Sencore FS74 CHANNELIZER SR. Tech Tips

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Automatic VS Manual Features Of The FS74 CHANNELIZER SR.™
Most of the setups for your FS7 4 tests are
done automatically to speed testing and
avoid measurement errors. This allows you
to quickly make important tests even under
inconvenient circumstances such as times
when you are up a pole . in a manhole . or
wearing gloves .
When you are testing non -standard signals
or wish to make non-standard tests ,
however . you can also use your FS7 4
manually. Your FS7 4 includes both the
automatic and the manual tests so you can
troubleshoot with maximum efficiency no
matter what the test conditions.
Th is Tech Tip explains how to obtain
maximum benefit from your FS7 4 tests by
using both the automatic and manual
features of your FS7 4.
Testing Signal To Noise Ratio
Automatic
Your FS74 uses an automatic , patented
method of measuring the noise on the same
channel that you are testing for S/N . so
you get a true , on-channel S/ N result. The
on-channel S/N ratio may be different than
the SI N ratio you measure when using a
noise reference at some other frequency.
This is because narrowband noise may be
present either on the channel you are
testing or at the frequency you are using for
an off-channel noise reference (Figure 1 ).
When testing at the subscriber tap , you 'II
want to do an on-channel S/ N test. This
automatic test allows you to check your
subscriber taps for S/ N up to 46 dB on
individual channels in order to get the best
indication of the signal quality that your
customer will see on a particular channel.
To do an automatic on-channel S/ N test
with your FS74:
a. Set the FUNCTION switch to the RF
VIDEO position and tune to the desired
channel.
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Signal Processor Noi se
Noise Reference
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50 100 150 200 25 0
Frequency In MH z
Fig. 1: System noise varies throughout the
band. Your FS74 allows you to measure
the noise on an in-use channel and locate
bad SIN levels that would be missed
without a true, on-channel SIN test.
b. Reset the FUNCTION SWITCH to the
NOISE REF position, check that the tuning
is still on your desired channel, and push
the NOISE REF STORE button after the
reading has settled .
c. Move the FUNCTION switch to the S/ N
position and read the true , on-channel S/N
reading on the meter (Figure 2).
Manual
You can also manually tune your FS7 4 for
an off-channel noise reference to test for
system noise levels on the trunk and feeder
lines when the expected S/ N ratio is
greater than 46 dB. With your FS74 you
can manually sample the noise on an empty
channel . such as the channel below your
lowest channel or above your highest
channel. You can use your FS74
off-channel test results to check the
broadband system noise , exclusive of
individual channel processor noise or
signal source noise.
To do a man ual off-channel S/N test with
your FS74:
a. Set the FUNCTION switch to the RF
VIDEO position and tune to the desired
channel . Note the RF signal level.
b. Retune to the empty channel you wish to
use for your noise reference (use the
FREQUENCY OFFSET control to tune up 2
MHz if there is a carrier present) . Note the
measured noi se level and add 13 dB to
compensate to a 4 MHz noise sample
bandwidth.
c. Calculate the difference between the
signal level and the compensated noise
level .
NOISE REF
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STORE
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Fig. 2: To measure SIN, set the function
switch to NOISE REF and tune to the
desired reference (a). Push the NOISE REF
STORE button to store the noise level (b).
Set the FUNCTION switch to SIN and read
the ratio on the meter scale (c).
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