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FAQ 211-2001-en

How does the TDT input loop-through system work?


Televes uses the loop-through combining system for most of its products in both input and output headend ends.

This technique, known also as the "Z System" has certain advantages from the installers point of view, such as:

  • Flexibility
  • Easy and fast installation:

All TDT module inputs are directional because they incorporate an active path for compensating the through losses of each module.

A simplified block diagram of the system would be:

Due to the directivity of the input TDT ports, they have been identified for ease of installation:

The module interconnection is made straightforward by means of F connector links.

Let us consider a 6 modules TDT headend.

The signal can enter in two different ways, depending on the module chosen to be connected to the downlead:

The SAT IF signal insertion loss through the loop-through front end path is so high (950 to 2150 MHz) that 20 dB between the low and high frequency levels can be achieved.

The insertion loss for an input channel at 2069 MHz should be 10 dB higher than at 1136 MHz.

Furthermore, the transponder level at the input of the TDT decreases with the frequency because of the coaxial cable attenuation from the LNB to the headend.

Therefore, the satellite signal will be inputting the headend with certain TlT attenuation, getting higher towards the other end of the headend:

  • Module 1 will be programmed for the highest transponder frequency;
  • Module 3 will be programmed for the lowest transponder frequency (central frequency of the transponder for all of them).

In general, it is possible to say that the programmed frequencies for each TDT will comply with the following frequency arrangements:

If (module 1) > If (module 2) > If (module 3)

or with this frequency arrangement if the satellite signal is connected to module 3:

If (module 3) > If (module 2) >If (module 1)

"If" being the satellite Input frequency.

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