Transmitting 7MHz Signal Wirelessly

Discussion in 'Wireless & RF Design' started by Haxord, Jul 22, 2013.

  1. Haxord

    Thread Starter New Member

    Feb 15, 2011
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    Hello,

    I am an Electronic Engineer who has not had much experience with practical wireless systems. I am currently facing a problem that I think I can solve using a wireless system.

    There is an RF signal being transmitted via cable from one device to another (one way). Due to constraints at the location of these devices the cable cannot be used. I want to transmit this signal wirelessly. The signal itself is transmitted periodically and I am told it is a 7 MHz signal (I assume its digital but I could be wrong).

    I would like some pointers on what technology is capable of transmitting this signal and reproducing it on the other end. References to books I could use or commercial off the shelf solutions would also be useful.

    I would like to also test the signal using a scope and a spectrum analyzer however I do not have access to these devices any more. Once I have an idea on how the problem can be solved I would gladly invest in these if required :)

    Any help/pointers will be appreciated. Thanks.
     
  2. Sue_AF6LJ

    Member

    Mar 16, 2013
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    You need to consult FCC Part 15, I don't believe it would be legal to transmit that signal in the open air unless you can meet the Part 15 requirements.
     
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  3. Papabravo

    Expert

    Feb 24, 2006
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    1,786
    You might want to consider combining that signal with a carrier signal in a frequency band where low power unlicensed operation is allowed. Furthermore, if the bandwidth of the information content is DC to 7 MHz., you're going to have to go up in to the microwave region to find a 7 MHz. wide channel. I can tell you from experience this is no place for amateurs and weekend dablers. The equipment is expensive and you may have to fund multiple tries before you get a workable solution.
     
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  4. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Without a real idea of the signal's complete spectra,it is impossible to help you.

    If "7MHz" is its total bandwidth & not its repetition rate,you may be able to use something like a TV transmitter.

    Anoiher possible way is modulating a laser,or using a TV OB link.

    If "7MHz" is its repetition rate,the bandwidth required will be very much greater.

    You cannot transmit the baseband signal,because :-

    (1) It won't work,
    &
    (2) It will interfere with licenced Broadcast & Communication stations.

    As Sue says,you need to consult the Radio Licencing Authority in your country.
     
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  5. Haxord

    Thread Starter New Member

    Feb 15, 2011
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    Thanks for the replies. I think I have more information about the signal. I found an active wired repeater that passes this signal and its specs is 7MHz +/- 500KHz. I assume this is a frequency modulated signal, is this correct? :confused:

    If yes, will there be any legal way of transmitting this signal using wireless?
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    So the signal occupies a spectrum from 6.5 MHz to 7.5 MHz. This covers both a military SSB allocation, an international SW band, and the entire 40M Amateur Radio band. You have no chance to transmit this signal in the US, as it is, without causing harmful interference. Doing so could result in substantial penalties, asset forfeiture, and jail time.

    What you need to do is find a 1MHz wide chunk of spectrum and combine your signal with a carrier signal to produce an output at the sum and difference frequencies.

    For example If you took a carrier of 505 MHz. and combined it with your FSK signal then the sum would be:
    505 + 7 = 512 MHz. ± 500 kHz.
    and the difference would be:
    505 - 7 = 498 HHz. ± 500 kHz.

    I'm not saying that operation at these frequencies would be legal. I'm only trying to construct an example of using a circuit called a 'mixer' to convert a signal from one frequency band to another.

    In short, a mixer takes input signals at two different frequencies. The mixer's outputs have energy at the frequencies of the two input signals and at the sum of the two input frequencies and the difference of the two input frequencies. Filters are placed on the output of a mixer to eliminate the undesired frequency components.

    In looking through RF spectrum allocation charts pay attention to the ISM (Instrumentation, Scientific, and Medical) bands. Power and range are usually limited so pay careful attention to transmit and receive antenna design.
     
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  7. Haxord

    Thread Starter New Member

    Feb 15, 2011
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    Is there no way of getting around the legal limitations by limiting power? I only really need it to travel approx 30 meters through two sets of walls.
     
  8. Sue_AF6LJ

    Member

    Mar 16, 2013
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    27
    You might want to investigate using the 900MHZ band, You actually may be able to find an off the shelf transmitter that would handle signals of that bandwidth, if not you will be looking to use some off the shelf gear for one of the UHF / microwave ISM bands.
     
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  9. Haxord

    Thread Starter New Member

    Feb 15, 2011
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    From what I have gathered, the ISM range in my area is 2400MHz to 2500MHz. Assuming my signal is 7MHz +/- 500KHz, will something along the lines of this work:

    http://dx.com/p/2-5ghz-2w-wireless-transmitter-receiver-kit-silver-black-155938

    If it will, are there any better transmitters/receivers that any of you know off? If not, will any sort of commercial off the shelf transmitter/receiver pair be able to transmit the signal?
     
  10. Sue_AF6LJ

    Member

    Mar 16, 2013
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    Unless you convert to baseband and modulate the transmitter then upconvert at the other end you will need a transmitter with a modulation bandwidth equal the the carrier frequency plus the modulation, and a little headway because operating with transmitter / receiver that is bandwidth limited to just outside of your modulation frequency will introduce problems associated with phase shift of the modulation signal at or near the "corner" frequency, that being the 3DB point where the modulation bandwidth starts to roll off.

    Another issue you need to consider is maintaining proper phase.
    FM telemetry / video transmitters have a specification called Mod Sense, a transmitter with negative mod sense will swing down in frequency when a positive modulating voltage is applied, conversely a transmitter with positive mod sense will swing up in frequency when a positive signal is applied.

    Another but less important issue today is modulation / demodulation comparability. If you are using a direct FM modulation scheme make sure your receiver is using a true FM detector, not a phase detector. The PM receiver can introduce errors in the data stream. If it is an analogue signal this is much lass of an issue.

    I know about this last issue due to my work at a defense contractor.
    We built (myself and 3 other techs and a half dozen assemblers) 175 direct FM telemetry transmitters for some of Jimmy Carter's aid to Israel, the transmitters worked flawlessly, the US government military engineering group who designed the system were using a phase mod receiver to receive the commands sent by those transmitters. They had an excessive number of errors and traced it down to noise in the PM receiver. The problem was brought to our attention and our engineering group identified the problem by simply substituting one of our FM bench receivers used in testing. No more errors.

    In accordance with predictable government procedure instead of replacing the receivers, the transmitter exciter were replaced with a phase modulated version. The US government paid for the R&D, engineering and gave us back the unusable exciters, which we gladly modified and used in other projects.
     
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  11. Haxord

    Thread Starter New Member

    Feb 15, 2011
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    I think I am going to investigate the possibility of maybe getting digital data from the FSK pulse. Then I would be back to familiar territory and can send the digital data using WiFi and reproduce the signal on the other end. I dont think timing/delay will be an issue because the system tolerates lost transmissions and its basically there just for authorizing the second device to operate every couple of minutes.

    Any hints on IC's or circuits to extract the digital data from the source signal? I was thinking of just using filters centered around the frequencies of interest, but im sure theres a better approach. Thanks
     
  12. Sue_AF6LJ

    Member

    Mar 16, 2013
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    Sounds good to me.
     
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