Preventing distortion and laser clipping in HFC networks

December 1, 2004.

Most Cable TV systems, both large & small use fibre optics technology. Optical and coaxial parts of the network coexist and interface with each other, which as a result produces a number of technical problems. The downstream traffic, for example, combines analogue TV channels and QAM multiplexes carrying both VIDEO and data that fill the spectrum up to as high as 870 MHz.

This complex signal is fed to an optical transmitter which converts it into light that will travel through the network optical fibre to an optical receiver at the other end. Typically, this will range from several to 30 or more Kms.

PROMAX-10 Premium full band spectrum
PROMAX-10 Premium
full band spectrum

In the same way the upstream traffic, which may have much less power than the downstream but is a more complex RF signal, feeds a return path optical transmitter. The resulting optical signal will travel back to an optical receiver located near the headend.

Optical transmitters are basically infrared lasers (1310 nm wavelenght is pretty common) where the invisible light is modulated by the RF signal. These optical transmitters introduce very little distortion provided that they work within certain operational limits.

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PROMAX-10 Premium
return path

In this sense one of the most critical parameters is the RF input power. If the RF input power goes beyond a limit which is specified by the laser transmitter manufacturer (normally in the range of +35 dBmV) then distortion will increase rapidly. If RF input power goes substantially above that limit then it can produce "laser clipping" and then distortion will be very high.

The RF signal fed to the laser transmitter coaxial input will actually be a combination of signals. If we look at the downstream we will have several analogue TV channels together with digital QAM multiplexes, and test pilots, all with different bandwidths and signal levels. Knowing the total power of such signals to prevent laser distortion or clipping is not easy by calculation.

Full band power function
Full band power function

PROMAX-8 and PROMAX-10 Premium series introduce a unique function FULL BAND POWER that measures the total power of the RF signal no matter how complex it is. Connecting the meter to the transmitter RF input test port is all that is required to prevent laser distortion that can be otherwise very difficult to identify.