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Thread: Is the video-line impedance of RMRC receivers verified?

  1. #1
    Sky Pirate & FPV Outlaw SecretSpy711's Avatar
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    Is the video-line impedance of RMRC receivers verified?

    Hello RMRC,

    I recently bought one of your 2.4GHz 14-channel SAW-upgraded receivers. I am having an issue when I plug my goggles into the receiver, I see vertical lines on the screen. When using a 5.8GHz relay, the lines seem to be gone, but my OSD text appears "smeared." But when using an ImmersionRC Uno instead of the RMRC receiver, the lines go away, regardless of whether I am using the repeater.

    I have not verified, but this seems like it could be an issue with an impedance mismatch on the video RCA plug in the receiver. I was just wondering if RMRC does any verification of this when received from the factory, or if the manufacturer you get them from has any data on it?

    The other parts of my video link are:
    Surveilzone HS1177 camera, which claims to be 75 ohms.
    ImmersionRC 2.4GHz 700mW vtx, which also claims to be 75 ohms on the video input.
    EagleTree Vector, which I assume uses 75 ohms but can't find it stated anywhere.

    Images of what I am seeing are posted here:
    http://fpvlab.com/forums/showthread....ot-expert-quot
    youtube.com/secretspy711
    COFPV.com

  2. #2
    Instructor Pilot Channel 1's Avatar
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    Quote Originally Posted by SecretSpy711 View Post
    But wheinstead of the RMRC receiver, the lines go away, regardless of whether I am using the repeater.
    Put an ohmmeter across the input, many times with video the termination is provided a resistor which will read around 75 ohms.

    Wayne
    Everybody loves a bunny.

  3. #3
    Sky Pirate & FPV Outlaw SecretSpy711's Avatar
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    Quote Originally Posted by Channel 1 View Post
    Put an ohmmeter across the input, many times with video the termination is provided a resistor which will read around 75 ohms.

    Wayne
    The input of what? Video receivers only have an output. If you do mean the output, the resistance appears to be infinite, except when I set the dial to 200MOhms, it starts at 4.0 and slowly decreases.

    If you mean the input of the repeater, then that is measuring 109 ohms.
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    Pilot HeliShredder's Avatar
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    You can't really measure the output impedance using an ohm meter. It works for inputs, as there is usually a 75 Ohm resistor to ground, but for the output the resistor is in series with the output output amplifier output, which can have a very high impedance when unpowered.

    To measure the output impedance, you would need to use an oscilloscope and measure the output amplitude with nothing connected and then connect a resistor with a known value from output to ground and see how much the amplitude decays. If the resistor is 75Ohm and the output impedance is 75Ohm, you will see 1/2 of the amplitude with the resistor connected.

  5. #5
    Sky Pirate & FPV Outlaw SecretSpy711's Avatar
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    Thanks Heli. I kinda gathered that the output had to be measured with an o-scope. Unfortunately I don't have one. But, are the input measurements with a DVM still valid?

    Turns out the vertical lines were being caused by the HS1177 camera itself. I tried many many different camera settings, tried powering it from 5v, 12v, and 16v, could not find a setting that fixed the problem. I switched to a trusty PZ0420H and lines went away. The Uno receiver, as well as using the IRC repeater vtx, are able to kinda mask/cope with the issue, whereas the RMRC receiver will show it.

    The OSD smearing is probably a combo of impedance mismatch and white-levels too high in the OSD (and the KVosd is somewhat known for it, smearing was never a problem on the Vector or older OSDPro). Again, the Uno receiver and repeater vtx are able to cope with it better.
    youtube.com/secretspy711
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