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Notching 1407kHz out of a composite video signal

  1. Jul 2, 2012 #1

    davenn

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    hi gang,

    got a friend who lifes relatively close to a 1407kHz (multi kW) AM broadcast transmitter
    he has a ~ 50metre run of 75 Ohm coax between a security camera and the recording/viewing equip. The 1407 kHz is getting into the system and causing the usual herringbone pattern on the displayed video

    I have suggested trying some clamp on ferrite chokes on the outside of the coax to see if that minimises the RF signal. Still awaiting for a response on that test.

    Secondly am looking at an inline notch filter (trap) a parallelled L/C combination and having a variable C so as to have some tuning ability of the notch freq.

    firstly, there's lots of filter calculators online, none of them I have seen so far take the feedline or load impedance into account. how critical is that ?

    In his case of course its a 75 Ohm system.

    And Yes I know that at a last resort he may have to do a 2.4gig radio link for the camera. Tho a few components and connectors for a filter would be cheaper to impliment ;)

    Dave
     
  2. jcsd
  3. Jul 3, 2012 #2
    www.nuhertz.com has a free version of their filter design tool. I think the free version limits it to 3rd or 4th order. It does allow you to specify input and output impedances.

    Best bet for common mode filtering at these low frequencies is with a big MnZn type ferrite toroid with multiple turns. Impedance goes up as square of number of turns and no big worries about inter-winding capacitance at 1.4MHz.
     
  4. Jul 3, 2012 #3

    davenn

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    hi ya

    thanks for that link am in the process of downloading the file

    I'm not too certain if sticking a notch filter inline is going to cause problems with the composite video signal....
    any thoughts ?

    Dave
     
  5. Jul 3, 2012 #4

    jim hardy

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    shielding for RF is a bit of an art.

    One book i remember suggested it's necessary to earth the shield rather more frequently than 1/4 wavelength, which at your ~1megacycle would be 75 meters, a bit longer than your run.
    1/10 was better they said, ~30 meters for yor friend.

    Is it a "home run" cable? Is there perhaps a connector midway where you could earth shield?

    Yungman probably knows way more than i.

    old jim
     
  6. Jul 3, 2012 #5

    Averagesupernova

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    I would by a cheap RF modulator and send the video that way instead of sending baseband all the way.
     
  7. Jul 3, 2012 #6

    davenn

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    Hi Jim

    Im not personally familiar with the setup my mate lives some distance from me but yes its a home location, so maybe multiple earthpoints may be possible
    -----
    Yes thats true, he may have no choice but to go to an RF link is all else fails.

    Dave
     
  8. Jul 3, 2012 #7

    vk6kro

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    If the equipment at either end is in plastic boxes, filtering or RF links probably won't help.

    Putting sensitive equipment in plastic boxes has become almost universal now and it is hard to find properly shielded metal cases on equipment any more.

    Your friend may have to look for high end professional gear to get rid of this problem, or be prepared to get his equipment custom mounted in metal boxes.
     
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