What are Pads and EQ's?
By Kevin Custer


Gerry Estay wrote:

> You have a great website.  I work with cable and am now just getting into line extenders and amplifiers for .500 cables.   Can you tell me what the difference are between forward eq, forward pads and cable simulators and when and how to use them?
 
 

Sure.

First and foremost:  A CATV plant is based on "unity gain" meaning the resulting signal (signal level and slope or tilt) from any subsequent amplifier or line extender (LE) should be the same as the first amplifier or LE.  An amplifier or LE will hereafter be referred to as an "active".

A Forward Equalizer (EQ) is a circuit that goes into the front end of an amplifier or line extender, (active), to compensate for the amount of cable loss ahead of it.  Because cable loses more signal the higher the frequency, a situation called cable tilt happens.  How much tilt the cable poses on the signal depends on how many feet and the size of the cable.  Properly selecting the forward EQ nullifies the effect of the length of cable ahead of the active. The result is you have the same output from from this active as the one before it.  An EQ has an equal but opposite reaction to to signal as coaxial cable.  EQ's are made in different sizes to accommodate for differing amounts of cable preceding an active.

A Forward PAD compensates for the difference in the amount of signal present on the input of the active and the amount of signal needed to properly balance (set the level of) that active.  Many times too much signal is present on the input of an active.  A pad will knock the signal down accordingly (the dB rating of the pad) to allow the level of the active to be set properly.  The loss that a pad poses on the signal affects all channels or frequencies to the same degree.  This loss of all frequencies the same amount is referred to as Flat Loss.  Line Splitters, DC's, etc. also have flat loss.

In modern CATV equipment, the pad value is usually determined by the amount of excess input level on the highest channel or the high pilot, and the EQ is determined by the amount of slope needed to properly balance the amplifier to the same levels of the preceding active.

Some modern actives, (like C-COR), have a set amount of equalization already 'built-in'.  Many times this built-in equilazation is done interstage, or between the input hybrid and the output hybrid.  This is done to allow a smaller input EQ to be used - because now doesn't need to compensate for all of the cable preceding the active.  In some instances, when a large amount of "flat loss" is present ahead of the active (and not that much cable loss) the active cannot be balanced correctly because it'll exhibit too much corrective tilt.  A Cable Simulator removes some of this inherent tilt so the active can be properly balanced.  A cable simulator is exactly what it states, it is a device that simulates coaxial cable loss.

> thanks

Hope this helps...

Kevin Custer
Chief Optical and RF Engineer
Masters Communications, Inc.
Somerset PA 15501
 
 
 
 
 
 
 

The information presented herein is Copyrighted 1999 to present, Kevin K. Custer
All Rights Reserved.