can a pwm create a radio wave?

Discussion in 'General Electronics Chat' started by leonheard, Apr 28, 2013.

  1. leonheard

    Thread Starter Member

    Jan 8, 2013
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    hi!

    correct me if am wrong...


    if i have a circuit: a DC power supply 5v, to turn ON a simple led and a switch, when i press the switch it will turn the led ON and it will create a radio wave...?

    soo... simply turning a device on and off many times will produce radio waves?!



    if i make an astable multivibrator, doest it create radio waves?!

    please help! :)
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Yes, a square wave signal has a lot of harmonics, that can even reach to the radio frequencies.
    It will specialy "transmit" when an inductive load is used like a motor.
    Thats why shielding and decoupling are important.

    Bertus
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Any AC voltage on a wire creates a radio (electro-magnetic) wave. The strength of the radiated wave depends upon numerous factors including the frequency, the AC voltage, the wire length, any shielding and nearby metal, etc.
     
  4. leonheard

    Thread Starter Member

    Jan 8, 2013
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    I already know that, but can i make a radio wave with a DC voltage? turning on and off many times ( astable multivibrator) ?
     
  5. leonheard

    Thread Starter Member

    Jan 8, 2013
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    why it need a inductive load?! if the astable is turning on and off many times per sec. isnt it enought?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    An inductive load will create spikes on the up or down going edge.
    This will make the radiation of RF stronger.
    Take a look at the subject of EMI (electro magnetic interference).

    Bertus
     
    leonheard likes this.
  7. Wendy

    Moderator

    Mar 24, 2008
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    Any AC signal can be transmitted, even what we normally consider audio. The submarine services use ELF (Extra Low Frequencies) because they penetrate deeper into water for example. When you pulse a DC voltage you are created a AC wave, that is a mixture of both.
     
  8. #12

    Expert

    Nov 30, 2010
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    You can radiate radio frequency waves with any fast switching. There are several problems with this. First, they are all over the frequency spectrum and cause interference with other devices. Second, they are useless for carrying information unless you are trying to do something like Morse Code in a Hollywood movie.

    I'm sure there are more disadvantages but I think this answers your implied question.
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yes, you can make radio waves by switching a DC.

    No, you should not do this.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Glad to know you know that. :rolleyes:

    If you switch DC then you no longer have DC. You have a square-wave containing many AC harmonics.
     
    leonheard likes this.
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I think there is a "disconnect" here (pun intended). I think the OP wants to create radio frequencies with a timer...
    ... some of the answers are referring to generating radio waves with physical switching (arcs from relays and/or commutators) that lead to radio frequency interference.
     
  12. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Hey I made a detectable AM signal with nothing but a 555 and an (expensive labratory) oscillator in series with it's power supply.

    "Detectable" means I could hear a tone on a radio set on or near the same bench.
     
  13. #12

    Expert

    Nov 30, 2010
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    The datasheet graph on an NE555 goes to 100KHz.
     
  14. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Yeah, and the 10th harmonic of 100 kHz is 1MHz ... well within the Broadcast band we call AM which is why the proximity of the receiver and the 555 was not large.

    Imagine receiving complaints about 60 Hz interfering with a signal at 10.2 kHz (170th Harmonic).
     
  15. leonheard

    Thread Starter Member

    Jan 8, 2013
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    interesting.... i didnt know that an astable multivibrator is no longer a DC.... many thanks 2 you all! :)
     
  16. Ramussons

    Active Member

    May 3, 2013
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    A square wave has odd harmonics. You can pick up the higher harmonics by a tuned LC tank circuit. A whole lot of frequency multipliers use this technique. A sine wave is distorted to generate harmonics for this purpose.

    Ramesh
     
    Last edited: May 6, 2013
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