CALCULATION OF MODULATOR NOISE AT THE HEADEND
Let us suppose that a typical modulator has an in band Carrier to Noise ratio of -65 dB and an out of band Carrier to Noise ratio of -65 dB. Let’s assume that there are 64 such modulators at the Headend. The noise of all these 64 modulators will add up, lets calculate. The exact mathematical formula for calculation of noise is: C/Nn = C/N1 - 10 Log n where: C/N1 is the Carrier To Noise ratio of the 1st Modulator n: The Total Number of Modulators at the Headend C/Nn : Resulting C/N of the total n modulators. However let us ignore elaborate mathematics. Fairly accurate estimates can be arrived by simple "Rule of Thumb". "Doubling the number of Amplifiers or Modulators increases the total noise by 3 dB i.e. the Carrier to Noise ratio reduces by 3 dB". The out of band noise of 1 modulator is assumed to be -65 dB.
Therefore 2 modulators would reduce the C/N to -62 dB.
4 modulators C/N -59 dB.
8 modulators C/N -56 dB.
16 modulators C/N -53 dB.
32 modulators C/N -50 dB.
64 modulators C/N -47 dB.
It quickly becomes apparent that the out of band noise is now a bigger concern than the noise generated by each modulator within its own channel bandwidth!
In practice, the situation sometimes is a little better because manufacturers often provide output filters on their modulators which reduce all signals including noise at frequencies far away from the operating frequency. High quality international modulators such as BARCO have an out of band C/N of approximately -90dB and an In band C/N of -70 dB. A Headend of 64 Modulators would therefore reduce this by 18 dB i.e. the C/N of just the modulators put together would have deteriorated to -52 dB. This of course excludes the noise contributed by the Dish + LNB and the Satellite Receiver!
THE AMPLIFIER
The input level signal to an Amplifier is of crucial importance to the noise performance. Low level signals deteriorate rapidly with the injection of noise. On the other hand noise contributed to high level signals plays a less significant role. Given this basic understanding, a good starting point is to always ensure that input level signals are maintained fairly high for each Amplifier. An input signal level of at least 65 dBU, preferably 70 dBU should be maintained at the input of each Amplifier, throughout the entire Cable TV network. Let’s take a closer look. To start with, we need to estimate the noise performance of a single Amplifier. Once this noise is estimated, it would be a relatively simple matter to estimate the overall noise performance of the cascade of similar Amplifiers on the network. Keep in mind that we need to estimate the noise performance of the cascade of Amplifiers i.e. the chain of Amplifiers where the output of one feeds the other. Let us assume that a Cable TV network has 3 trunk lines. The first trunk has a chain (cascade) of 8 Amplifiers, the second trunk with 12 Amplifiers and the third trunk with 9 Amplifiers. The network therefore has a total of 29 Amplifiers but the worse noise will be at the end of trunk 2 which has 12 Amplifiers in cascade. Hence the total noise of 12 Amplifiers needs to be estimated, not of 29 Amplifiers.
NOISE OF THE FIRST AMPLIFIER
Most Hybrid Amplifiers utilize the Motorola 5342 (450 MHz) or 6342 (550 MHz) or their equivalent Philips ICs. These Hybrid ICs have similar noise figures of 5.5 dB. Let’s take this to be 6 dB. If we use this as a starting point, we would arrive at reasonably good estimates for cascades of Hybrid Amplifiers.
The noise performance of one Hybrid Amplifier is given by the equation:
C/N1 : C/N Ratio of the 1st Amplifier
C/N1 = ( i/p in dBU - 1 ) - Noise Figure of Amp
= ( i/p in dBU - 1 ) - 6
Clearly, the noise performance is better if the level of the input signal is higher. Let us assume that the input signal with the Amplifier is 70 dBU. C/N1 = (70-1) - 6
This yields a noise performance of the first Amplifier of 63 dB. However, if a network installs amplifiers so that each amplifier receives an input signal of 55 dB, the Noise of the first amplifier itself will be 48 dB. This clearly shows how the noise performance can be adversely affected, if levels are not correctly maintainer throughout the network. It is not necessary to install expensive amplifiers to keep noise to a low level. It simply requires proper signal levels.
