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| TA0321 TECHNICAL ARTICLE Lightning protection for LNB supply and control voltage regulator Introduction 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. For example: Ku Band downlink frequency is 10.7 GHz-12.75 GHz and uplink frequency is 17.3 GHz-17.8 GHz. C Band downlink frequency is 3.7 GHz-4.2 GHz and uplink frequency is 5.925 GHz-6.425 GHz 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 left-hand simultaneously per frequency. UPLINK DOWNLINK January 2006 Rev 1 1/12 www.st.com 12 1 What is a Low Noise Block (LNB)? TA0321 1 What is a 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 22kHz 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 22kHz tone PPM encoding, as discussed later in this paper. OMNI-LNB architecture Figure 1. RF Input Antenna feed system Power supply and control Waveguide transition and Depolarizer X Y H LNA and Polaraization LHCP matrix V RHCP Polarization switch LNB Back-end IF Output 2/12 TA0321 2 Polarization selection 2 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 different ways of polarization Linear Horizontal Vertical 11.7 GHz 12.2 GHz Circular Right hand Left hand 12.7 GHz Right hand circular RHCP Left hand circular LHCP Horizontal HP Vertical VP The "universal" LNB switches the polarization by looking at the voltage that it receives from the receiver. Generally, only two signals 13 V and 18 V are used with one type of antenna. 13 V(1) - Vertical Polarization or Circular Right-Hand Polarization (RHCP) 18 V(2) - Horizontal Polarization or Circular Left-Hand Polarization (LHCP) Also, 1 V can be added from a receiver to any of above voltages to compensate for the voltage drop in the coaxial cable. 1. actually from 11.5 to 14V 2. actually from 15.5 to 21V 3/12 3 22kHz Tone and DiSEqC (Digital Satellite Equipment Control) Encoding TA0321 3 22kHz Tone and DiSEqC (Digital Satellite Equipment Control) Encoding In addition to selecting the polarization, the LNB needs to select the operating band. Indeed, each reception band is divided in two bands: Low Band (10.7-11.7 GHz) High Band (11.7-12.75 GHz). This is done with the use of a 22kHz tone frequency. A 22 kHz pulse-position modulated signal of about 0.6 V amplitude is superimposed on the LNB's DC power rail. Its coding scheme also 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 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 22 kHz oscillator has to 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 Parameters Operating band 13 V VDC 18 V Horizontal polarization Horizontal polarization 22 kHz Tone present High Vertical polarization 22 kHz Tone absent Low Vertical polarization 4/12 TA0321 3 22kHz Tone and DiSEqC (Digital Satellite Equipment Control) Encoding 3.1 22 kHz Wave shape and details Carrier frequency Carrier amplitude Modulation mark period Modulation space period 22 kHz 2 kHz over line and temperature 650 mVpp 250 mV 500 s 100 s 1 ms 200 s 3.2 Modulation method Figure 3. Modulation scheme Figure 4. Timing diagram for tone burst control signal 5/12 4 The Need for Lightning Protection TA0321 4 The Need for Lightning Protection The LNB is remotely powered from the satellite receiver. The same coaxial cable that carries the IF signal from the LNB to the receiver carries power from the receiver to the LNB. The 13V to 18V DC is generated by a dedicated IC, a LNB Voltage Regulator. 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 IEC61000-4-5 standard: tr/tf = 8/20 s Vpp = 3 kV to 6 kV R = 12 Ipp = 250 A to 500 A IEC61000-4-5 current waveform I I I/2 t Figure 5. 8 s 20 s 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 can not 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. 6/12 TA0321 5 STMicroelectronics solution 5 STMicroelectronics solution To offer Satellite Set Top Box manufacturers the best trade-off between cost and lightning protection of the LNB voltage regulator, STMicroelectronics has introduced the new dedicated and optimized LNBTVSx-22x devices. 