Howto:Improve signal quality

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If you are going to use more than 1 capture card in 1 system (like a pvr-500 which is 2 pvr-150's) or you are having problems with the quality of the signal there are some things you can do to improve the signal quality. Below are some steps graciously provided by Andy Walls, thanks!


Summary

The objective is to increase the desired signal power received at your tuner(s) and minimize degradation of the signal power to noise power ratio (SNR) in the video distribution system that is under your control (i.e. the wiring in your home). The actions in the following section will help you accomplish this by

1. Minimizing reflections that create artifacts or contribute to the noise floor by properly matching impedances.

2. Maximizing signal power transfer to all connected tuners by properly matching impedances.

3. Minimizing signal attenuation by minimizing cable and insertion losses.

4. Increasing signal power with amplification.

5. Minimizing potential intermodulation product contributions to the noise floor in the tuner caused by frequencies not of interest.

6. Minimizing electromagnetic interference (EMI) with shielding.

7. Minimizing EMI by avoiding ground loops.



Actions

  • If possible, replace all 300 ohm twin-lead transmission line runs, not solely dedicated to FM radio, with 75 ohm coaxial cable. Although 300 ohm twin lead wire has superior loss performance (only 1.1 dB per 100 feet at 100 MHz) compared to coaxial cable, since 300 ohm twin lead wire is not shielded it is much more susceptible to EMI. Unless you're sure you're in an EMI free environment for the VHF/UHF TV frequency bands, shielded, 75 ohm coaxial cable is a better choice. (Most PC's generate square wave signals internally with fundamental or harmonic frequencies in the VHF/UHF bands, and hence are a potential source of EMI.) The rest of the actions below assume the use of 75 ohm coaxial cable.
  • Ensure you are using 75 ohm splitters that cover your desired frequency range (55 - 802 MHz for the current US NTSC broadcast allocation). Using splitters of the proper impedance and frequency range is required for maximizing signal power transfer and minimizing reflections.
  • Use as few splitters, wall plate feed-throughs, and connectors as possible. Continuous cable runs are best. Every connection incurs an insertion loss. The insertion loss for splitters can be estimated to be anywhere from 0.5 to 3.5 dB before the power is split.
  • Ensure that all unused cable runs, wall jacks, or splitter outputs are properly terminated in a 75 ohm load. Again, to maximize signal power transfer and minimize reflections.
  • Replace RG-59 cables with RG-6 (or RG-11) cables. All three cables are type are required by MIL-C-17 to have an impedance of 75 +/- 3 ohms. According to MIL-C-17/2A, RG-6A/U is permitted to have a maximum loss of only 6.5 dB per 100 feet at 400 MHz. According to MIL-C-17/29C, RG-59B/U is allowed to have a maximum loss of 9 dB per 100 feet at 400 MHz. According to MIL-C-17/6B, RG-11A/U is permitted to have a maximum loss of 5.2 dB per 100 feet at 400 MHz. RG-6A/U is rated to operate up to 3000 MHz, while RG-59B/U and RG-11A/U are only rated to operate up to 1000 MHz. The goal is to reduce signal power loss through the cable.
  • If you have cable, if feasible, install an isolation transformer to avoid connecting the cable company's coaxial cable's shield to the shield of the coaxial cable in your home to avoid ground loops. Electric current on the coaxial cable shield due to a voltage difference between your home ground and the cable company ground is a source of EMI. http://en.wikipedia.org/wiki/Ground_loop_(electricity)
  • Make sure you have an in-line lightning protection/grounding block installed in the cable close to where the signal comes into your location. Grounding the shield of the coax protects your gear from electrical transients induced by nearby lightning. Grounding the shield of the coaxial cable near the antenna also appears to improve the EMI rejection provided by the coaxial cable, than if only grounded at the tuner end.
  • Add a broadband TV amplifier, that covers your frequencies of interest, as close as you can to where the signal comes into your location. Ideally place it before any splitters or long cable runs, but not before the split to your cable modem if it doesn't cover all the frequencies your cable modem uses.
  • If you install a broadband amplifier, but don't use your cables to receive and distribute broadcast FM radio signals, and the amplifier has an FM trap, set the FM trap to "IN" to reduce intermodulation products potentially induced in the tuner by the FM broadcasts in the 88-108 MHz band. Strong FM interferers will show up as a herring bone pattern on analog VHF channels. (The preselector in your tuner should do a fair job of filtering out FM broadcast signals already, but there's no need to make it's job harder by sending it amplified FM broadcast signals.) Set the trap to "OUT" if amplifying a cable television feed. Cable uses frequencies in the FM broadcast range for TV channels.
  • If you install a broadband amplifier that has too much gain for your needs, the excessively amplified signals will overdrive the front end of the tuner. If the amplifier overdrives the tuner, TV frequencies not of interest will show up as intermodulation products on the tuned channel as unwanted noise or interference. On analog channels an overdriven tuner may show a weak image of another TV channel in addition to the tuned channel.
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