NOISE OF THE CASCADE
The exact formula to calculate the Noise of a cascade of amplifiers is: C / Nn = C / N1 - 10 log ( n ) n = No. of Amplifiers in Cascade Further, the BIS specifies that C/Nn should not be worse than 43 dB. Of course this formula is daunting for anyone short of an engineer with a Scientific Calculator! Let’s take a simpler approach.
Once the noise of the first Amplifier is determined it is simple to estimate the total noise of the Amplifier cascade, without even a calculator! Similar to the case of Modulators, the noise Performance is reduced by 3 dB for a doubling of the number of Amplifiers. Hence:
2 Amplifiers 60 dB
4 Amplifiers 57 dB
8 Amplifiers 54 dB
16 Amplifiers 51 dB
32 Amplifiers 49 dB
In most actual systems, the cascade will not exceed 12 to 16 Amplifiers. Hence, for a 16 Hybrid Amplifier cascade, with an input signal level of 70 dBU for each Amplifier, the output signal will have a carrier to noise ratio of 51 dB. The BIS specifies that the carrier to noise ratio to the last customer should not be worse than 43 dB. Of course this 43 dB is a noise contribution of the Amplifier cascade, the Modulators and the Dish Antenna, LNB + Satellite Receiver, etc. However it is clear that the Amplifiers in such a cable network will not contribute significantly in terms of noise.
CONCLUSION
It is often believed that good noise performance can only be achieved by expensive trunk Amplifiers. The facts are quite different. In fact as shown in Table 2, the noise performance of the same amplifier reduces drastically, and can be used for a cascade of just amplifiers, if fed an input signal of 55dB per amplifier. The same product, if fed an input of 70dBU can be used for a cascade of amplifiers.
The key to good noise and distortion performance is the proper adjustment of signal level. Good noise performance dictates that the input levels be maintained fairly high. An input signal level of at least 65 dBU preferably 70 dBU for Hybrid IC based Amplifiers is ideal. The next article in this series will examine distortion in Amplifiers and once again look at the optimal settings for the lowest practical distortion.
Let us suppose that a typical modulator has an in band Carrier to Noise ratio of -65 dB and an out of band Carrier to Noise ratio of -65 dB. Let’s assume that there are 64 such modulators at the Headend. The noise of all these 64 modulators will add up, lets calculate. The exact mathematical formula for calculation of noise is: C/Nn = C/N1 - 10 Log n where: C/N1 is the Carrier To Noise ratio of the 1st Modulator n: The Total Number of Modulators at the Headend C/Nn : Resulting C/N of the total n modulators. However let us ignore elaborate mathematics. Fairly accurate estimates can be arrived by simple "Rule of Thumb". "Doubling the number of Amplifiers or Modulators increases the total noise by 3 dB i.e. the Carrier to Noise ratio reduces by 3 dB". The out of band noise of 1 modulator is assumed to be -65 dB.
Therefore 2 modulators would reduce the C/N to -62 dB.
4 modulators C/N -59 dB.
8 modulators C/N -56 dB.
16 modulators C/N -53 dB.
32 modulators C/N -50 dB.
64 modulators C/N -47 dB.
It quickly becomes apparent that the out of band noise is now a bigger concern than the noise generated by each modulator within its own channel bandwidth!
In practice, the situation sometimes is a little better because manufacturers often provide output filters on their modulators which reduce all signals including noise at frequencies far away from the operating frequency. High quality international modulators such as BARCO have an out of band C/N of approximately -90dB and an In band C/N of -70 dB. A Headend of 64 Modulators would therefore reduce this by 18 dB i.e. the C/N of just the modulators put together would have deteriorated to -52 dB. This of course excludes the noise contributed by the Dish + LNB and the Satellite Receiver!
