FM frequency

Discussion in 'Wireless & RF Design' started by nerdegutta, Oct 30, 2010.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi,

    I have built a FM transmitter. When I connect it to the computer speaker, and power it up, I hear it in my radio. And by turning the variable capacitor, the frequency changes.

    On my frequency counter, DVM13MFC2 from Velleman, the frequency shown is from 30-55MHz, according to the turning on the capacitor.

    The question is: Why do I hear it on the radio? Does the radio pick up the 2nd harmonic or something? I'm a little bit in the wild here...:confused:



    I'm aware of the law and regulations in the 88 - 108 MHz FM range.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It is most likely the 3th harmonic.
    What are the dimension of the coil and the value of the Ctrim?

    Bertus
     
  3. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi,

    The coil is an air core coil.

    Diameter : 10 mm
    Length : 7 mm
    Turns : 4
    Wire: copper 0,5 mm
    Antenna tap in center.

    The ctrim is 6-70pF, I guess...
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you see this thread?
    http://forum.allaboutcircuits.com/showthread.php?t=7130

    To calculate the value of the coil, take a look at this page about aircoils:
    http://info.ee.surrey.ac.uk/Workshop...air_coils.html

    To calculate the resonance frequency take a look at this page of the AAC eBook.
    http://www.allaboutcircuits.com/vol_2/chpt_6/5.html


    I used the calculator from this page for the value of the aircoil:
    http://www.mantaro.com/resources/imp...calculator.htm

    I have done some little calculations:
    L = 100.440 nH (when dia = 6 mm and lenght = 10 mm and 6 turns)

    Lower frequency ( C = 30 pF) = 91.6 Mhz
    Higher frequency ( C = 5 pF) = 224.6 Mhz
    Middle frequency ( C = 15 pF) = 129.7 Mhz

    Greetings,
    Bertus
     
    Last edited: Oct 31, 2010
  5. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    That's weird. I do not get the same numbers you do.

    Here's what I got:

    L=0.001N²r² / (228r + 254l)

    L=0.001 x 6² x 0.003² / (228 x 0.003 + 254 x 0.01)

    L=0.001 x 36 x 0.000009 / (0.684 + 2.54)

    L=0.000000324 / 3.224

    L=0.000000100

    Where do I take the wrong turn? :confused:

    I do not get the same numbers with this either...
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As you can see I can even go wrong.
    Your calculations are correct.
    I found an other calculator page that also gives the value you calculated:
    http://www.mantaro.com/resources/impedance_calculator.htm
    The answer for the transmitter tread is 100.440 nH

    Your coil is : 136.939 nH

    I will correct the page also in that thread.

    With your coil and capacitor the frequency will be between:

    Ctrim = 6 pF : freq = 175.6 Mhz
    Ctrim = 70 pF : freq = 51.4 Mhz

    Bertus
     
    Last edited: Oct 31, 2010
  7. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Thank you. I can now stop pulling my hair... :)

    I got the same result with this formula:

    [​IMG]

    Now I'm back on track. :D

    So, by manipulating the coil, I can narrow down the frequency. Nice!
     
  8. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hm... How did you get this? I've tried

    http://www.allaboutcircuits.com/vol_2/chpt_6/5.html

    with no luck. *start-pulling-hair*
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The highest frequency is :
    1 / (2 * pi * sqr(136.969 * 10^-9 * 6 * 10^-12)) =
    1 / (2 * 3.141592 * sqr(821.634 * 10^-21))=
    175.582 Mhz.

    The lowest frequency is :
    1 / (2 *pi * sqr(136.969 *10^-9 * 70 * 10^-12)) =
    1 / (2 * 3.141592 * sqr(9.58273 * 10^-18))=
    51.413 Mhz.

    As you can see the spread in frequency is large due to the large change in capacity.

    Bertus
     
  10. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Thank you, Bertus. Now I got the same numbers.:)
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    If you make the FM transmitter on a pcb then add about 10pf of stray capacitance to everything.
    If it is made on a breadboard then add about 30pf to everything plus add stray series inductance to each wire. No wonder its frequency is too low.

    Most cheap FM radios are overloaded by a little FM transmitter close-by. Then the signal is received all over the dial and beyond.
     
  12. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi,

    all my RF projects are built on PCB I etch myself. I've read about stray capacitance, skin-effect and "micro-phony" in a few books.

    So far I've only noticed "micro-phony", but as long as the PCB is laying still, I'm not to bothered with it. I guess it'll be better if I mount the pcb in a metal casing, or if I put aluminum foil inside a plastic box I have. (as long as the foil is not connecting/touching the pcb.)

    I didn't know I should add 10pF.

    Thanks.
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As Audioguru said the frequency will go lower when the stray capacity is larger.
    The calculated frequencies are in ideal cases where there are no stray capacities.

    I have attached a PDF with a lot of information on RF prototyping.

    Bertus
     
  14. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    2,515
    785
    Thanks.

    This is a good document.
     
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