Measuring System Performance: Measuring Signal Levels
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specified levels for signals at the high and low end of the spectrum, used for
Automatic Gain Control (AGC) or Automatic Slope Control (ASC),
respectively. In the amplifier balancing process, these signals are measured
and adjusted to specification.
In practice, before adjusting compensation of the amp, first check that there
are no problems in the system requiring repair. Gain control affects all
frequencies about equally; but slope control affects lower frequencies more
than the high end.
To balance the amplifier for Tilt:
1. Observe the nine Tilt channels by pressing the Tilt measurement
mode key. To include or exclude a channel in the nine Tilt channels,
you must choose desired video carriers as Tilt Channels in the EDIT
CHANNEL PLAN submenu of the CHANNEL PLAN menu.
2. Select the TILT display, and then select the HI pilot channel.
3. Use the up and down diamond keys to adjust the reference level of
the graph.
Adjust amplifier tilt as follows:
1. Switch off the amplifier, AGC, and ASC.
2. Adjust the HI pilot channel to the specified level, using the Manual
Gain control.
3. Adjust the LOW pilot channel to specified level using manual Slope
control.
4. Because of the interaction between these controls, repeat the process
until you have the prescribed balance.
5. Switch AGC and ASC on and wait briefly to be sure AGC and ASC
maintain the balance.
Scan Measurements
The Scan measurement mode displays the absolute carrier levels across
part or all of the CATV spectrum. The SDA-5000 displays a bar graph
showing the video and audio levels of carriers within the span of selected
frequencies. The narrow vertical marker designates levels of the carrier
being measured. Frequency, video, and audio levels, as well as the Delta
between levels, are displayed on the bottom half of the screen.
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