THE AMPLIFIER
The input level signal to an Amplifier is of crucial importance to the noise performance. Low level signals deteriorate rapidly with the injection of noise. On the other hand noise contributed to high level signals plays a less significant role. Given this basic understanding, a good starting point is to always ensure that input level signals are maintained fairly high for each Amplifier. An input signal level of at least 65 dBU, preferably 70 dBU should be maintained at the input of each Amplifier, throughout the entire Cable TV network. Let’s take a closer look. To start with, we need to estimate the noise performance of a single Amplifier. Once this noise is estimated, it would be a relatively simple matter to estimate the overall noise performance of the cascade of similar Amplifiers on the network. Keep in mind that we need to estimate the noise performance of the cascade of Amplifiers i.e. the chain of Amplifiers where the output of one feeds the other. Let us assume that a Cable TV network has 3 trunk lines. The first trunk has a chain (cascade) of 8 Amplifiers, the second trunk with 12 Amplifiers and the third trunk with 9 Amplifiers. The network therefore has a total of 29 Amplifiers but the worse noise will be at the end of trunk 2 which has 12 Amplifiers in cascade. Hence the total noise of 12 Amplifiers needs to be estimated, not of 29 Amplifiers.
NOISE OF THE FIRST AMPLIFIER
Most Hybrid Amplifiers utilize the Motorola 5342 (450 MHz) or 6342 (550 MHz) or their equivalent Philips ICs. These Hybrid ICs have similar noise figures of 5.5 dB. Let’s take this to be 6 dB. If we use this as a starting point, we would arrive at reasonably good estimates for cascades of Hybrid Amplifiers.
The noise performance of one Hybrid Amplifier is given by the equation:
C/N1 : C/N Ratio of the 1st Amplifier
C/N1 = ( i/p in dBU - 1 ) - Noise Figure of Amp
= ( i/p in dBU - 1 ) - 6
Clearly, the noise performance is better if the level of the input signal is higher. Let us assume that the input signal with the Amplifier is 70 dBU. C/N1 = (70-1) - 6
This yields a noise performance of the first Amplifier of 63 dB. However, if a network installs amplifiers so that each amplifier receives an input signal of 55 dB, the Noise of the first amplifier itself will be 48 dB. This clearly shows how the noise performance can be adversely affected, if levels are not correctly maintainer throughout the network. It is not necessary to install expensive amplifiers to keep noise to a low level. It simply requires proper signal levels.
NOISE OF THE CASCADE
The exact formula to calculate the Noise of a cascade of amplifiers is: C / Nn = C / N1 - 10 log ( n ) n = No. of Amplifiers in Cascade Further, the BIS specifies that C/Nn should not be worse than 43 dB. Of course this formula is daunting for anyone short of an engineer with a Scientific Calculator! Let’s take a simpler approach.
Once the noise of the first Amplifier is determined it is simple to estimate the total noise of the Amplifier cascade, without even a calculator! Similar to the case of Modulators, the noise Performance is reduced by 3 dB for a doubling of the number of Amplifiers. Hence:
2 Amplifiers 60 dB
4 Amplifiers 57 dB
8 Amplifiers 54 dB
16 Amplifiers 51 dB
32 Amplifiers 49 dB
In most actual systems, the cascade will not exceed 12 to 16 Amplifiers. Hence, for a 16 Hybrid Amplifier cascade, with an input signal level of 70 dBU for each Amplifier, the output signal will have a carrier to noise ratio of 51 dB. The BIS specifies that the carrier to noise ratio to the last customer should not be worse than 43 dB. Of course this 43 dB is a noise contribution of the Amplifier cascade, the Modulators and the Dish Antenna, LNB + Satellite Receiver, etc. However it is clear that the Amplifiers in such a cable network will not contribute significantly in terms of noise.
CONCLUSION
It is often believed that good noise performance can only be achieved by expensive trunk Amplifiers. The facts are quite different. In fact as shown in Table 2, the noise performance of the same amplifier reduces drastically, and can be used for a cascade of just amplifiers, if fed an input signal of 55dB per amplifier. The same product, if fed an input of 70dBU can be used for a cascade of amplifiers.
The key to good noise and distortion performance is the proper adjustment of signal level. Good noise performance dictates that the input levels be maintained fairly high. An input signal level of at least 65 dBU preferably 70 dBU for Hybrid IC based Amplifiers is ideal. The next article in this series will examine distortion in Amplifiers and once again look at the optimal settings for the lowest practical distortion.
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