Wavetek 147 Specifiche Scaricare il pdf (Pagina 364)

Application Notes: Proofing and Testing

340

Terminal Isolation [76.605(a)(9)]

Terminal isolation shall be at least 18 dB. In reality, it’s usually much better

than this. Depending on the source of the purchase!

Ground Loops & Voltage Potential Differences

Use a device called a 188A to test for dangerous voltage potentials before

touching anything.

You could also use a voltmeter in the ammeter mode. Connect one lead to

the cable sheath/connector and another lead to a known good ground. If

current is flowing, there is an obvious voltage potential difference. You may

want to establish a ground before disconnecting the only ground you have

going. Call the electric company if the ground is in question.

Common bonding is required for the elimination of ground loops, but if the

power ground becomes dislodged a more serious problem could occur. If the

power company entrance into the house becomes disconnected from

corrosion or whatever, the house powering will use the cable grounding as

its neutral. Eventually the dielectric will melt.

Assure the house is grounded and bonded correctly for safety and

elimination of ground loops, which can cause hum related problems and

impulse burst noise.

To eliminate hum bars, try common mode coiling, ghost busters, or drop

isolators. Drop isolators have capacitors installed to block the voltage from

traveling on the sheath or center conductor while still allowing RF to pass.

We could also use common mode coiling, which is used to eliminate

common mode currents from entering through a breach in the cable sheath.

Take 10 feet of cable and coil into 7 turns approximately 5-6 inches in

diameter. If impulse noise or hum gets in at the TV tuner or from the power

ground, it will travel on the cable braiding until it dissipates or is induced onto

the center conductor. Coiling the drop cable prior to insertion into the house

appliance will "choke" out this reverse ingress before it finds that breach.

SigVision also makes a snap-on ferrite bead to attach to the coax, which will

impede common mode currents. One way to make a drop isolator would be

to connect two 75/300 ohm transformers back-to-back. This may be good for

troubleshooting, but may not conform to bonding ordinances and will cause

signal loss and leakage.

Hum [76.605(a)(10)]

Acterna equipment can measure hum on an unscrambled, active channel.

This is a peak-to-peak measurement. The definition of FCC hum is any low

frequency disturbance (< 1 kHz) and must be < 3%. This is usually caused

1 2 ... 359 360 361 362 363 364 365 366 367 368 369 ... 409 410

Nessun commento

Wavetek 147 Specifiche Pagina 364