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Baluns. I got this one wrong.... (Patreon)

Published:
2023-11-22 03:37:36
Imported:
2023-11

Content

I was expecting these devices to be galvanic isolators as well as impedance matchers, but I was wrong.

These are VERY useful devices that allow the transmission of video and power from a camera to a monitor or recording device, using plain CAT5 style network cable.

That saves cash, cable complexity and possibly allows the use of existing cables for video, audio and other data like DMX.

https://www.youtube.com/watch?v=BtcVHG38cTs

Files

Baluns - I got this one wrong (with schematic)

I got this completely wrong. I expected it to be for galvanic separation and impedance matching between the 75 ohm CCTV output and the 100 ohm CAT5/6 cable. In reality it's just for impedance matching. In my defense, it is called a balun (balanced to unbalanced) and I'd describe it more as an impedance matcher. If you want to know what that's about then here are some Wikipedia links with enough formulas to satisfy the most geeky data-nerd. For the rest of us it will make some pretty tough reading. https://en.wikipedia.org/wiki/Characteristic_impedance https://en.wikipedia.org/wiki/Impedance_matching https://en.wikipedia.org/wiki/Electrical_termination

Comments

Anonymous

Baluns were a very common consumer product here in North America for a long time and while they are usually installed at the factory these days, still are used on TV aerials. In North America, we used two different kinds of aerial connector and aerial cabling for many decades. The original connection was for 300 ohm twin-lead cable. These were screw terminals. The newer (and now predominate) connection is the 75 Ohm F-Type for coaxial cable. We also never eliminated VHF TV broadcasting in the states. Old TVs have a wide range of connections (and so did aerial systems). Odd combinations did occur sometimes. For example to plan an old Atari Games console (this also applies to old home computers as such as the Commodore 64 as well). We had the CATV and VCR feed going to the input by a 75 ohm to 300 ohm balun, through the selector, to a 300 ohm output to a 300 to 75 ohm balun to match the 75 ohm input on the set. Additionally, the game/computer input also passed through a 75 ohm to 300 ohm balun inside the selector. Later game consoles with automatic selectors usually used 75 ohms. A hack I use for my Commodore computer is to use a F-Type femaie to RCA type male to connect a coax line direct to the RF out as I am not switching an antenna on my TV. If you are to do this with NES and later games consoles (NES, SNES, Master, Mega Drive/Genesis, etc) you technically want to use the switch or put a capacitor inline on the center conductor of the coax as these consoles superimpose a small amount of DC power to operate the automatic selector.

Anonymous

Part of the magic has to do with the CAT5 cable itself. The cable has 4 twisted pairs of wire (all about 100 Ohms impedance), but the twist rates are not the same. One pair has 1 twist/meter, another has 2 twists, third has 4 twists, while the 4th pair has 8 twists/meter. These twists are key to ensuring there is no transformer-like crosstalk coupling between pairs. The different twist rates ensure that anything that gets coupled in one half of any pair gets canceled in the second half. The twisted pairs are a type is transmission line, one where both conductors are isolated from the ground. Relative to ground, one is the mirror image of the other with respect to voltage and current. This is a balanced pair line (that happens to be twisted). The video signal is sent as a single-ended signal (one side of the circuit is grounded, making it unbalanced), and is frequently 50 or 75 Ohms impedance. The coaxial cable is providing a ground shield around the center conductor, protecting it from electrostatic coupling of noise (or other signals). A Balun (BAL-UNbal) is simply a transformer that can couple balanced and unbalanced circuits together. Coincidentally, transformers can also be used to match impedance between circuits. There's lots going on in that apparently simple system (unbalanced video > balun > twisted balanced pair > Balun > unbalanced video). And there are 4 pairs in the cable, which are rated for 10's or 100's of MHz, and are all very effectively isolated from each other.

Anonymous
>>122821816

Let's not forget the 300 Ohm twin lead / 75 On coax connection issue faced by every video tape/cassette recorder (VCR/VTR) connecting to a TV that may have had one or both of the twin-lead or coaxial connector, and may have in turn been connected to an antenna (twin/coax) and/or cable (coax).