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ISL6421A Single LNB Controller with I2C Interface for Advanced Satellite Set-Top Box Designs
Application Note January 2, 2007 AN1161.2

Introduction
Communication Satellite Frequency Allocation
Communication satellites operate within two frequency bands for TV/Broadband service broadcast signals, C Band and Ku Band. The C Band overall frequency spectrum is 4.0GHz to 8.0GHz, while the KuBand overall frequency spectrum is 11.7GHz to 18.0GHz. Within these bands, each satellite will have specific uplink and downlink frequency allocation. For example, the North American DBS system has categories assigned as follows: Ku Band high power downlink is 12.2GHz to12.7GHz and 17.3GHz to 17.8GHz as the uplink frequency, C Band downlink frequency is 3.7GHz to 4.2GHz and 5.925GHz to 6.425GHzas uplink frequency. Also, to use the frequencies that are available for satellite broadcast as efficiently as possible, and to accommodate an additional number of channels within a given frequency band, the transmission signal can be formatted to be either vertical and horizontal, or circular right-hand and circular lefthand simultaneously per frequency.

The “universal” LNB switches thepolarization by looking at the voltage that it receives from the receiver. 12V - Horizontal, 18V - Vertical 13V - Circular right-hand, 20V - Circular left-hand Generally, only two (12V and 18V or 13V and 20V) will be used with one type of antenna. Also, 1V can be added from a receiver to any of above voltages to compensate for the voltage drop in the coaxial cable, i.e., it could be 13V (12V), 14V (13V),19V (18V) or 21V (20V) instead.

22kHz Tone and DiSEqC™ (Digital Satellite Equipment Control) Encoding
In addition to selecting the polarization, the LNB needs to select the operating band. This is done with the use of a 22kHz tone frequency. A 22kHz pulse-position modulated signal of about 0.6V amplitude is superimposed on the LNB’s DC power rail. Its coding scheme allows the remoteelectronics to perform more complex functions like varying the down conversion frequency to select one of multiple LNB’s for dual-dish systems or physically rotating the antenna assembly. Traditionally, when other encoding functions do not require using 22kHz tone, simple presence or absence of this tone selects the operating band by changing the local oscillator frequency of the LNB. The complex encodingof the 22kHz burst is done with a more sophisticated communication bus protocol named the DiSEqC standard (Digital Satellite Equipment Control). The open DiSEqC standard developed by the European Telecommunication Satellite Organization is a well accepted worldwide standard for communication between satellite receivers and satellite peripheral equipment. The 22kHz oscillator has to be a tonegenerator with specific rise and fall time. The wave shape will be a quasi-square wave. (Sine with flat-top). The required frequency tolerance is ±2kHz over line and temperature variations. Burst coding of this signal is accomplished by input from the microcontroller at the DSQIN pin of the IC or by setting the ENT bit of the system register through the I2C bus as detailed in the ISL6421A datasheet.22kHz WAVE SHAPE AND DETAILS (See Figures 1 and 2) Carrier frequency: 22kHz ±2kHz over line and temperature Carrier amplitude: 650mVpp ±250mV Modulation mark period: 500µs ±100µs Modulation space period: 1ms ±200µs

What is a Low Noise Block (LNB)?
An LNB is a low noise block module, placed on the focus of the dish antenna (parabola) that provides the following functions: • Down conversion of theincoming signal from GHz range to the 910MHz to 2150MHz (for Europe) range called “first conversion signal.” This conversion allows the signal to be carried by an inexpensive coaxial cable towards the receiver. • Signal amplification with good noise figure. The LNB improves the first conversion signal level through the use of a built-in low noise amplifier. • Selection of Vertical or Horizontal...