it indicates the noise present within the useful video bandwidth of a tv channel.

see picture1 to identify how it looks like in the tv screen, and compare to picture 2, which is a free of distortions good quality picture.

picture 1


picture 2

a tv channel contains video, audio and guardband.

video bandwidth is part of this channel that for a pal b/g is approximately 5 mhz.

for a cascading system of identical amplifiers the c/n is calculated in two steps:

1. determine the c/n for one amplifier
2. do the same for a number of amplifiers in cascade, all of them working in the same conditions.

for one amplifier the formula to be applied is:

c/n_{1 amp} = 58 + input level to the amplifier - n.f.

that depends on the input level to the amplifier following a ratio 1:1.

example1

amplifiers featuring:

• input level = 10 dbmv
• n.f. = 8 db
• then c/n = 58 + 10 – 8 = 60 db

increasing the input level 1 db, the c/n improves 1 db (ratio 1:1)

for more than one amplifier, the formula is:

c/n _{n amps} = c/n_{1 amp} - 10 log n

example2

two amplifiers in cascade like the above mentioned.

c/n _{2 amps} = 60 - 10 log 2 = 57 db

remark: in a logarithmic scale, +-3 db, doubles or halves the level

then for this particular case the c/n has been impair to the half.

example 3:

let us take now 4 amplifiers in cascade

c/n _{4 amps} = 60 - 10 log 4 = 54 db

therefore each time the cascade doubles, the c/n gets worse by 3 db

as cascades increase, input level also must increase for compensating the contribution of noise of each amplifier.

the use of amplifiers with low nf is a very important factor as the number of amplifiers in cascade increases.