fm transmitter(completed) questions

Discussion in 'The Projects Forum' started by poolman966554, Feb 14, 2014.

  1. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    hey all, ive completed my max2606 fm transmitter project.. It is very stable vs the other schematics ive tried to replicate.
    incase anyone curious.. http://forum.allaboutcircuits.com/showthread.php?t=94229
    Ive been had it energized for 5 days continuously now, still exactly 87.5fm. The range however, seems a bit disappointing. I can get the transmitter to work about 7 feet max with perfect conditions.
    Id like at least 40 feet to reach my shed, where i tinker on stuff.. I originally tried this transmitter i found for the stability, as this is for a friend, and she wont want to(cant) "fine tune" it all the time from drift as the others ive built suffer from..

    My antenna is a 12" piece of wire(suggested to not break fcc laws?), that seems to hardly anything.. no antenna gets around 3' range(rotated just right).

    now i said 40 feet, which seems low in the realm of transmitters imho.. bestbuy and walmart sell ones(many actually) for 60 bux( minimum price it seems) with fancy illuminated led displays that transmit about 150ft.. my buddy has the cheapest one the sold, and weve tested it down my drive way

    Ive found this..
    http://www.electronics-lab.com/projects/rf/015/FM_transmitter_Schematic.png

    may i ask whats going on here? as in what is the benefit of having the max4467 feeding the tune terminal on the max 2606? vs the circuit ive replicated here: http://www.electroschematics.com/wp-content/uploads/2008/04/stereo-mini-transmitter-max2606.gif

    Any way i can boost the power of my existing circuit? maybe a different antenna?? if not , maybe a small fm booster circuit compatible with a max2606? Id hate to scrap the board design time, soldering etc..

    Thanks guys!
     
  2. Experimentonomen

    Member

    Feb 16, 2011
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    Study antenna design.
     
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  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    a full wave length at 87.5 Mhz is 3.42857 meters.

    One quarter wavelength would be 2 feet, 9 and 3/4 inches.

    make your antenna a full quarter wavelength and see how much your range increases from that.
     
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  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A better antenna will help. A 1/4 wave vertical is better than the 12" wire you have; a 1/2 wave dipole would be better. Beyond that, you will need a directional antenna, the most common of which is a yagi.

    The Max 2606 is intended to be used as an oscillator, not a transmitter. Without an amplifier stage, RF power is in the range of .0001 watts. In the US, the maximum permitted RF power for an unlicensed Part 15 transmitter was generally accepted to be .1 watts. Now, instead of a wattage restriction, the Part 15 rules impose a distance limit. Here's a quote, "On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters)." Either way, you need an amplifier section to significantly increase your range.
     
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  5. poolman966554

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    Jan 22, 2014
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  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The MAX4467 is a preamp; it amplifies the signal from the microphone before the signal goes to the oscillator. It has essentially no effect on the RF power of the MAX2606. You need an RF amplifier stage after the MAX2606.
     
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  7. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    thanks for the explanation tracecom.

    Ive worked out the range. i can get around 40 feet or so with longer wire. good enough for my shed, hopefully good for a car too.

    Ive just ran into a new problem i havent considered.. Temperature..

    It swings the pots like crazy. I dialed in the transmitter at 87.7 at room temp, then set it outside whiile transmitting. every 30 sec id change station on reciever to follow transmission. it seemed to stabilize around 89.7(at 26f outside)

    I found out theres a temp coefficient to resistors that i failed to realize in this design. How would i go about correcting this? i found Phase locked loop on wiki, but idk how to make one, or incorporate it into the current schematic..
    the type of pots i used incase it matters:
    http://www.digikey.com/product-detail/en/3310Y-001-103L/3310Y-001-103L-ND/1088215

    What are my options here??
    Thanks for your time!
     
    Last edited: Feb 15, 2014
  8. wayneh

    Expert

    Sep 9, 2010
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    I'm sure you'll get more elegant answers but a brute force "solution" would be resistors with low temperature coefficients (low drift). Where you need a pot, use the smallest value pot you can, along with a fixed resistor. A drift in the pot will have less relative effect.
     
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  9. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    seems like an idea that could work.. to get value of resistor for station 87.3,
    i should set the pot to 87.3 (disconnect batt) and measure resistance between supply and output to pot to get a starting point?
    ive got a decent meter.. a uei dl389 but idk if even a fluke could get leads/contact resistance accurate enough.

    as with anything else, id like to get this calculation correct the first time. Ive got a radioshack around here but what they stock in resistors is kinda bah..

    although i do have a few car amplifiers(really old) ive been harvesting for parts here and there. so with what i got, i might be able to obtain a series and/or parallel setup to hit a more exact ohm rating....
    would those resistors be a good candidate? temp coefficient wise? i know thats a vague question, but wouldnt it make since to manufacture an amp with resistors that arent temp sensitive? obviously winter , it wouldnt work, or when overheated it wouldnt work right?

