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Frequency response ?

Discussion in 'LEDs and Optoelectronics' started by Externet, Feb 9, 2019.

  1. Externet

    Externet

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    Hi.
    For plain indicator LEDs, nothing fancy, rarely published in specs... What could be the typical maximum frequency response ?
    Do they go beyond 10MHz ? Varies with color/chemistry ?
     
  2. Bluejets

    Bluejets

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    LED is d.c.
    It will only show average as far as I am aware.
     
  3. duke37

    duke37

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    There was an article in RadCom a while ago describing how to send signals a long way. They used SHF and flat lenses intended to be used for rear view with caravans. The sending and receiving LEDS were the same. Alignment was achieved with visible light and several km were managed.

    Visible LEDs appear to be on continuousy when pulsed above 100Hz or so. It is interesting to wave an LED clock around a see where the image is generated.
     
  4. Externet

    Externet

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    Perhaps am misunderstood. Up to what frequency can be pulsed ? Trise+Tradiative+Tdecrease+Toff
    [​IMG]
     
    Last edited: Feb 10, 2019
  5. duke37

    duke37

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    LEDs are used in internet fibre optics. Don't know the details.
    White LEDs using fluorescence might be quite slow.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    This abstract and some of the comments following it may be useful.
     
  7. Externet

    Externet

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    Thanks, fellows.
    Canibalized a Toslink Tx jack from my junk box to use that LED, supposed to be faster. Will see...
     
  8. Nanren888

    Nanren888

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    Might make a big difference how you drive it.
    I guess the ones from CD writers might also be fast? (No clue of the light output)
    I guess we're assuming you are going to try signalling?
    Got a fast receiver?
     
  9. Externet

    Externet

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    CD writers use laser, not LEDs.
     
  10. BobK

    BobK

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    Laser diodes, which are LEDs sandwiched between mirrors.

    Bob
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Average to the naked eye, but pulse when "seen" by a suitable optical receiver.

    The frequency response seems to be a parameter often missing from datasheets. Here's an overview with some good data. Figure 4-7 shows an example of an LED with a -3 dB cutoff frequency of 50 MHz.

    LEDs are the standard transmitters in photocouplers. Data photocouplers can be had for data rates up to 50 Mbps (equiv. to 25 MHz), possibly even more. At high speeds a suitable driver circuit is of the essence. Here's an example for a 270 Mbps LED driver (assuming you get the required very fast LED).
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    There's more to it. The construction of laser diodes differs considerably from LEDs, so does the typical way they emit light.
     
    hevans1944 likes this.
  13. hevans1944

    hevans1944 Hop - AC8NS

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    Absolutely! The first laser diodes were edge emitters, manufactured by cleaving the semiconductor diode wafer material. Not a very bright or spatially coherent source of laser radiation. Lately VCSELs (Vertical Cavity Surface-Emitting Laser) have been developed with much "better" optical characteristics. These employ a traditional mirrored cavity to promote a more coherent beam output that is more easily focused. This Wikipedia article has a lot of good information on various diode laser types and how they are constructed.
     
  14. poor mystic

    poor mystic

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    Hi guys
    Obviously, the diodes used in high speed data transmission have entirely different characteristics to the ordinary diodes described in Duke37's data sheet. Still i like the idea of using such ordinary devices because they are adequate up to useful speeds and they are not lasers.
    I'm very nervous of lasers. I think they should all be locked up where nobody can see them :) BTW... radio is a good option if you need greater data rates.
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    Greater data rates than what?

    You are aware that both radio and light are electromegnetic radiation, are you? Only light has a much, much higher frequency and therefore is capable of much higher bandwidth transmission and therefore higher data rates than radio,
     
  16. poor mystic

    poor mystic

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    I am well aware that light can be modulated and demodulates at far greater.rates than can radio; my only objection to lasers is that they are more dangerous than radio, to experimenters.
     
  17. hevans1944

    hevans1944 Hop - AC8NS

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    Just because something is "dangerous" is no reason to avoid using it. Life itself is dangerous because (last time I looked) there have been NO known survivors. In this electrical/electronics hobby we encounter many dangerous things, for which education and the practice of safety precautions is necessary to prevent mishaps.

    Years ago my enjoyment of electronics required that I learn to use devices called thermionic electron emitting, glass enveloped, vacuum tubes. These were available in a huge variety exhibiting different characteristics and multiple functions. Some were quite large (almost thirty inches in diameter) and very heavy, requiring special restraining techniques to safely support them for viewing in a home environment. They all employed potentially deadly voltages with lethal current capabilities, usually several hundred volts and at least as many milliamperes, but some employed tens of thousands of volts and several milliamperes of current. If struck a sharp blow, they would shatter and implode with great force, scattering sharp shards of glass and dangerous chemicals throughout their immediate vicinity. None of this stopped me from approaching these dangerous beasties and learning how to maintain and repair color television receivers.

    Lasers are NOT intrinsically more dangerous than radio. The dangers associated with using lasers are simply different than the dangers associated with radio. The experimenter must learn how to use either technology in a safe and appropriate manner, preferably by education and not by trial-and-error. For example, never stare into the output aperture of a laser with your remaining good eye. Never stand in the near-field vicinity of an end-fed radio transmitting antenna, not even once, when 1500 watts of peak envelope power is being applied. Stuff like that.
     
    Harald Kapp likes this.
  18. BobK

    BobK

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    Yeah, like “don’t piss on the third rail.”

    Bob
     
    hevans1944 likes this.
  19. poor mystic

    poor mystic

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  20. hevans1944

    hevans1944 Hop - AC8NS

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    This would be an example of a total failure of operational security. What organization would go to the trouble of "air gaping" their hardware and then create an open window for the world to look in on? And what kind of op-sec protocols would tolerate the unmonitored presence of thumb drives and removable disks of any kind when there is a possible purpose of uploading malware? Might as well provide unrestricted access to your local area network!

    Back in the day, "black" systems were placed inside Faraday cages with controlled and monitored admission through a bank-vault type of door with copper finger-seals around the edges. All the power lines and telephone lines that penetrated the Faraday cage had special filters installed to suppress unauthorized signals from entering or leaving the Faraday cage environment. Anything connected to a computer (keybaords, monitors, removable storage devices) had to pass a TEMPEST emission test. Then, and only then, could the room become a certified SCIF (Sensitive Compartmented Information Facility).

    So, no air gap back then, but only because we didn't know (yet) just how sneaky attacks could be. Below is an image of a typical entrance to a SCIF. Doors could and would be more elaborate, depending on the nature of the classified material being safeguarded by the SCIF. The U.S. Government became downright paranoid about SCIF security dating back to at least when the Soviets implanted numerous "bugs" in the new U.S. Embassy they "kindly" built for our use in the 1960s.

    [​IMG]

    I would have to classify the link posted by @poor mystic in his post #19 as "fake news". Just try flying a drone anywhere near, say, the NSA building in Ft. Meade to see what sort of response that elicits.
     
    Harald Kapp likes this.
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