24 Maret 2008

Undesrtanding Modulators

This article explains in simple terms, the basics of modulation as well as the different types of Modulators along with block diagrams and technical details of each Modulator topology. Authored by Mahendra Tanna, Tanna Electronics & SCaT Magazine.


The Modulator is an indispensable part of any Headend or control room. Most CATV technicians accept the use of Modulators and the fact that each channel requires a Modulator at the Headend. However often, details of why modulation is necessary to begin with or the differences between various Modulators topology such as Saw Filter, single PLL, Double PLL and Frequency Agile Modulators, is often not fully understood.This article sets out to address these matters, in simple, easy to understand language.

While we all understand that a Modulator is required to modulate each channel at the CATV Headend, why do we need to modulate ? If we were to pass a single RF signal on a Cable TV network, the coaxial cable is well suited for the application.
However, even one channel actually consists of two signals, in its simplest form the audio signal and the video signal. Therefore we would actually require two cables to carry a single television
channel ! The situation would become completely unmanageable even for a modest number of channels say 36 channels..... Each subscriber would have to be provided 72 separate coaxial cables, 2 for each channel.
The Modulator actually receives the video and audio signals of a TV channel and modulates them or makes them change either the frequency or amplitude of an external, unrelated high frequency signal.
This high frequency signal is the "Carrier" which carries with it the video or audio signal. If the video signal alters the amplitude (or strength) of the carrier signal, the modulation is referred to as Amplitude Modulation (AM). If the sound or audio signal alters the frequency of the carrier, it is referred to as Frequency Modulation (FM).

PAL-B/G Modulators provides for amplitude modulation of the video carrier and frequency modulation of the sound carrier. The PAL-B system dictates that the audio carrier is at a frequency of 5.5 MHz above the video carrier. It also allocates 7 MHz per channel in which both the audio and video carriers are accommodated. The PAL-B frequency plan is applied for carriers upto 300 MHz. The UHF frequency band starts at 300 MHz. It would become increasingly difficult for TV tuners to accurately tune into a 7 MHz channel. Therefore, for frequencies above 300 MHz, the PAL-G frequency plan is used. PAL-G is identical to PAL-B in all respects except that 8 MHz is allocated per channel. Note that the spacing between the audio and video carriers remains at 5.5 MHz for both PAL-B and PAL-G.


If a large number of channels are to be closely packed together, it is very important to ensure that the carrier (video and audio) frequencies must be very well defined. Further the exact frequency should also not drift or change over a period of time. If the carrier frequency of 1 channel drifts, it could over lap with or cause disturbance to the adjacent channel. To ensure stable carrier frequencies, most modulators utilise a crystal oscillator. Crystal oscillators are low
cost and provide excellent stability. Infact, we all know the accuracy of a digital watch or clock, which is driven by a crystal oscillator.

The signal generated by a modulator has certain unwanted components. Simple Modulators also referred to as VSB or Vestigial Side Band Modulators include an output for about 1.25 MHz before the video carrier. This "lower side band" (lower because it is lower in frequency than the video carrier) will interfere with the sound carrier of the previous channel.

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