Carrier-to-Noise Ratio (CNR) is a measurement of the noise power in a specific bandwidth relative to the video carrier level in the same bandwidth for NTSC/PAL Channels. CNR is specified over a 4MHz band—the video portion of the channel.
Carrier-to-Noise Ratio (CNR) is an important parameter to CATV products, such as optical receiver.
How to calculate Carrier-to-Noise Ratio (CNR)?
CNR adds on a power basis. If the amplifier input level and the 3 noise figure are known, CNR can be calculated with the following formulas.
CNR for a Single Amplifier:
CNR = Input Level (dBmV) + 59 – NF
(NF =Noise Figure)
To add similar CNR figures
CNRs = CNR₀– 10logN
(N=Number of CNR Figures)
To add dissimilar CNR figures
where:
CNR₀, CNRn = CNR (dB)of a Single Amplifier (n=1, 2, 3,…N)
CNRs =System CNR (dB)
59 =Thermal Noise in 4MHz Bandwidth(dBmV)
N = Number of Amplifiers in Cascade
NF =Noise Figure (dB)
Note: Input level must be in dBmV.
If the Noise Figure does not include the equalizer at the amplifier input, add 1dB to the Noise Figure.This will decrease CNR by about 1dB.
Note: Every time you double a cascade of similar amplifiers, CNR degrades by 3dB.
To graphically add Carrier-to-Noise Ratio (CNR) figures
Use the Combining CNR or CSO Ratios figure to estimate the combined effects of two amplifiers with known CNR values.
1.Calculate the CNR difference for the two amplifiers.
2.Locate the point corresponding to the difference between the two CNR values on the line graph.
3.Identify the “subtraction factor.”
4.Subtract the”subtraction factor”from the lowest CNR value to obtain the combined CNR value.