SOURCEFORGE » RALF » 2015 Evaluation and test of an optical / L-band tranceiver

Proposal of structure: 

Introduction - tell what is the aim of the project

State of art - talk about how L-band reception systems work currently

Project - Present our results, discuss them point by point

-Linearity - what is, show results, compare to the technical specification provided.

-Intermodulation - what is, show results, explain and compare to coax (theoretically)

-Phase/noise - what is, show results, explain and compare to coax (theoretically)

-Application

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h1. 1. Introduction

Earth stations are based on an indoor/outdoor unit architecture. The outdoor unit comprises antennas and the RF frontend (amplifier and up/down converter). The indoor unit includes the receiver, modem and network/application appliances. On typical consumer systems, the outdoor and indoor units are connected by means of a 75-ohm coaxial cable. The cable conveys the intermediate frequency (typically L band) signal between the indoor/outdoor units. 

This approach, while being cost effective, is not optimal from a signal quality standpoint and might severely impair the end-to-end link budget. For example, typical attenuation values for a coaxial cable at a frequency of 1 GHz are in the order of 15 dB/100 m.

The objective of this project is to test and evaluate a system that makes possible to convert the L-band RF signal from/to optical and use an optical fibre (up to 10 km) as primary interconnection media.  The signal is converted from optical back to RF in the indoor unit. As such, it is directed to DTH (Direct-To-Home) TV systems.

h1. 2. Elements and technical features  

As discussed in the introduction an optic system will be implemented throughout this project. Therefore, trying to substitute and improve the classical performances of the 75-ohm coaxial cable. In order to do so the schema below represents the systems used and the configurations adopted:  

!schema.jpg!

As it has been presented the project consist of an indoor and outdoor parts. This first one is composed by the reception L-band antenna, the coaxial cable relying the antenna and the modulator transmitter (allows to convert from L-band to optic frequencies) and the Monomode fiber optic, FO (in our installation 30m of monomode FO have been used). 

On the other side, indoor part, consists of the receiver L-band/optic demodulator and the coaxial cable to rely the demodulator with the TV. 

_Remark: As the aim of the project is to characterise the fiber system we have substitute the TV by an spectrum analyser in order to evaluate the performances._    

h1. 3. Measures

h2. 3.1 Linearity

h2. 3.2 Intermodulation

h2. 3.3 Phase/Noise

h1. 4. Results

h1. 5. Application

This section explains how the Outdoor system has been installed and the results obtained measuring a real TV signal from a Satellite. 

h2. 5.1 Preparation

The optical transmitter has been put in an hermetic box.

Its interface consists of :

* The power supply plug to be connected to the mains ;

* The optical fiber harness (2 fibers) : only the red fiber is actually connected to optical transmitter ; the black fiber is not used, it will enable to have a b ;

* The type F coaxial cable to connect the Antenna Low Noise Block, LNB, to optical transmitter.

Optical transmitter is set to :

* To supply 18 V to LNB, which selects Horizontale polarization ;

* To provide 0 kHz tone to LNB, which set its Local Oscillator to 9.75 GHz to transpose lower Ku Band from  10.7 to 11.7 GHz into IF Band.

* This hermetic box has been mounted on Antenna mast. This Antenna is pointing to Astra 1KR/1L/1M/1N colocalized satellites on 19.2° East.

h2. 5.2 Trials

h3. 5.2.1 Inter Frequency Spectrum with Optical Link

RF output from Optical receiver has been analyzed with Rohde & Schwartz FSV spectrum analyzer :

!rsz_1combine_images.png!

_IF Band, 1 Channel (with Optical Link)_

It is possible to distinguish several channels within the Inter Frequency, IF, band. There is a zoom on left side. The 3 dB channel bandwdith is 22 MHz.

The power is about -32 dBm for the channel centered around 1141 MHz.

!rsz_tv_channel.png!

_TV channel at 1141 MHz_

h3. 5.2.2 Link Quality with Optical link

First, optical link performance enables to have a good TV quality (for example, Eins Plus on 10744H). More deeply, physical link quality has been assessed with the information provided by IPRICOT SCB router.

This data has been compared with data from website like [www.lingsat.com] :

!table_channels.jpg!

!rsz_ipr_sc.png!

Physical link parameter at 10744 MHz

!rsz_ipr_sc2.png!

Physical link parameter at 11244 MHz

h1. 6. Conclusions