| 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. |