Controlling Distortion in a CATV Network 2

THE WIDEBAND / BROADBAND AMP
The most basic CATV Amplifier is the Wideband Amplifier sometimes also referred to as the Broadband Amplifier. This is typically constructed using 5 or 6 transistor stages of amplification. Each of these transistor stages uses several discreet components. As a result of this, the characteristics of the amplifier can vary quite significantly from piece to piece. However, these amplifiers are relatively low cost and have proved popular for use in small networks carrying 24 or less CATV channels over relatively short distances.

THE HYBRID AMPLIFIER
Most modern CATV Amplifiers now utilize a Hybrid IC manufactured either by Motorola or Philips.These Hybrid ICs provide all the essentials of a good CATV Amplifier in a easy to use, hybrid module. The manufacturer needs to arrange a few basic components around the Hybrid IC to obtain a quality CATV Amplifier. Of course, considering the very high frequency of operation, the actual component layout, quality of components as well as even the material used for the PCB, effects the performance of the finished product. Nevertheless, Amplifiers manufactured from Hybrid ICs provide very consistent specifications that do not vary significantly from piece to piece. Further, the Hybrid ICs are designed to provide a significantly higher output level for the CATV signal, with low distortion.

POWER DOUBLER AMP
As we will see later in the article, the distortion generated by an Amplifier depends largely on the output signal level. Just as a car would strain to run at a higher speed, an Amplifier generates more distortion if operated at a larger output level. Circuit designers then thought of using 2 amplifier output stages to share the output. As a result, each output stage bears only half the load. Alternatively, for the same distortion, the power doubler amplifier can provide twice the output (i.e. 6 dB more) than a conventional Hybrid IC.

LINE LENGTH & DISTORTION
In a CATV network, the output of 1 amplifier, after being attenuated by the distribution cable feeds the input of another CATV Amplifier. This is referred to as "Cascading" of Amplifiers. In large networks, it is not uncommon to encounter a cascade of 10 to 12 Amplifiers before the signal reaches the end subscriber from the control room. In a cascade, not only the output signal but also the distortion of one Amplifier is feed into the input of the next Amplifier which amplifies both the signal and distortion!

One can very easily appreciate that unless the distortion; particularly in the first few amplifiers; is kept at a minimum, the picture at the customer end will be significantly deteriorated. Because of this larger networks need to pay greater attention to minimizing distortion from the Amplifiers.
Again, it is for this reason that large networks require to deploy superior performance "Trunk Amplifiers" on the trunk line. Somewhat lower performance and of course cheaper "Line Extender Amplifiers" can be used on the branch and distribution routes.

ESTIMATING DISTORTION
The distortion of an Amplifier can be calculated quite accurately from the data sheets. Since most Hybrid Amplifiers use similar Hybrid IC modules, the data sheet of the Hybrid IC module is an excellent starting point to calculate and predict amplifier distortion. The Cross Modulation (X-Mod) distortion in any amplifier is given by the formula
                            X-Mod
= X-Mod Spec + 2(Rated output - Actual output)
Let us try and understand what this formula conveys. The Cross Modulation of the Hybrid IC is specified for a particular output level (usually the maximum output, e.g. 108 dBU) and number of channels on the system (usually 69 channels for 550 MHz). As one can expect the amplifier would generate less distortion for a lower output level. How much lower, is what the formula will reveal? The formula actually indicates that Distortion deteriorates by 2 dB of every dB increase in the output level!
Note: While referring to the Maximum output level, we refer to the Channel with the highest level.As an example, if the output of the amplifier is 102 dBU at Channel 2 & 92 dBU on channel s-20, the max output for the calculations should be taken as 102 dBU.
Let us assume that the Motorola Hybrid IC 5342 is rated to provide -58 dB distortions with an output level of 108 dBU. Let us assume that all amplifiers in the network are set so that their maximum output is 100 dBU putting these values in the formula, we obtain the X-Mod distortion of the first Amplifier as
        X-Mod 1 = 58 + 2(108 - 100)
                        = 58 + 16
                        = 74 dB
Thus the first Amplifier generates -74 dB of distortion.
The BIS specifications dictate that the customer should not receive more than -54 dB of distortion. Hence more amplifiers can be cascaded till the distortion reaches -54 dB. The formula that yields the maximum number of Amplifiers that can be used before distortion exceeds permissible limits is :
System X-Mod = X-Mod of first Amplifier - 20 log (n) where n is the number of Amplifiers.
Plugging in the values we have obtained above:
                54 = 74 - 20 log (n)
Using a scientific calculator yields n = 10 i.e. a maximum of 10 Amplifiers can be used in cascade if the output level of each amplifier is kept within 100 dBU. A cascade of 10 amplifiers is fairly reasonable for most networks. If the distribution network requires more than 10 Cascaded Amplifiers, then the output level of all amplifiers should be suitably reduced. A max output level of 95 dBU will permit a 40 Amplifier cascade!
The use of a scientific calculator may prove daunting for many readers hence we have formulated a simple method for calculation of distortion without the use of anything more than a paper and pencil.