PUBLICATION TSM 20-399 TECHNICAL MANUAL 10 WATT VHF AMPLIFIER FOR FRONTIER SERIES DIGITAL TV TRANSMITTER/TRANSLATOR LARCAN INC.
CONTENTS 1 GENERAL .......................................................................................................................................................................... 3 1.1 SAFETY NOTICES .......................................................................................................................................................... 3 1.1.1 Beryllium Oxide Warning .................................................................................................................
LIST OF FIGURES Figure 1 Bandpass Filter Response ...........................................................................................................................8 Figure 2 Bandpass Filter Schematic ...........................................................................................................................9 Figure 3 RF Directional Coupler Schematic .............................................................................................................
1 GENERAL 1.1 SAFETY NOTICES This section has been written to provide general guidance and information for the operation, maintenance, and service personnel who are familiar with the hazards of working with high-powered and high-voltage electronic circuits. This manual does not detail all of the safety precautions which should be observed when servicing this or any other electronic equipment.
While all practical safety precautions have been adopted to safeguard personnel from possible injury, both supervisory and operating personnel are urged to ensure that the safety rules detailed below are followed as an established routine at all times.
circuits. Make a habit of removing all jewelry and storing it in a safe place as soon as you enter the transmitter building. Before beginning work, shut off all AC to the transmitter power supplies. Tag and lockout the switches. Your safety routine must ensure that the person placing the tag and locking the switch is be the only person who removes it and reactivates the switch.
1.3 TRADEMARK INFORMATION All material in this manual is copyright LARCAN INC. and reproduction in whole or in part in any form for any purpose other than exclusive use of the equipment owner, without prior written authorization from LARCAN INC., is prohibited. Trademarks used in this document are the property of their respective owners and are mentioned in the text for discussion purposes; any such mention is not necessarily an endorsement of the trademark or its owner.
2 INTRODUCTION This manual describes the LARCAN 10 Watt VHF amplifier designed for ATSC channels 2 through 13. Model part number 21B2935G2 is for channels 7 through 13, 21B2935G1 for channels 2 through 6. These amplifiers are used in the LARCAN MXi FRONTIER series transmitters and translators. LARCAN all-solid-state 10W VHF amplifier were designed to operate conservatively at 10W average digital ATSC power with superb performance, reliability and operating economy.
3 MASK FILTER AND OUPTPUT 3.1 HELICAL RESONATOR BANDPASS FILTER LARCAN bandpass filter implementations generally consist of a cascaded series of coupled resonators. Some use helical resonators; essentially a self supporting high Q coil (the helix) mounted inside a metallic shield enclosure. One end of the coil is solidly connected to the shield enclosure and the other end is open circuited except for a small trimmer capacitance to ground.
Figure 2 Bandpass Filter Schematic This is the electrical equivalent of a series of five coupled helical resonators. Similar lower power filters are built using conventional air wound coils and ceramic trimmer capacitors, and these will be described next. 3.
one way only, as indicated in Figure 6. The low pass section (L1, L2, C1 through C5) of the filter is designed to cut off at a frequency lower than the second harmonic, so that only the fundamental is able to pass through, but harmonics are blocked. The four notch sections are tuned to reject fV–9, fV–4.5, fV+9, and fV+13.5 MHz (down typically 18, 30, 24, and 26dB respectively - sometimes one of the tuned circuits at fV–9, either L3 or L5, is readjusted to fV–4.
through R4. Our target is directivity of 30dB or better on each sampling port, and coupling (forward direction) for J1 and J2 about 36dB down, J5 and J6 about 46dB down. No user adjustments are possible nor recommended. Very little can go wrong with the directional coupler other than from the antenna being hit by lightning, and inspection is all that is recommended.
Figure 4 Generic Helical Resonator Bandpass Filter Assembly TSM 20-399 Rev 0, Feb 2014 12 10W VHF Amplifier
Figure 5: Filter Schematic TSM 20-399 Rev 0, Feb 2014 13 10W VHF Amplifier
Figure 6 Low Band Bandpass and Notch Filter Assembly TSM 20-399 Rev 0, Feb 2014 14 10W VHF Amplifier
Figure 7 Low Band Bandpass and Notch Filter Schematic TSM 20-399 Rev 0, Feb 2014 15 10W VHF Amplifier
Figure 8 RF Directional Coupler TSM 20-399 Rev 0, Feb 2014 16 10W VHF Amplifier
Figure 9 Low Band Coupler PC Board Assembly TSM 20-399 Rev 0, Feb 2014 17 10W VHF Amplifier
4 RF POWER AMPLIFIER AND HEATSINK ASSEMBLY The complete RF amplifier chain consists of a preamplifier and a final output amplifier mounted on a convectioncooled heatsink on the front panel of the unit.A thermal cutout switch removes the power to the drive stages should the temperature of the heatsink exceed a safe operating level. 4.1 PREAMPLIFIER The preamplifier stage consists of a gain-block hybrid amplifier.
Figure 10: Preamplifier Assembly TSM 20-399 Rev 0, Feb 2014 19 10W VHF Amplifier
Figure 11: Preamplifier Schematic TSM 20-399 Rev 0, Feb 2014 20 10W VHF Amplifier
Figure 12: Final Amplifier Assembly (sht 1) TSM 20-399 Rev 0, Feb 2014 21 10W VHF Amplifier
Figure 13: Final Amplifier Assembly (sht 2) TSM 20-399 Rev 0, Feb 2014 22 10W VHF Amplifier
Figure 14: Final Amplifier Schematic TSM 20-399 Rev 0, Feb 2014 23 10W VHF Amplifier
5 METERING AND CONTROL 5.1 SYSTEM The Amplifier unit is very straightforward in operation. A front panel switch is provided to turn on and off the 48V (also herein referred to as 50V) power supply. Voltage is applied to the amplifier and preamp, and via a thermal switch to a small gain stage. The output samples of forward and reflected power are fed to the RF Detector circuit which feeds the output power meter and also provides a protection and monitoring circuit in case of VSWR.
5.3.3 Forward Power Calibration Set the exciter RF output for the transmitter to run at its operating power. Adjust R32 for a forward power meter reading of 100%. NOTE: Before proceeding to the next step, ensure that the Pin Attenuator Board has been setup according to the procedure on the Pin Attenuator Board section of this manual. 5.3.4 Reflected Power Calibration 1. Temporarily defeat the VSWR cutback protection (if it has been initially setup) by adjusting R36 fully clockwise (CW).
Figure 15: System Block Diagram TSM 20-399 Rev 0, Feb 2014 26 10W VHF Amplifier
Figure 16:Metering Board Assembly TSM 20-399 Rev 0, Feb 2014 27 10W VHF Amplifier
Figure 17: Metering Board Schematic TSM 20-399 Rev 0, Feb 2014 28 10W VHF Amplifier
TSM 20-399 Rev 0 Feb.
