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NOT 560-2002-en



they are cause by non-linearity of the amplifiers – see next picture 1


an amplifier has a gain/frequency response not lineal along its amplification bandwidth.

this fact causes frequencies inter-acting each other, adding and subtracting yielding new frequencies.
this is due to the beating of the original frequencies against each other.

these beat products occur mainly on the vicinity of the video carriers, although beat distortions might interfere with the audio carriers and color subcarriers as well.

a good amplifier should amplify only the incoming signals, and nothing else.

actually an amplifier is not a linear device.
they present non-lineal aspects causing distortion to the original signals well as generating spurious frequencies.

signals in the amplifier tend to inter-act each other.
one of these processes is called heterodyne process: "two or more signals entering the amplifier can beat each other producing a third frequency".

this effect is very common in tv and radio receivers.
when this distortion occurs, appear non-desirable products occupying a portion of the band that might be used to transmit other cable channels.

s.o. occurs when one frequency adds or subtracts from the other (see figure 1).
whenever a distortion product bleeds into an allocated tv channel, distortion will result if the beat product is strong enough.

the tv set is not able to discriminate between modulation (video information) and the interfering carrier (beat product).

in case the beat product is high enough (see figure 2), it will affect the quality of the picture.
s.o. beats may occur 1.25 mhz above the video carrier or 1.25 mhz below the video carrier.

cable systems should space all uhf b/g channels at exact 8 mhz increments, thus giving a subjective improvement to third order.

second harmonics (sh) are included in second order products.

these are frequencies twice the frequency of the original signals sent via the amplifier unit (if video carrier is 175.25 mhz, the second harmonic is two times this frequency: 350.50 mhz), and therefore they can interfere other channels in the distribution network.

single-ended amplifiers produces many second harmonics (sh) of all the carried channels (see figure 3).

these sh bleed in the s bands and that is why this type of amplifiers should not be used whenever these bands are carrying tv channels.

state-of-art amplifier designs use a dual output transistor, known as a push-pull configuration (see figure 4).

this output configuration eliminates the even harmonics (also the second ones) by producing 180┬║ out of phase each other output transistor distortions.
out of phase even harmonics are almost completely suppressed in such a way that they cannot interfere other channels carried by the coaxial cable.

for s.o. calculations in cascaded configurations, two steps are followed:

  1. first of all calculate the s.o. distortion for one amplifier 
  2. and then apply for n amplifiers.

the manufacturer gives the specifications for s.o. distortion at a given output level:
e.g. manufacturer specs are –67 db s.o. at 46 dbmv.

for an output level about 33 dbmv, s.o. is the following:

s.o.1 amp = 67 + (46 – 33) = 80 db

for the total cascade of n amplifiers

s.o. total cascade = s.o.1 amp + 10 log n


let us consider 15 amplifiers in cascade.

s.o. 20 amps = - 80 + log 15 = - 80 + 11.76 = - 68.24db

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