Model 5112 Operation Manual
Applied Instruments Inc
3 SYSTEM CARRIER-TO-NOISE MEASUREMENT
Carrier-to-noise ratio is a measurement of a carrier amplitude relative to the noise floor of
the transmission system. This noise floor is created by the summation of natural thermal
noise (-59.75 dBmV @25C. in a 4 MHz BW) and the noise figure of each active device.
Since all the inbound amplifiers "funnel" into the headend, the return portion of a two-
way system is the main contribution to the noise floor. This is especially true in block
translated systems where a portion of the inbound noise is converted, along with the
desired carrier, to the outbound leg.
Figure 1 is a display of a carrier surrounded by broadband noise. The term "apparent" is
used as the noise level presentation is typically a factor of the resolution bandwidth of the
test receiver. To evaluate the carrier-to-noise ratio, insert a test carrier at the point of
origin and adjust to system amplitude.
NOTE: If the test carrier is not at system level, record the difference in dB and add this
quantity to the final reading.
Now connect a spectrum analyzer or signal level meter to the system test point.
NOTE: For tap levels below +15 dBmV, the receiver may require pre-amplification to
assure "sample noise" levels are significantly above the receiver noise floor.
Tune the receiver to the test carrier frequency and record the level in dBmV. Next,
remove the test carrier or tune the receiver about 2 MHz off center frequency. Remove
attenuation as needed and assure that the noise level changes 1dB per dB of attenuator
change. Read the level of noise and apply bandwidth and other noise correction as
described in the receiver manual. Record this level. To obtain the system carrier to noise
relationship, subtract the corrected noise level from the recorded signal level of the test
carrier. If the test carrier was inserted at a level lower than system level, add this
difference to the resultant. Repeat the above process at other frequencies in the spectrum.
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