your double-sided pcb is larger than my single-sided stripboard circuit.
    Try yours to see if it works.
     
  9. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Let me think...copper strips on the bottom...jumpers on the top...isn't that two sides? :)
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You are right.
     
  11. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I have a working breadboard; a photo is attached.

    The power is presently batteries, but the transmitter itself works. I still have to learn how to tune it, but I am happy to have FM that sounds ok. I am using my MP3 player as the audio source, so R1 is not included. The attenuator/mixer for the stereo output is outboard.
     
    Last edited: Dec 2, 2012
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your masterpiece looks good.
    The preamp transistor might saturate if its supply is a little more than 5V and it might be cutoff if it is less than 5V. Therefore I dropped the 9V battery voltage down with a low-dropout 5V regulator IC. The regulated voltage also keeps the RF oscillator frequency from changing when the battery voltage drops.

    The oscillator is tuned for the frequency you want. If a cheap radio is near it then it will show the transmitter all across the dial so space them apart or use a high quality home stereo.

    I tuned my output for a peak voltage on a simple Field Strength circuit.
     
  13. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I don't think I have a 1 mH coil, but I'll look. Thanks.
     
  14. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The coil can be less than 1mH for the FM broadcast band. Try making it with 10 to 20 turns and an air core.
     
  15. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I just noticed it's a germanium diode, which I am sure that I don't have.

    As it is right now, I have a pretty solid signal within about a 5 meter radius, but outside of that it fades in and out. However, I will have to say the audio is quite good.

    I think I read on another forum that you don't think much of the Rohm BA1404. Unfortunately, I had already ordered a kit using that IC. What's wrong with it, in your opinion?
     
    Last edited: Dec 2, 2012
  16. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The Rohm BA1404 had very poor performance which is why Rohm replaced it with the much better BH141x line of FM stereo transmitter ICs. The BA1404 has not been made for nearly 10 years. Kit manufacturers gobbled them up when they were cheap without knowing that the much better ICs were coming.

    Its radio frequency drifted all over the place and changed as the battery voltage ran down. Its 19kHz pilot was a square-wave producing harmonics that beat against the stereo subcarrier sidebands producing odd sounds. Its audio did not have a brickwall filter so high frequencies beat against the 19kHz pilot and produced odd sounds. Its audio distortion was pretty bad with a maximum of 3%. Its stereo separation used a trimpot to adjust a minimum of only 25dB. Maybe I missed a few problems.

    Here is the article "Stereo Multiplexing for Dummies" that I posted on the other website:
    http://transmitters.tripod.com/stereo.htm

    Here is what a Radio Pirate says about the BA1404:
    a word about the BA1404 chip:
    Many of the FM kits listed below use Rohm's BA1404
    integrated circuit, which is esentially an FM transmitter
    in a single 18-pin chip. The BA1404 has some
    limitations in sound quality. The separation between the
    left and right channels and the overall audio distortion
    are not up to "broadcast standards." To get a clear idea
    of how bad it is, obtain a studio reference CD (a.k.a.
    audio system test CD) that has a "sweep" on it -- a
    sweep is a tone that steadily rises in pitch from very low
    to very high -- and play the sweep through any BA1404-
    based transmitter, while listening on a high-quality
    receiver. Blecccch!
     
    Last edited: Dec 2, 2012
  17. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    what gives more noise:

    the square wave pilot tone,

    or the lack of MPX filter?
    Is it possible to build MPX filter with 700uH toroid coils?
     
  18. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The audio channels need a "brickwall filter" so that harmonics above 15kHz do not beat with the 19kHz pilot.
    The 19kHz pilot must be a sinewave because the harmonics of a square-wave would beat with the stereo subcarrier sidebands.
    The MPX should be "smoothed" so that the signal does not deviate too far and cause interference to adjacent stations. The smoothing is best done with over-sampling because a filter causes a phase shift that turns stereo into mono.
     
  19. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I spent quite a lot of time trying to tune the transmiter, but was foiled by its sensitivity to stray capacitance. Just placing my hand near the PCB, the power wires, or especially the MP3 player caused the frequency to shift. Of course the power wires are too long waving in the breeze, but the cable from the MP3 player is shielded.

    What sort of assembly error might I have made that would cause this frequency instability? And are the two variable capacitors normally interactive?

    Thanks.
     
  20. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    it makes sense to make the antenna coil tuneable.

    If you decided for a frequency, and hooked a wire antenna, you can change the coil value, and get less distortion. They are easier to adjust than the variable caps.

    I think what I have here are television UHF coils.
     
Loading...