INTRODUCTION
While most of us have a general idea of distortion and know that it deteriorates the signal, it may be useful to re-state what distortion is. When a signal is processed through an electronic circuit, the output of the circuit often contains unwanted signals which were not part of the input. This unwanted output signal’s frequency is usually related to the input signal's frequency. As a simple example, if a 100 MHz signal is fed into a CATV Amplifier, the output signal may contain unwanted signals at 200 MHz, 300 MHz, 400 MHz etc. These signals are referred to as distortion. It is important to note that only active electronic circuits generate distortion. Passive elements or circuits such as coaxial cables or even traps and filters do not generate any distortion.
TYPES OF DISTORTION
Several types of Distortion exist. The example given above with a 100 MHz input signal is demonstrated graphically.
The original 100 MHz input signal is referred to as the "Fundamental". The 200 MHz distortion is the "Second Harmonic" since it is twice the frequency of the original. Similarly the 300 MHz distortion is the "Third Harmonic". This example of a single frequency input signal yielding multiple harmonics is the simplest case of distortion and is termed as "Harmonic Distortion" since the distortion products are harmonics or multiples of the original (fundamental) signal. In the real world, it is unlikely that any amplifier would be fed a single frequency input. Even a single
TV channel contains 2 carriers via the video and audio carriers. Let us assume that 2 frequencies f1 and f2 are fed into the amplifier. Besides simple, harmonic distortion, other types of distortion such as f1 + 2xf2, f2 + 2xf1 and various other multiples of f1 and f2 will be created. These are shown in Figure 2. Lets assume that f1 = 50 MHz and f2 = 80 MHz. Distortion products will be generated at 210 MHz, 180 MHz, etc. These distortion products are collectively termed as "Intermodulation Distortion" since they are the result of the interaction or modulation of both the input frequencies. Readers will realize that a Cable TV system with even 10 to 12 channels can very quickly generate a large variety of intermodulation distortion products. It is therefore crucial to maintain a low level of intermodulation distortion.
There are various types of intermodulation products depending on the harmonic multiples that are selected. These are CSO (Composite Second Order) distortion and CTB (Composite Triple Beat) distortion. In typical CATV networks, particularly those with a large number of channels, CTB is the distortion which rises very rapidly and is the predominant distortion. Therefore, usually the CTB specifications of an Amplifier are to be considered since this is usually the worse case distortion.
Fortunately for technicians involved in the installation (rather than the design) of CATV networks, the measures that reduce CTB are also effective for reducing most other types of distortion such as CSO or of course the Grand Daddy of all distortions - Harmonic Distortions. As indicated above, harmonic distortion is the simplest form of distortion. The second harmonic of the 450 MHz signal will be at 900 MHz. However, the bandwidth of most cable networks does not exceed 890 MHz. Hence the harmonic distortion of any channel above 450 MHz will not affect the picture quality on a CATV network.
However intermodulation distortion products occur both above and below the input frequencies.
Cable networks have now realized that improper setting up of channel X, Y or Z modulators often creates distortion in the picture on channels E2 to E4. Switching off the offending channel x Modulator magically clears the picture on channel E2 ! Given this basic primer on the different types of distortion, lets take a look at the various types of amplifiers typically used in a cable network.
While most of us have a general idea of distortion and know that it deteriorates the signal, it may be useful to re-state what distortion is. When a signal is processed through an electronic circuit, the output of the circuit often contains unwanted signals which were not part of the input. This unwanted output signal’s frequency is usually related to the input signal's frequency. As a simple example, if a 100 MHz signal is fed into a CATV Amplifier, the output signal may contain unwanted signals at 200 MHz, 300 MHz, 400 MHz etc. These signals are referred to as distortion. It is important to note that only active electronic circuits generate distortion. Passive elements or circuits such as coaxial cables or even traps and filters do not generate any distortion.
TYPES OF DISTORTION
Several types of Distortion exist. The example given above with a 100 MHz input signal is demonstrated graphically.
The original 100 MHz input signal is referred to as the "Fundamental". The 200 MHz distortion is the "Second Harmonic" since it is twice the frequency of the original. Similarly the 300 MHz distortion is the "Third Harmonic". This example of a single frequency input signal yielding multiple harmonics is the simplest case of distortion and is termed as "Harmonic Distortion" since the distortion products are harmonics or multiples of the original (fundamental) signal. In the real world, it is unlikely that any amplifier would be fed a single frequency input. Even a single
TV channel contains 2 carriers via the video and audio carriers. Let us assume that 2 frequencies f1 and f2 are fed into the amplifier. Besides simple, harmonic distortion, other types of distortion such as f1 + 2xf2, f2 + 2xf1 and various other multiples of f1 and f2 will be created. These are shown in Figure 2. Lets assume that f1 = 50 MHz and f2 = 80 MHz. Distortion products will be generated at 210 MHz, 180 MHz, etc. These distortion products are collectively termed as "Intermodulation Distortion" since they are the result of the interaction or modulation of both the input frequencies. Readers will realize that a Cable TV system with even 10 to 12 channels can very quickly generate a large variety of intermodulation distortion products. It is therefore crucial to maintain a low level of intermodulation distortion.
There are various types of intermodulation products depending on the harmonic multiples that are selected. These are CSO (Composite Second Order) distortion and CTB (Composite Triple Beat) distortion. In typical CATV networks, particularly those with a large number of channels, CTB is the distortion which rises very rapidly and is the predominant distortion. Therefore, usually the CTB specifications of an Amplifier are to be considered since this is usually the worse case distortion.
Fortunately for technicians involved in the installation (rather than the design) of CATV networks, the measures that reduce CTB are also effective for reducing most other types of distortion such as CSO or of course the Grand Daddy of all distortions - Harmonic Distortions. As indicated above, harmonic distortion is the simplest form of distortion. The second harmonic of the 450 MHz signal will be at 900 MHz. However, the bandwidth of most cable networks does not exceed 890 MHz. Hence the harmonic distortion of any channel above 450 MHz will not affect the picture quality on a CATV network.
However intermodulation distortion products occur both above and below the input frequencies.
Cable networks have now realized that improper setting up of channel X, Y or Z modulators often creates distortion in the picture on channels E2 to E4. Switching off the offending channel x Modulator magically clears the picture on channel E2 ! Given this basic primer on the different types of distortion, lets take a look at the various types of amplifiers typically used in a cable network.
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