Generic CATV Amplifier AGC Setup
By Jerry K. Thorne
All CATV amplifiers with AGC/and or automatic SLOPE control have an operational sweet spot that will provide the optimum performance, not just in RF linearity, but in carrier-to-noise (C/N) and composite-triple-beat (CTB) levels.  The most common problems with AGC amplifiers comes from not providing the correct input signal levels to operate at the "sweet spot" of the amplifier.  This article will help the reader to identify that sweet spot.
If you have a good bench sweep setup, you can test your AGC amplifiers to be sure they have the correct maximum and minimum gain and slope ratings.  Depending on the brand and type of amplifier under test, the factory specifications could list as much as 8 dB of total gain/slope range.  How you utilize this range of gain and slope is what this article is about.  Now that I have mentioned the words "gain" and "slope", I want to warn you that not all amplifier manufacturers mean the same thing when they put the GAIN and SLOPE labels adjacent to their controls.  I will attempt to explain those labels with the diagram below.
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Depending on whose amplifier you use, the marking of gain and slope controls may produce one of the response changes seen above.  It is up to the technician to confirm which control produces the desired result.  The example is for a 550 MHz trunk amplifier, but applies to other bandwidths also.  The difference would be the pivot point tuned to 450 MHz, 750 MHz or 870 MHz.  Do NOT think that GAIN always affects ALL channels or just the LOW frequency channels.  Depending on the type of amplifier being used, GAIN could be the control for the high frequencies.   Enough about that, let's move into what really counts here, proper year-round amplifier performance.
For those of you with underground cable systems, you probably wonder why I wrote this article.  The reason you probably do not have AGC level control problems should be obvious.  Buried cable remains at a relatively constant temperature and therefore, the loss of that cable does not vary with ambient air temperature.  But, for aerial plant, the loss of the cable can vary by about 1 per cent for every 10 degrees Fahrenheit.  Now I know that doesn't sound like much, but do the math.  For a cable system with 20 amplifiers in cascade and each amplifier spaced at 25 dB of cable loss at 70 degrees F, if the temperature changes 40 degrees in one day, you could see the total loss of the cable change by 20 dB.  Remember you have 20 spans of 25 dB each, or 500 dB of cable loss from the headend to the end of the cascade.   Four percent of that is the 20 dB of AGC signal level control needed.  For a system that has winter temperatures down to zero degrees F and as high as 100 degrees F, you now see that the total AGC action needed from winter to summer is 50 dB.  If half of the amplifiers are AGC amplifiers, then each AGC amplifier will need a minimum of 5 dB range.  Most CATV automatic amplifiers have from 6 to 8 dB gain control range.
If you have not already read my article:  Set your amplifier to the "Sweet Spot", now is a good time to read it by clicking HERE.   Once you understand the principles of the amplifier "sweet spot" you will find this section on AGC and SLOPE setup will be quite logical.  Since all AGC/SLOPE amplifiers have a definite gain and slope control operating RANGE, it is up to the technician to be sure the amplifier has input and output levels that remain WITHIN that RANGE!  How to set up the amplifier in that range is discussed on the next page.
Amplifier AGC Setup - Page 2 Return to Techarts Menu