SECOND ORDER DISTORTIONS (S.O.)
they are cause by non-linearity of the amplifiers – see next picture 1
this fact causes frequencies inter-acting each other, adding and subtracting yielding new frequencies.
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.
signals in the amplifier tend to inter-act each other.
this effect is very common in tv and radio receivers.
s.o. occurs when one frequency adds or subtracts from the other (see figure 1).
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.
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.
the manufacturer gives the specifications for s.o. distortion at a given output level:
for an output level about 33 dbmv, s.o. is the following:
for the total cascade of n amplifiers
let us consider 15 amplifiers in cascade.
s.o. 20 amps = - 80 + log 15 = - 80 + 11.76 = - 68.24db