| OK, Jerrold SJ fans, you have your pad and EQ selected on that
70-degree day for that magic +10 dBmV input to the trunk module. For systems further
north, the nominal temperature may be a day which ranges from 40 to 50 degrees Fahrenheit.
The next thing to do is set both switches to manual and check the maximum settings
and the minimum settings for both gain and slope controls to verify that the +3 dB and -5
dB range is available. If it is there, then you need only set the manual controls to
the correct output levels, then set each of the auto controls, ONE
at a time, to the same levels and you're finished, right? Not quite.
Let's test the amplifier AGC action. |
| Just one more test for the SJ guys. Take a push-on fitting
out of your pocket, screw a terminator on it. Don't do anything with it yet. With
all this talk about input and output levels, push-on fittings, etc. - - we never mentioned
the Jerrold SPD-30 test probe, did we? You have been using it on the trunk input and
output test points, and on the test point on the feeder maker, too? What, you use
the push-on fitting directly to your meter? No wonder you have AGC problems, all
your readings were 3 dB OFF! Oh, but for those of you who did know to use the SPD-30
probe and measure 0 dBmV and +4 dBmV to set your trunk output level, then all is forgiven. |
| What that was all about is painfully obvious to those of you who
have made that mistake. If you double-terminate a 75-ohm transmission line, you
reduce the measured signal level by 3 dB. And that is what we are going to do with
that push-on fitting and the 75-ohm terminator. If you have a trunk amplifier like
the Jerrold SJ, Pathmaker, Magnavox, etc. that requires an RF test probe, then you have
direct access to the input and output transmission lines (minus AC power) and can do the
next test. On the Jerrold SCD-2W chassis, or on a Pathmaker 750 diplex filter, the
INPUT test point can be double-terminated with the push-on fitting and terminator,
effectively reducing the RF input to the amplifier by 3 dB. If the AGC/Slope is
working correctly, it should hold the same output level within 1/2 dB or so. Now
take the push-on fitting and put it away. Did the RF output levels remain constant
after removing the terminator? IF yes, then you're finished now. |
| Oh, for those of you who have Magnavox, S-A, or other trunk
modules that do not have the ability to break into the RF path after the pad and EQ and
before the trunk module, there is some truth here for you, too. If you have that
bench sweep handy, sit down there with a zero EQ and a zero dB pad in the input of the
trunk module you want to test. If you don't have a bench sweep, use the flat headend
test point used to set up all the signals and feed that to the amplifier to be tested.
|
| With the input at 10 dBmV flat and outputs of 30/35, be sure that
your amplifier gain is set to 20 dB of gain at 55.25 MHz and 25 dB gain at 550 MHz (or
wherever you're headed). Do this in manual gain and then check for the 3 dB reserve
gain and 5 dB reserve attenuation described above for those hot and cold days.
Reset the manual settings for 20 dB and 25 dB and leave it there. When the module
gets to the field, the tech should NOT change the manual settings at all. Make sure
the input EQ and pad are installed to produce the desired output levels of 30/35 (or
whatever for feedforward, push-pull, etc.). When that is done, you have again made
things right for the C/N performance and AGC dynamic range that will work year
round. Turn on the AGC and slope controls one end at a time if you have dual pilots,
and adjust the AUTO controls to the correct output levels and walk away, after you close
the housing of course. |
| If you have a system spacing that is different, 22 dB for older
systems, 25dB and 30 dB gain for a system amplifier, D/A, or mini-bridger, use a bit of
common sense about setting the levels to apply to your specific situation. The main
thing is to be sure the input stage has received a FLAT RF input from the pad and EQ
chosen. After that, be sure that you have enough reserve gain and reserve
attenuation to handle the temperature changes for your area. |
| There you have it, the secrets are out now, your signal levels
should be great year round. You have now been introduced to the sweet spot for
automatic trunk modules. With any amplifier, the goal is to have input levels that
are a reasonable level above the noise figure of the input gain stage, and output levels
that are within the AGC gain range. If the system has the right choice of output
technology for the cascade required, then the composite-triple-beat performance will be
there at the end of the line. |
| To know you have the right technology for the job, system
distortion calculations can be performed using a popular spread sheet program that has
been around since the introduction of fiber optic lasers and nodes became popular around
1990. An example of that type of spread sheet can be seen HERE
to see what I mean about choosing the correct input and output levels on trunks, bridgers
and line extenders to meet the desired goals of C/N and CTB at the end of the line. |