Combining an RF signal with PWM

Discussion in 'General Electronics Chat' started by fz3012, Jun 28, 2014.

  1. fz3012

    Thread Starter New Member

    Jun 28, 2014
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    Hi everyone,
    I am working on Visible light communication (VLC) where LED light is switched at high frequencies to transmit data. I am currently working on OOK modulation.

    I need a component to combine very fast RF signal (~10MHz) to a relatively fast PWM signal (~1kHz). I am having a hard time in finding an equipment which can be used to combine them.

    I am attaching a block diagram to show what I am looking for.
     
  2. dougc314

    Member

    Dec 20, 2013
    38
    11
    Try a device called "Analog Switch" such as Texas Instruments TS5A3167.
     
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  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    An AND gate it would work also. Are we missing something??

    Mark
     
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  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
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    Why such a high RF carrier on a light beam ?
    OOK = on-off modulation ?
     
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  5. dougc314

    Member

    Dec 20, 2013
    38
    11
    The way its drawn, the LED drive is a AC signal superimposed on a DC bias, so it's not necessarily logic levels. That's why I suggest a switch. The circuit could be simplified to a logic type oscillator that drives 1 input of an and gate with the PWM driving the other. Then a resistor between the and out and the anode of LED (cathode to ground) would complete the circuit.

    Whether he can actually use either an and gate or the suggested switch depends on voltage levels and desired currents. As drawn its very generalized and we can't tell for certain.
     
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  6. fz3012

    Thread Starter New Member

    Jun 28, 2014
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    There is no constraints in my desired voltage level, the current is limited to a maximum of 1A for the LED to be safe. I was looking for a device which combines two AC signal (one of them biased) and also has SMA connection since I am using AWG. A switch can also be used but that would cause more difficulty in understanding the duty cycle of the PWM which is a key variable in my experiment. SO something like a multiplexer or a combiner? Which is available commercially?
     
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Is it a true AC signal or a pulsed DC signal? Can the output of an AND gate feed a FET or two to drive the LED? Is an LED the actual load or are you using it as a example?

    Not sure why the switch makes it more difficult to understand the duty cycle. Must be me.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What is the purpose of the bias-T? Is it only to get a power source to the missing circuit, or is the DC offset important for LED operation?

    Can on or off keying happen in the middle of a 10 MHz carrier cycle, or must the on and off transitions be delayed to the next zero-crossing?

    Is the DC injected by the bias-T stripped off before the combined waveform gets to the LED, or is the DC bias important to the LED operation?

    AND - what are the voltage levels of the PWM signal and the carrier?

    ak
     
  9. fz3012

    Thread Starter New Member

    Jun 28, 2014
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    The bias T combines the OOK modulation with the DC voltage which is needed to run the LED. The DC offset is important for the LED to run.

    The OOK can work either ways. I am looking to transmit data as well as having dimming control.

    The DC bias is important to the LED operation.

    The logic levels can be 1 or 0. Or can take any value depending on the data I want to transmit and the dimming level I need to achieve.
     
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