~800MHz sniffer

Discussion in 'Wireless & RF Design' started by cam0, Mar 25, 2010.

  1. cam0

    Thread Starter New Member

    Mar 23, 2010
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    0
    Hi,

    I've a project in mind, but I'm stuck at one point. Let me explain you my problem. I want to "sniff" a signal being emitted around 800MHz (Between 766MHz and 833MHz, I'd like to tune it), which has an FSK modulation enveloping a digital signal. I got classes about DSP, where I think I am ok, but I am bugging with the signal acquisition [in practice]. The USRP from ettus would be a very good solution, but it is too expensive for my wallet :/. I've been looking for classical FM receiver (superheterodyne, etc), but 800MHz seems to already be a "high frequency" that requires special hardware.

    Can you help me a bit ?

    Thanks
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    That is a bit too high a frequency for on-the-cheap experimentation. Lumped circuit elements don't behave at those frequencies and microstripline techniques come into play. The CAD tools for doing this kind of design are both propritary and expensive. The test equipment is beyond the budget of even a modestly wealthy hobbyist. A used VNA might cost $20,000 to $30,000. A new one in excess of $100,000.00 In the words of my New Jersey cousin "fageddaboudit".
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    UHF TV receivers tune up to around 850MHz (at least here in the UK)

    You may be able to just use a portable TV, or build something based on an analog TV tuner module and a home built IF & demodulator - or even just feed the tuner IF out to a scanner, I think typically the IF is at about 50MHz..
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    It is true that with prebuilt modules you can cobble something together, but without schematics or test equipment it will be a challenging task. In addition I don't think a TV tuner will do much with an FSK signal. Life is too short to live like that.
     
  5. cam0

    Thread Starter New Member

    Mar 23, 2010
    25
    0
    Thanks for your replies. So the USRP (or a good oscillator) might be the cheapest solution :S. I'm gonna think about it.

    Thanks !
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    All the tuner is doing is acting as a wideband down converter - if it can downshift a video signal with several MHz bandwidth it can certainly handle FSK...

    It's the most basic superhetrodyne concept - shift the wanted RF down to a practical frequency for filtering and demodulation. 50MHz is not difficult to work with, 'dead bug' construction is fine.

    Just for info, 800 MHz is still easily within the range for DIY construction - there is a ham radio band around 1300MHz which I've designed and built gear for, and some hams build gear up in the GHz range with nothing but minimal test equipment.
    You do not need to spend a lot of money, but patience, techniques and experience are everything at UHF & Microwave frequencies.

    For someone new to RF construction, the use of a converter to get the signal down to a frequency where prototyping techniques are practical is simply a way of making the project possible rather than saying forget it.
     
    Last edited: Mar 27, 2010
    cam0 likes this.
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