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Thread: Bias Tee Mystery

  1. #1

    Default Bias Tee Mystery

    I recently bought a set of 5MP wired security cameras (Night Owl brand) and wanted to have a camera at the end of the driveway. It is about 200 feet. I homebrewed up a set of bias tees to run the DC power over the coax so I didn't have to run extra wire and ran into a mystery. The cameras use a single BNC connector for the video and have an NTSC sticker on them. I jumped on wiki and assumed the frequency was around 4MHz so I made the bypass capacitor a 1nF (102) ceramic cap in both tee's. No signal through them. Without DC bias, shorting the capacitor allowed the signal to pass through. I tried cap after cap until I had a reliable signal through the tee. The value I ended up with was 100uF electrolytic. This is when I got really puzzled and connected the camera directly to my spectrum analyzer. There is a broad peak around 6.1MHz and another with several sidebands at 24MHz. Now im even more puzzled because that 102 cap should have done just fine at a higher frequency. Along with the 6.1MHz signal, there was a ton of much lower frequency stuff happening so I am totally lost as to what these cameras really put out. I am assuming the data is the 24MHz stuff and the camera is possibly doing some back and forth digital communication on the lower frequencies which explains why I needed a bigger cap.

    It works, but now I have another concern. Being I have that electrolytic cap in there, I worry that when I connect the power to the bias tees that the momentary displacement current through the capacitor will damage the camera. Has anyone done any work with these type of cameras? Anyone know if this lower frequency stuff is in fact control signals (I have spectrum images) and if I will cook the camera with that displacement current as the caps charge when plugging in the DC power?

    Thanks, 73!

    DC-9 MHz.jpg

    DC-31 MHz.jpg
    Last edited by brandon lind; Mon 19th Oct 2020 at 04:22.

  2. #2

    Default

    Update: I took a closer look with the oscilloscope and there is a ton happening in the kHz region. There are 24 bit data streams running at a rate of about 1.6Mbps separated by about 45microsecond timing pulses. There is also a 24MHz CW signal that is both duration and dc offset modulated throughout everything. It seems that the camera control signals fit into a 2.6ms period between 64ms of many digital video frames (still separated by the 22kHz timing signals). Now I have no clue about digital television signals but I need to learn because I just saw something very cool.......

    That video portion I mentioned, I zoomed in on one of the 22kHz spaced data portions and watched it as the girlfriend walked in front of the camera (scope tee'd into the video stream) and could see where in the camera view she was (left to right) on the oscilloscope trace! The change in the data chunk corresponded to where she was!

    Long story short, I have my answer. My capacitor must be big enough to pass that 22kHz timing pulse cleanly which explains why I needed such a ridiculous value cap to make it work. Also, being the camera signal voltage never exceeds 1.55v, I think I will zener the camera side of the bypass cap to stop anything higher than that from getting in when the bias tee capacitor initially charges from application of the 12v camera power and change inductor values correspondingly (maybe even add a parallel tank in the DC leg to reject 22kHz assuming an inductor alone to block 22kHz will be size and resistance prohibitive). Now my brain is in motion detection mode wishing my c# was better
    Last edited by brandon lind; Tue 20th Oct 2020 at 01:03.

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