Wavetek 147 Especificações descarregar pdf (Página 339)

Application Notes: Forward Sweep and Balance

315

• Disable all audio readings; only use single insertion points

(CONFIGURE; SWEEP TRANSCEIVER.)

• Place sweep points in the lower sideband of analog channels, especially

sync suppressed scrambled channels, and in the guardband of digital

channels. 1.1 MHz below the analog channels seems to be the "sweet

spot”. Dwell times are set at 4 ms for a standard channel, 2.8 ms for a

sweep point, and 158 ms for a scrambled or digital channel.

• This new plan doesn’t have to be loaded or enabled in the field unit for

sweep to be achieved.

Frequency Response Identification

• Low End Roll-Off

• Usually caused by loose seizure screws, bad EQs/CSs, or diplex filters.

• Standing Waves

• Created from reflections from impedance mismatches. This is seen

when viewing a sweep display from a resistive/bi-directional test point.

Directional test points have enough isolation to block the reflected wave

(depending on the severity) from adding in and out of phase with the

main wave, which creates the standing wave appearance. They can

even be created between the test point and test equipment. If you use

the formula 492*Vp/f, it will tell you the "ballpark" distance in feet to the

fault. Vp is the velocity of propagation of the cable and is typically .87 for

most foam dielectric, hardline cable. Using this formula, "f" is the

distance in MHz between two peaks on the sweep display; 492 is derived

by the speed of light, which is 984 Mft/s and the fact that the reflection is

180 degrees out of phase for 984/2 = 492.

• Suck-Outs

• Caused by grounding issues or multiple impedance mismatches at

perfect intervals. Sometime this also manifests itself as spikes because

of signals adding in-phase. Spikes are also caused by oscillations.

• High End Roll-Off

• Associated with bad accessories, water, or amplifier band edge roll-off.

Other Applications

Using the Portable Receiver Transmitter for Forward Activation.

Acterna’s portable transmitter and receiver can be utilized to perform the

following:

• Segmented sweeping; Moving the transmitter out in the field to sweep

certain spans or legs of a cable plant.

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Wavetek 147 Especificações Página 339