Wednesday, October 16, 2013

Hump Day...Been Busy at Work

Been really busy at work rebuilding the CCTV system. Although Honywell was the prime contractor (they did our fire and other alarm systems) I think they subbed this out to Larry The Cable Guy!

We kept having problems with noise in the system, dead jacks on the patch panels, and overall degradation of the picture the last few years. I first looked at it last year, made some adjustments, and the picture improved enough that we just let it slide. A few weeks ago I finally convinced TPTB to just let me go nuts on it.

The building where all the cameras are located is where I started. This CCTV system is only used during spacecraft fueling and payload encapsulation activities, so it's a fairly small system, with 8 cameras. Each camera feeds a Video Distribution Amplifier with three outputs. One output goes to an RF modulator where we put the video on a "Cable TV" channel, and the other two go to a patch panel. The output of each modulator then goes to a channel-specific bandpass filter, through a splitter (more on this later), and then to a 12-way combiner. The output of the combiner then went to a 30dB amplifier/line driver, with a 20dB pad on the input, and into 850 feet of RG-11 cable to get over to the building where it gets patched to the end users. The bad thing about the system design is that it passes through the building where my office and all the end users are, continues on to the main office building where the patch panel is, and then comes back to my building where it finally gets distributed to the end users!

An extra 600 feet of cable running between two buildings that the signal goes through, just because somebody wanted the patch panel in the Telco Room!

ANYWAY......The two unused outputs of the Video Distribution Amplifiers weren't being used, and were unterminated, so I put 75 Ohm loads on them. Unterminated inputs and outputs are bad juju on cable systems, as they cause reflections, which causes "ghosting" on the system, just like when your outside TV antenna was aimed wrong in the old days.

After terminating all the unused inputs/outputs, I went and adjusted all the modulators for the same output level so I'd have a baseline for future reference. I also found that there was NO reason to use a splitter on the output of the bandpass filter, as one of the splits was ONLY used locally, at the patch panel, as a monitor point. So, I removed the splitters and replaced them with a "tap" (actually a Directional Coupler) which "taps" off a small amount of the signal, rather than dividing it into two equal signals. They system was also designed with an inordinate amount of "flexibility", allowing you to patch anything to anywhere, something which had never been used, and never will be used. This involved running all the signals through a large enclosure with dozens of splitters, all sucking out their share of the signal, and all going to the patch panel unterminated.

I basically took an axe to it!

When I was done with that, I took a look at the amp with the pad on the input.

With a 20db pad (attenuator) on the input, the amp was producing roughly 10dB gain to drive the cable, almost not worth it, as the amp adds noise, and overdriving it even just slightly, resulted in the noise floor coming up significantly, and spurs popping up all over the place.

After axing out all the unnecessary splitters, replacing the output splitters with taps, terminating the 4 four unused ports on the 12 way combiner, replacing most of the cables and connectors with high quality parts, removing the line driver amp, and readjusting the modulators, I had a MUCH cleaner signal, and had 18dB more signal into the cable!

Almost all of the connectors were poorly crimped on, some falling apart as I removed them, and the cable was "Consumer Quality" RG-59. RG-59 cable is OK in your house (well....."kinda sorta" OK), but I won't use it on a "Production" system. The center conductor is copper-plated steel, and in the presence of moisture, the steel starts to rust, the copper plating blisters off, and the conductivity goes sharply down. I replaced it with Belden RG-6QS, rated (and swept) to 3GHz. The Belden cable has a solid-copper center conductor, two layers of braid, and two "foil" shields. It's really excellent cable for this type of use, and I mated it with Thomas & Betts "Snap-N-Seal" F-Type connectors, which have internal o-rings to weather proof them.


Now I'm working on the next part of the system, which is the patch bay located in the main office building. It was originally located there as they wanted to be able to send the video to certain offices and conference rooms during certain operations, something which has never been done.

And I'm finding the same things; cheap connectors and cable that fall apart, unneeded splitters sucking out signal, another unneeded amplifier, and four unneeded bandpass filters.

I removed all the equipment we don't use, all of the cables and splitters, and started putting the rack back together today. We still haven't decided if we're going to move the rack over to the building where all the end users are located, but I think we should, as it would eliminate 600 feet of cable run, and leave us with five cables between the buildings that we can use to run the signal back to the two places where it's needed, the server room, and one office that would be used as an "Incident Command Center" in case we had an accident during spacecraft fueling operations.

At least I'm being left on my own to do the job, as nobody there understands things like this, usually good for job security!

5 comments:

  1. That's the way I like jobs. Tell me what you want done then leave me alone to do it. I'll tell you when I'm done. And yess Belden makes very good cable and RG6 should be used almost anywhwere a good signal is needed. I use it for my Free to Air TV antenna reception.

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    1. I'm using Belden 7916A. I bought a 1000' spool from Solid Signal, as the cost for getting the couple of hundred feet I needed to rewire my house was MORE than buying the whole spool!

      I also use it for my scanner radios and antennas as it's low loss, and the small impedance mismatch between 75 Ohms and 50 Ohms doesn't matter.

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  2. AND you're having fun... :-) Not a bad thing!

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  3. No rest for the wicked, eh? But what they said, above - best jobs always involve a clear objective & no interference.

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  4. yep, always good to avoid micromanagement!

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Keep it civil, please....