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 4.0 GHz-8.0 GHz, while the Ku Band overall frequency spectrum is 10.7 GHz-18.4 GHz.
Within these bands each satellite has a specific uplink and downlink frequency allocation. The C Band downlink frequency is 3.7 GHz-4.2 GHz and uplink frequency is 5.925 GHz-6.425 GHz. The Ku Band downlink frequency is 10.7 GHz-12.75 GHz and uplink frequency is 17.3 GHz-17.8 GHz.
To use 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 left-hand simultaneously per frequency.
Designers of protection circuitry must ensure that this functionality is not compromised in any way.
Low noise block (LNB)
An LNB is a low noise block module placed on the focus of the satellite dish antenna (parabola) that provides the following functions:
- Down conversion of the incoming signal from GHz range to the 910 MHz-2150 MHz (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 polarization.
- Selects operating band by switching its internal oscillator from low band to high band when the LNB "receives" a 22 kHz tone. Specifically, the local oscillator (LO) frequency changes from 9.75 GHz to 10.6 GHz (C Band-LO frequency 9.75 GHz, Ku Band-LO frequency 10.6 GHz).
- Miscellaneous functions based on 22 kHz tone PPM encoding, as discussed later in this paper.
Figure 1. OMNI-LNB Architecture
Polarization selection
Polarization is a way to give a specific direction to a transmission signal. It increases the beam concentration.
The signal transmitted by satellite can be polarized in one of four different ways: Linear (horizontal or vertical) or Circular (right-hand or left-hand). Consequently, the satellite can broadcast both H and V or LH and RH polarized signals on one frequency.
Figure 2. The four methods of polarization
The "universal" LNB switches polarization by looking at the voltage that it receives from the receiver. Generally, only two signals 13V and 18V are used with one type of antenna. The 13V signal (range 11.5 to 14V) uses vertical polarization or right-hand circular polarization (RHCP). The 18V signal (range 15.5 to 21V) uses horizontal polarization or left-hand circular polarization (LHCP). Also, 1V can be added from a receiver to any of above voltages to compensate for the voltage drop in the coaxial cable.
22 kHz tone and DiSEqC encoding
In addition to selecting the polarization, the LNB must select the operating band. Each reception band is divided in two bands: Low Band (10.7-11.7 GHz) and High Band (11.7-12.75 GHz). This is done with the use of a 22 kHz tone frequency. A 22 kHz pulse-position modulated signal, amplitude approximately 0.6V, is superimposed on the LNB DC power rail. Its coding scheme allows the remote electronics to perform more complex functions. Traditionally, when other encoding functions do not require the 22 kHz tone, simple presence or absence of this tone selects the operating band by changing the local oscillator frequency of the LNB.
The complex encoding of the 22 kHz burst is accomplished with a more sophisticated communication bus protocol named the Digital Satellite Equipment Control (DiSEqC) standard. 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 22 kHz oscillator must be a tone generator with specific rise and fall time. The wave shape will be a quasi-square wave (sine with flat-top). The required frequency tolerance is 2 kHz over line and temperature variations.
Table 1. Band and polarization selection table
22 kHz Wave shape and details
Modulation method
Figure 3. Modulation Scheme
Figure 4. Timing diagram for tone burst control signal
The Need for Lightning Protection
The LNB is remotely powered from the satellite receiver set-top box. The same coaxial cable that carries an IF signal from the LNB to the receiver carries power from the receiver to the LNB. A dedicated IC, an LNB Voltage Regulator, generates the 13V to 18V DC. This device can be damaged by any lightning strike on the coaxial cable or the antenna that can generate high current, high voltage surge at the voltage regulator.
This surge can be simulated according to the IEC 61000-4-5 standard:
Figure 5. IEC 61000-4-5 current waveform
In case of lightning events, the current surge at the LNB voltage regulator (IC) inputs ranges from 250 A (when 3 kV is applied) to 500 A (when 6 kV is applied). This IC cannot withstand such high value energy.
To comply with this IEC regulation and to protect the LNB voltage regulator IC against any damage from lightning events, a dedicated and optimized protection device is required in front of the voltage regulator.
Design solution parameters
Look for a solution based on a segmented approach to provide the best suitable protection device relative to the various LNB voltage regulator absolute maximum ratings capabilities. Depending on the LNB voltage regulator used in the application, and depending on the applied lightning surge test level, a different solution may have to be implemented to optimize the cost and robustness of the total solution.
Features
The following features are important considerations in the selected solution:
Figure 6 illustrates surge tests +4 kV (Standard IEC61000-4-5 with a series resistor. For a trade-off between cost and lightning protection of the LNB voltage regulator, see a proposed application diagram (See Figure 7).
Click here for Figure 6
Click here for Figure 7
About the Author
Richard Renard is a product marketing engineer in the ASD & IPAD Division, MPA Group, at STMicroelectronics. He can be reached at: Richard.renard@st.com