5.1 A segmented & differentiated approach The product strategy is based on a segmented approach to provide the best suitable protection device relative to the various LNB voltage regulator absolute maximum ratings capabilities. It means that depending on the LNB voltage regulator used in the application and depending on the lightning surge test level applied, a different LNBTVSx-22x has to be implemented to optimize the cost and robustness of the total solution. As an example: When the LNBP21 voltage regulator is used and if the box needs to comply with the level 4 of the IEC61000-4-5 (4 kV), the best suitable protection device to be implemented is the LNBTVS4-222S. See Table 3 for recommended fit. 5.2 Features 3 kV, 4 kV and 6 kV protection (8/20 s) Axial & SMD package Low Vf Low Clamping factor Fast Response Time UL Recognized Surge tests +4 kV (Standard IEC61000-4-5 - with series resistor of 12 ) Figure 6. Device only Device on test board LNBP20 7/12 5 STMicroelectronics solution TA0321 Table 2. Type LNBTVSx-22xx range IRM @VRM Max VBR @ IR Min Typ Max V 20 20 20 20 20 20 20 20 20 V V V mA 1 1 1 1 1 1 1 1 1 W 1500 1500 1800 1800 2000 2000 3000 2000 3000 PPP VCL @ Ipp VCL @ Ipp 10/1000 S 10/1000 s 8/20 s (1) Max V 33.2 33.2 33.2 33.2 33.2 33.2 33.2 33.2 33.2 A 45 45 55 55 60 60 90 60 90 Max V 35 35 32 32 32 32 30 35 32 A 250 250 331 331 331 331 331 500 500 T Max 10-4/C 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 C Typ pF 3000 3000 3500 3500 5500 5500 6000 5500 6000 Unidirectional LNBTVS3-220 LNBTVS3-220U LNBTVS4-220 LNBTVS4-220S LNBTVS4-221 LNBTVS4-221S LNBTVS4-222S LNBTVS6-220S LNBTVS6-221S A 1 1 1 1 1 1 1 1 1 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 22 23.1 24.2 1. IEC61000-4-5 R = 12 Note: In some tests, some customers apply an external power supply voltage in the range of +28 V DC to the coaxial cable. In this case, it is recommended to used a TVS with VRM > 28 V (SM15T33A or SM15T36A). 5.3 Ordering information scheme LNB Low noise block regulator Transient voltage suppressor VPP 3 = 3 kV 4 = 4 kV 6 = 6 kV VBR 22 = 22 V VCL 0 = 35 V 1 = 32 V 2 = 30 V Package U = SMB S = SMC Blank = DO201 TVS x - 22 xx 8/12 TA0321 5 STMicroelectronics solution 5.4 Application diagram Gate Sense StepController LNBP/S Feedback LT1 LT2 L Vup Vcc Byp OUT Preregul. U.V.lockout +P.ON Enable I V Linear Post-reg +Modulator +Protection R EXTM SCL Diagnostics SDA ADDR DSQIN IC 22 kHz Oscill Tone Detector DETIN DSQOUT LNBTVSx-22xx Table 3. Recommended fit between LNB Voltage Regulator and LNBTVSx-22xx lightning protections LNB voltage regulators LNBP20, LNBP11x, LNBP8/9 LNBS21, LNBP21 LNBH21, LNBH221, LNBH22, LNBEH21, LNBEH221 LNBH23, LNBH24 Recommended lightning protection LNBTVS3-220/x, LNBTVS4-220/x, LNBTVS6-220S LNBTVS4-222S LNBTVS4-220/x, LNBTVS4-221/x, LNBTVS6-221S LNBTVS3-220/x, LNBTVS4-220/x, LNBTVS6-220S 9/12 5 STMicroelectronics solution TA0321 5.5 Benefits Simple and Low Cost Solution Replace current solutions at lower cost and better performances High Reliability Protection Solution Dedicated and Optimized for LNB Voltage Regulator protection Complies with LNB supply voltages Complies with IEC61000-4-5 standard Available in Axial and SMD package Compatible with all LNB Voltage Regulator types and references Criteria Lightning Protection Reliability, longevity Coverage vs LNB Reg. Cost Design and safety Benefits Up to 6 kV - 500 A No diode paralleling Various P/N Low Dedicated for LNB Reg. protection LNBTVSx-22xx Competition ++ ++ ++ ++ ++ + + + 10/12 TA0321 6 Conclusion 6 Conclusion As explained and demonstrated, all LNB power supplies have to be protected against lightning. The best in class solution for cost effective and reliable protection are the new LNVTVSx-22xx devices from STMicroelectronics. All devices are available and are in mass production. 7 Revision history Date 09-Jan-2006 Revision 1 Initial release. Changes 11/12 7 Revision history TA0321 Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners (c) 2006 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com 12/12 |
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