Modulation for short distance

Discussion in 'Wireless & RF Design' started by electrocute, Dec 14, 2007.

  1. electrocute

    Thread Starter New Member

    Dec 14, 2007
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    Which Modulation Do you think is best for sending encypted signals over a short distance of about a few hundred metres. Should it be PWM or PAM or PCM? As Communication is not my forte as I am a instrumentaion student so please help
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    What's preventing you from using AM? Without knowing more about the kind of signal, AM would seem to be simplest.
     
  3. Dave

    Retired Moderator

    Nov 17, 2003
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    What are the practicalities of encryption on AM?

    Dave
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
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    Some frequency bands do not allow encryption at all. Ham radio is one.

    You can transmit very simply with the arduino, reception is tricky though.

    What medium are you sending that over? Wire, air, sound....
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    There's encryption, and there's encrypting. The Navy sends encrypted data by letting bit pairs select tones. The non-harmonically related tones are mixed into a horrible-sounding composite and filtered out by the receiver. Then it decrypts the words following the scheme of the day.
     
  6. Eyas

    Member

    Dec 23, 2007
    10
    0
    good day everyone,

    one advantage of using digital communication over analog communication is that, when the signal is received, the data can be read even if there is some noise with it "PCM" because it is either 1 or 0. however, PWM and PAM can be effected by the noise. Also, if the data to be sent is originally analog like a human voise, you either use a FM or an AM. It is recommended to use FM since the noise mostly affect the amplitude not the frequency of the signal. this is a small project which is a FM transmitter which can transmit to a small distance. go to http://www.boondog.com
    Click on Tutorials, then Wireless FM Transmitter.

    or see the attachment.

    THANK YOU FOR TAKING THE TIME TO READ THIS LOOOONG REPLY AND GOOD LUCK :)
     
  7. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    All signals can be affected by noise, even digital. You can see by Shannon-Hartley's law that noise is a direct factor in maximum bit-rate.

    Secondly, the PCM signal is not a single bit signal, it is a binary representation of the sampled intelligence. Usually an 8-bit quantization. Then, this is serialized, usually into high-bandwidth channels, and transmitted. Since PWM, PCM, and PAM are digital signals, they all have the distinct noise immunity and therefore attractive for communications.

    FM and AM are the main two methods for transmitting both analog and digital signals. Amplitude Shift Keying (ASK) is a very common technique and there are chipsets which allow a simple 1Mb/s interface. There also exists a whole bunch of different modulation schemes that are based on AM FM PM, such as BPSK, QPSK, QAM, etc.

    FM signaling exchanges bandwidth for a better signal-to-noise ratio, whilst AM cannot obtain a better SNR without increasing tranmission power or improving an older style modulation (reg. AM compared to SSB).

    Steve
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    If you are encrypting the data yourself, then have a data stream, just send it over an ASK link. In the past, I have used some devices from RF Monolithics, namely their 1Mb/s 900MHz ASK transcievers. Since it is an asynchronous link, you will have to work at it to recover you data.

    In my particular situation, I used a simple UART-style serial link with start, stop, and data bits. There is a catch though, with these modules, you cannot have more than 2 or 3 ones or zeros in a row. You need to correct for this in software, I remember that it wasn't a big deal to do.

    If you need to really encrypt this data, you can look at microchip. They sell some really serious stuff, it is quite involved.

    Steve
     
  9. Eyas

    Member

    Dec 23, 2007
    10
    0
    Hi scubasteve 911 OR Steve,

    Thank you very much for the valuable information you post.
     
  10. abdnasir

    New Member

    Apr 14, 2007
    1
    0
    Eyas, Can U Please Upload Again The Fm Proj. Because I Couldnot Dowmload. Thanks
     
  11. oister

    New Member

    May 11, 2008
    5
    0
    one we did in our project was pwm
     
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