Automatic Gain Control And Manual Gain Control - Harmonic PWRLink II 4100S Series Product Manual

Transmitter Setup
In the case of modulated carriers: There are typically two different targets
when setting up the transmitter with modulated carriers:
Target 1:
• Achieve the maximum possible modulation for the transmitter with better
CNR, typically 2 dB higher than for unmodulated carriers
• The average CSO and CTB will be the same as the unmodulated case
• Preferably no digital loading on top of the video carriers
This target can be achieved by setting up the transmitter with modulated
carriers in the CW Mode. In this case, if you use Autosetup ⊕ CW Mode ⇒
Pad value will be 5.5 + (18-15) = 8.5 dB.
Target 2:
• Achieve the same CNR as you would with unmodulated carriers
• The average CSO and CTB will be better than they would be with
unmodulated carriers
• Digital loading
This target can be achieved by setting up the transmitter with modulated
carriers in Video Mode. In this case, if you use Autosetup ⊕ Video Mode ⇒
Pad value will be 5.5 + (18-15) + 2 = 10.5 dB.
For further information about using modulated carriers and how this changes
the RF level, see Appendix C, Using Modulated Carriers.
4.3.2

Automatic Gain Control and Manual Gain Control

The AGC function operates by measuring the input RF level to the transmitter
and continuously adjusting the RF pad to maintain constant drive level to the
laser. When you select AGC, the input level to the transmitter laser remains at
its previously established value.
When you use AGC mode with the PWL 4100S, Harmonic recommends that
you drive the transmitters 3 dB harder so that they can better compensate for
the increased variation in the RF input signal. For example, for 80 NTSC
(System M) channels, use an RF input level of 18 dBmV/channel.
In MGC mode, the internal RF pad value of the transmitter is kept at a fixed
value regardless of the input RF level variation.
NOTE: You can adjust the drive level of the laser in either AGC or MGC
4–6
mode by changing the RF pad. For instructions, see Section 4.2 Fine-
Tuning Each Transmitter on page 4-3. After you have adjusted the pad,
the transmitter will continue to operate in the same mode (AGC or
MGC).
Copyright © 2006 Harmonic Inc.