Model 5112 Operation Manual Applied Instruments Inc
3 SYSTEM C RRIER-TO-NOISE ME SUREMENT
Carrier-to-noise ratio is a measurement of a arrier amplitude relative to the noise floor of
the transmission system. This noise floor is reated by the summation of natural thermal
noise (-59.75 dBmV @25C. in a 4 MHz BW) and the noise figure of ea h a tive devi e.
Sin e all the inbound amplifiers "funnel" into the headend, the return portion of a two-
way system is the main ontribution to the noise floor. This is espe ially true in blo k
translated systems where a portion of the inbound noise is onverted, along with the
desired arrier, to the outbound leg.
Figure 1 is a display of a arrier surrounded by broadband noise. The term "apparent" is
used as the noise level presentation is typi ally a fa tor of the resolution bandwidth of the
test re eiver. To evaluate the arrier-to-noise ratio, insert a test arrier at the point of
origin and adjust to system amplitude.
NOTE: If the test arrier is not at system level, re ord the differen e in dB and add this
quantity to the final reading.
Now onne t a spe trum analyzer or signal level meter to the system test point.
NOTE: For tap levels below +15 dBmV, the re eiver may require pre-amplifi ation to
assure "sample noise" levels are signifi antly above the re eiver noise floor.
Tune the re eiver to the test arrier frequen y and re ord the level in dBmV. Next,
remove the test arrier or tune the re eiver about 2 MHz off enter frequen y. Remove
attenuation as needed and assure that the noise level hanges 1dB per dB of attenuator
hange. Read the level of noise and apply bandwidth and other noise orre tion as
des ribed in the re eiver manual. Re ord this level. To obtain the system arrier to noise
relationship, subtra t the orre ted noise level from the re orded signal level of the test
arrier. If the test arrier was inserted at a level lower than system level, add this
differen e to the resultant. Repeat the above pro ess at other frequen ies in the spe trum.
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