TV & SAT TV
Maximum output level in active elements of community and cable networks
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Maximum output level. One of the important parameters of amplifiers is their maximum output level. It is the level which can appear in the amplifier output without exceeding an acceptable level of nonlinear or crossover distortions.
Every amplifier brings in some distortions of the signal, which are caused by non-ideal amplitude-frequency characteristics. This parameter is very important, because exceeding this level we risk significant deterioration in signal quality, which is especially harmful when transmitting many channels and using cascade amplifiers.
What is a decibel?
To understand the meaning of maximal output level it requires us to know what is decibel. Definition of decibel:
GdB = 10 log(X2/X1)
You should pay attention to the notation GdB, which means G (number) of decibels, because it is not a special unit but numerical ratio.
So, if you are interested in power:
GdB = 10 log(P2/P1)
Power can be also expressed using current or voltage, then:
GdB = 10 log(P2/P1) = 10 log((U2/U1)2) = 20 log(U2/U1) and
GdB = 10 log(P2/P1) = 10 log((I2/I1)2) = 20 log(I2/I1)
Obviously, for the reason of matching, the impedance in both patterns has been reduced (we usually talk about its real part - resistance marked with R).
We remind that:
P = U2/R = I2R
Standard input and output impedance for community and cable television equipment is 75 ohm.
The level expressed in dBmV is the reference to standard reference power, in this case (1mV)2/75 ohm. It means that dBmV is a unit of power, not voltage.
GdBmV= 10 log(P2/((1mV)2/75 ohm))
The level expressed in dBuV is the reference to standard reference power, in this case (1uV) 2/75 ohm. Also dBuV is a power, not voltage unit.
GdBuV= 10log(P2/((1uV)2/75 ohm))
L(dBuV) = L(dBmV)-60, it means that 0dBuV = -60dBmV
L(dBmV) = L(dBuV)+60, it means that 0dBmV = 60dBuV
Values given in dB, e.g. amplification or distortions factor, tell us about relation of two quantities expressed in logarithmic measure. For example, amplification is the relation of output power level to input power level. Distortion factor is the relation of the power of desired signal to the power of distortions.
Power level expressed in dBmV is absolute value of power, related to standard reference level. Defining dBmV we'll say that:
0dBmV corresponds to 1mV on 75 ohm impedance,
0dBmV is equal to power (1mV)2 dissipated in 75 ohm impedance.
Distortions and noises.
Two groups of factors have influence on the quality of signal transmitted in the network. These are external factors (e.g. electromagnetic interferences) and internal factors. The internal factors are different kinds of noises and distortions. Noise is generated by different types of casual physical phenomenons occurring in electronic parts e.g. disordered movement of electrons. Distortions are caused by (generalizing) nonlinearity of semiconductor elements, and also passive elements, especially such as different types of contacts and connectors.
The values given below are relations of useful signal to parasite signal. Assuming that the useful signal has greater power than the undesired signal, logarithm of that ratio always has positive value.
If we find negative values in any data, it means that parasite and useful signals were compared. Then, to use bellow written patterns, you only should ignore the minus sign - it is the result of natural property of logarithms.
One of the most important parameters is the ratio of useful signal power to the power of noise, taking into consideration the thermal noise.
C/N - carrier power to noise power ratio.
For a single amplifier it is expressed in dBmV:
C/Ni = 58 + input level - noise figure
or in dBuV:
C/Ni = -2 + input level - noise figure
For cascade k of amplifiers:
C/Nk = C/Ni - 10log(k)
Noise figure also depends on operating temperature - it means that the value given in catalog should be increased in some cases.
Maximum output level
Another parameters defining conditions and capabilities of amplifier's work are a whole group of maximum output levels.
A multitude of these parameters is a result of the fact that maximum output level may be defined in different ways, taking into consideration various types of distortions.
When adjusting working output level you should set the lowest calculated level among the levels determined for required margin from intermodulation distortions of third and second orders and TIM (transient intermodulation distortion).
Independently from definition details, maximum output level informs us how high can be the signal on amplifier's output, without exceeding certain level of undesired components.
Following norms informing about these distortions are commonly used:
- of the third order
- DIN45004B and identical: EN 50083-5
- EN 50083-3,
- of the second order:
- DIN45004A1 and identical: EN 50083-5
- EN 50083-3
During transmission of many channels in a network, also the following norms are useful:
- CTB (Composite Triple Beat)
- CSO (Composite Second Order),
In these ones, the value of distortions is measured in the channels 19/29/42; there are also standards used only in a single country, e.g.:
- 1R8-15W, where the level is measured at specific values IMA3=72 dB and IMA2=69dB,
- DIN45004K, which is used for channel amplifiers.
Measurements performed accordingly to the norm DIN45004B are the commonest, therefore it is good to know rough calculations allowing to compare them with the results obtained using other standards.
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