    Thanks for the idea!
    Anyone else? im a newb trying to learn all i can!
     
  10. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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  11. KJ6EAD

    Senior Member

    Apr 30, 2011
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    It's the 2.2nF capacitor that's causing the temperature dependent frequency drift. Many hobbyists built FM stereo transmitters using the Rohm BA1404 chip and had the same problem. The currently available car transmitters mostly use the Rohm BH1415 or BH1417 chips which have stable crystal/PLL-based tuning to avoid the problem.
     
  12. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    So this max2606 transmitter is garbage then.. So how about the previous link i posted?
    http://hllye.com/uploadfile/200812/18/5210216307.jpg

    This is using the BH1417 as you suggested. There is a pot on pin 5.. will that cause drift?

    Ive found two other schematics using the BH1417
    This one omitted the pots on the input signal, and the one on pin 5 but added a adjustable inductor: http://www.electroschematics.com/wp-content/uploads/2009/11/BH1417-pll-stereo-transmitter.jpg

    and this:
    http://2.bp.blogspot.com/-zEg_fWKOH...I0/s1600/BH1417+PLL+Stereo+FM+Transmitter.png

    These three schematics seem very similair to each other but different.

    Which transmitter would you build if you were to make one for yourself?
    Looks like im back at square one.. lol
    Thanks
     
  13. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Of the three, I like the first best, if only because the schematic is a little easier to read and well annotated. With all of these the frequency drift problem is minimal and the pots have no bearing on frequency. They're used for input attenuation (volume control) and modulation depth. The frequency is crystal controlled and selectable by dip switch settings.

    The 10dB T attenuator made up of two 39Ω and one 56Ω resistor may reduce the output power too much for the transmitter to have the required range. Use an online calculator such as this one:

    http://chemandy.com/calculators/matching-t-attenuator-calculator.htm

    to create some lower attenuations such as 6dB and 3dB using 50Ω for the input and output impedance but beware that reducing the attenuation puts you at risk of transmitting above legal limits. It also will, of course, increase power consumption and heating of the components. The chip is capable of nearly 450mW output power but is often neutered down to as little as 1% of that by the output attenuator.
     
    Last edited: Feb 16, 2014
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  14. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    Thanks for the info! i hope this one fits the bill as this will be the 3rd attempt at a decent transmitter. third time's a charm right? lol

    EDIT: off the ground side of switches(D0-3) what is TPG? And TP1 on comp out?(pin5)
     
  15. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Those are just test points. See my edits above for some important notes on output attenuation.
     
  16. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    so from what i just read, lowering db results in unfocused radiation, and more is same power, but more focused. Is this right?

    If so, gain only controls radiation pattern, vs actually amplifying it?
    like a flashlight bulb with a reflector vs without one? same power just focus?

    This is a new can of worms for me here.. not sure how to use the attenuator calculator. What would i input to find db output of posted schematic?

    I input 39ohm for input and output, and played with the required attenuation and watching the "ideal series in/out resistor" values till they got to 39.
    That equaled 21db. cant be right. Ideal shunt resistor was only 7 ohms. schematic shunt is 56ohm.

    Its just not making any since to me. Can someone please explain how this calculator works?


    I def dont want to break any fcc laws, but i would like to still have legal transmission range.
    Thanks for your time!
     
  17. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    Also, can someone explain what the value of L1 is on pin9??
     
  18. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Your first three questions are only relevant to a discussion of antenna design. Since we already know you'll be using a quarter wave dipole antenna, let's focus on the attenuator matter for now.

    The input and output impedances are characteristic impedances of the antenna and output stage of the IC, sort of an industry standard of 50Ω. Other than that, you just enter an arbitrary attenuation value in dB and let the calculator work out the resistor values. The values will display as "ideal" figures for which no actual component can be purchased so you then find the nearest standard component values and enter those in the lower part of the calculator to see what attenuation and impedance results.

    Standard resistor values such as 15Ω and 68Ω appear in the E12 series on this chart:

    http://www.logwell.com/tech/components/resistor_values.html
     
    Last edited: Feb 19, 2014
  19. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    ok.. So i input the standard 50 ohm in input and output resistor and select required db? like this??
    http://i.imgur.com/fD6xPav.jpg?1

    the closest resistors i could find to the ideal values are then added to the lower half..
     
  20. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
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    That's the basic idea. 41.2Ω is not a standard value. I might have tried 39Ω.

    Why did you choose an attenuation higher than 10dB instead of lower? More attenuation reduces your already weak signal.
     
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