ATX Chromadigm-HS Quick Start Manual - Page 11

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BLOCK DIAGRAM & OPERATION

Block Diagram & Operation

The following block diagram depicts the RF and optical signal flow through the Chromadigm CHS/CHQ transmitter.

The CHS/CHQ transmitter has four ITU C-Band laser transmitters, each operating at a discrete wavelength, multiplexed

together onto a single optical output. There are five RF input ports, a Broadcast (BC) RF input and four Narrowcast (NC)

input ports. The RF signals that are common to each of the four wavelengths are connected to the BC input port. The

common RF loading into the BC RF input port should occupy the RF spectrum to at least approximately 270 MHz to avoid

SRS crosstalk degradation. The four NC RF input ports are utilized for channels which are unique to each individual ITU

laser or wavelength and should be placed above 270 MHz.

The BC signal first passes through a variable RF attenuator. The attenuator is either automatically set through the feedback

loop when operated in the AGC mode or may be set in the manual mode via the chassis front panel menu display, LAN or

Local RS232 connection. Next the BC signal is amplified and split into four paths, one for each transmitter wavelength, and

then combined with each of the four NC input signals. The NC RF input signal first passes through a variable RF attenuator.

The attenuator is either automatically set through the feedback loop when operated in the AGC mode or may be set in the

manual mode via the chassis controller interfaces, same as the BC operation. The NC channels are then combined with the

BC channels creating a full-spectrum of channel loading from 54-1002 MHz. The combined signal is coupled to a -20 dB

front panel RF test point. The signal is amplified and then passes through a directional coupler used to sample the RF signal

for the AGC control loop back to the input variable attenuators and the through path is applied to the optical laser.

Our patented clipping mitigation circuit is also within the RF path to mitigate instantaneous peak bursts of the analog or QAM

modulated signals which normally would overdrive the laser briefly resulting in bit errors of the QAM signals. It is important

to note that the five RF variable attenuators are controlled together (ganged together) by the AGC feedback or by manual

adjustment of the attenuator setting. Changing the attenuation value steps each attenuator together by the selected amount.

It is therefore important that the RF levels connected to the chassis are closely balanced between each NC port. The

microcontroller will adjust the four attenuators based on the sampled port with the highest composite total power reading.

The laser modulates the RF signals onto an optical signal with an assigned wavelength between ITU C-Band channel 23 to

53 then the optical signal is chirp cancelled and passes to the output port. An optional optical express port may be added for

multiplexing additional forward path wavelengths or extracting return path wavelengths for connection to other equipment.

Chromadigm CHS/CHQ Transmitter – Quick Start Guide

Chromadigm CHS/CHQ Transmitter Block Diagram

CHAPTER 3:

BLOCK DIAGRAM & OPERATION

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