| From the "Sweet Spot" article, the first thing to
remember is that all amplifiers have an input noise figure that is reasonably flat to 1 or
2 dB different in the higher bandwidth amplifiers. This is shown in the hybrid noise
figure specifications by comparing the noise figure at 50 MHz with the noise figure at 550
MHz, 750 MHz, or 870 MHz. With this information in mind, it should become obvious
that choosing an input equalizer for any amplifier is all about presenting relatively FLAT
RF input signals to the first gain stage of the amplifier. If the EQ value cannot be
chosen to provide a perfectly flat input response, then choose an EQ that will provide
about 1 dB more attenuation at 50 MHz compared to the upper frequency limit of the system.
This will provide signal levels that follow the noise figure curve of the input
amplifier as described above. |
| What did that accomplish? For one thing, the right
equalizer will insure a uniform carrier-to-noise ratio for all channels in the cable
system. The choice of the correct pad value at the input of the amplifier will then
assure that the input signals are within the range for the AGC to maintain a constant
output level when the ambient temperature changes with the day/night and seasonal
variations. Are you working at a system that has winter pads and summer pads?
If you are, this article is for you! |
| By the way, for those of you who learned on a Jerrold SJ system
as I did, you will have the input EQ and pad on the connector chassis or "mother
board". The point here is that SJ systems have the luxury of removing the
active trunk amplifier briefly to see that the EQ and PAD are indeed providing a FLAT
input to the trunk module at the optimum RF input level. For the SJ systems, the
correct input level was +9 to +10 dBmV on ALL channels. When the noise figure of the
input gain block inside the trunk module is taken into consideration, that 10 dBmV of
signal is about 3 to 4 dB above the noise figure of the input gain block. At that
level, the C/N degradation from each amplifier is acceptable. To drive the amplifier
harder will improve the C/N of the system, but will reduce the AGC/Slope control range due
to the gain reduction that must be made on the gain and slope controls to provide the
desired output levels. |
| Now, what output levels are we going to use? That depends
on whether or not your system has push-pull, power-doubled, our feedforward trunk modules.
We discussed all three types of amplifiers in the "Sweet Spot" article.
Each type has an optimum output level by design. For Jerrold SJ push-pull
trunk modules, output levels of 32 dBmV flat or 30 dBmV at channel 2 and 32 dBmV at
channel 61 (450 MHz) were common. The power-doubled SJ modules would usually be
setup at 30/34 or 30/35 levels. Why the output slope? To improve the composite
triple beat specification of course. The modules also had more gain because they
were designed to allow drop-in bandwidth upgrades from 300 MHz at 22 dB spacing to 450 MHz
with 25 dB spacing, which is approximately the same distance in FEET of a given coaxial
cable. |
| Ok, here we go for the meat of this topic. If you have that
25 dB trunk spacing we mentioned earlier, then there is a good chance that your module is
going to put out +35 dBmV at the highest frequency and expects to see 10 dBmV input
levels. If your system is in the middle latitudes of the USA, those hot and cold
winter temperatures of ZERO in February and 100 degrees F in August could be a reality for
your system. Now, ask yourself this question, if these are the maximum and minimum
temperature extremes for the cable system, what is the nominal temperature? Do not
think that it is in the middle of these two extremes. What you are asking is this:
What temperature range exists for the greater part of the year? It could very
well be 60 to 70 degrees Fahrenheit. |
| Way back on page 1 of this article I mentioned that AGC
amplifiers could have as much as 8 dB of gain range, although 6 dB would work in our later
example. But for sake of the next argument, we will use the 8 dB range. |
| It's springtime and a young cable man's thoughts turn to sweep
and balance season. For some of you, the boss could have scheduled sweep and balance
for the fall, but that's OK too. The reason is simple, for the three months of
spring, the temperatures are likely to be similar to the three months of fall. Hey,
that means that we could be having SIX months of cool temperatures and three months of hot
or cold temperatures. Now you see why the "nominal temperature is not half way
between the hottest and coldest days. And that is why the AGC setup which works best
is NOT half way between maximum and minimum gain settings, but is more than likely 3 dB
below maximum gain. For that amplifier above with 8 dB gain range, that leaves 3 dB
of reserve gain for the hot days of summer and 5 dB of reserve ATTENUATION for the cold
days of winter. With this setup, the idea of winter pads and summer pads is not an
issue. When the system is setup correctly, pads and EQs will NEVER be changed after
the initial construction and balance of the system. The final setup of your
amplifier is on page 3. |