Larcan MXI1503U Television Broadcast Translator User Manual Heading 3

Larcan Inc Television Broadcast Translator Heading 3

Power Amplifier

MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY CONTENTS 1 POWER AMPLIFIER HEATSINK ASSEMBLY..........................................................................................................1 2 FRONT END MODULE ASSEMBLY 21B1473G7........................................................................................................2 3 DRIVER MODULE 21B2708G3......................................................................................................................................3 4 PA PALLET ASSEMBLY 21B2729G1............................................................................................................................4 5 FIVE-WAY SPLITTER/FIVE-WAY COMBINER.......................................................................................................5 6 DIRECTIONAL COUPLER 21B2746G1........................................................................................................................6 7 BENCH TEST PROCEDURES........................................................................................................................................7 7.1 FRONT-END MODULE, 21B1473G7, BENCH TEST PROCEDURE....................................................................................7 7.2 DRIVER PALLET, 21B2708G3, BENCH TEST PROCEDURE.............................................................................................7 7.3 PA PALLET 21B2729G1 BENCH TEST PROCEDURE......................................................................................................7 FIGURES FIGURE 1 MXI SUB-ASSEMBLIES .................................................................................................................................................1 FIGURE 2 21B1473 MXI FRONT END UHF DTV DRIVER ASSEMBLY ..........................................................................................8 FIGURE 3 11A1354 MXI FRONT END UHF DTV DRIVER SCHEMATIC.........................................................................................9 FIGURE 4 21B2708S DRIVER PALLET ASSEMBLY ......................................................................................................................10 FIGURE 5 21B2708S DRIVER PALLET SCHEMATIC .....................................................................................................................11 FIGURE 6 21B2729A1 PALLET CIRCUIT ASSEMBLY...................................................................................................................12 FIGURE 7 21B2729S1 PALLET CIRCUIT SCHEMATIC ..................................................................................................................13 FIGURE 8 21B2444A1 5 WAY SPLITTER ASSEMBLY ..................................................................................................................14 FIGURE 9 21B2444S1 5 WAY SPLITTER SCHEMATIC..................................................................................................................15 FIGURE 10 21B2445A1 5 WAY COMBINER ASSEMBLY..............................................................................................................16 FIGURE 11 21B2445S1 5 WAY COMBINER SCHEMATIC .............................................................................................................17 FIGURE 12 21B2446S1 DIRECTIONAL COUPLER ASSEMBLY ......................................................................................................18 FIGURE 13 21B2446S1 DIRECTIONAL COUPLER SCHEMATIC.....................................................................................................19 PUB11-014 Rev 1 July 2011 11-014-i MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY 1 POWER AMPLIFIER HEATSINK ASSEMBLY The MXi Power Amplifier Heatsink Assembly 41D2272G1 consists of a fan-cooled heatsink and ten printed circuit board sub-assemblies. The first module is a pre-amplifier, known as the Front-End Module. This first stage is operated in Class A mode. Next is the Driver pallet, which is biased so as to pre-correct for the non-linearity in the final amplifier stage. The next stage is a five-way splitter that splits the RF power so it can be fed to the PA pallets. The final amplifier stage is the PA pallets. After the pallets is the five-way combiner which combines the output power from the PA pallets. The last sub-assembly on the heatsink is the directional coupler which detects the forward and reflected power and provides samples to the control board and a RF test point on the rear panel. Also mounted on the heatsink is a thermal switch, which protects the amplifiers from over-temperature conditions such as the absent of cooling or amplifier over dissipation. This heatsink assembly is mounted in the Amplifier Chassis Assembly 41D2270G1 or G2. Descriptions of each stage are detailed in the following pages. Figure 1 MXi Sub-Assemblies The above picture illustrates the arrangement of the sub-assemblies on the amplifier heatsink assembly. From right to left are the Front-End and Driver modules. Centered and towards the back are the Splitter, PA modules and the Combiner. At the very back is the Directional Coupler. Strategically installed on the heatsink, is a thermal switch which protects the amplifier from over temperature conditions. Cooling for the heatsink is provided by a fan array assembly consisting of four 4-inch axial flow +12VDC muffin fans. This array is situated near the front section of the MXi housing underneath the MXi Amplifier Controller PC board. The fans blow air into the finned portion of the heatsink, which exhausts through the rear. PUB11-014 Rev 1 July 2011 11-014-1 MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY 2 FRONT END MODULE ASSEMBLY 21B1473G7 Drawing Reference: Figure 2 and Figure 3 The front end module performs several distinctive functions. Firstly, it serves as a pre-amplifier, which boosts the signal from the up-converter by approximately 9dB. This module also contains a phase shift circuit and a variable attenuator; both adjustments are available and set to maximum and do not need to be adjusted in the MXi amplifier. These circuits are only used for adjusting phase and gain balance between two paralleled amplifiers. The RF signal is fed to the amplifier through hybrid HY1 and attenuator P-AT1. Refer to Figure 3. HY1, along with associated components CR1, CR2, C1 through C6, R1, R2 and RV1 act as a phase shifter. P-AT1 is a variable attenuator. The attenuation is controlled by RV2. Typically, RV2 is adjusted for minimum attenuation (fully clockwise) during the bench testing of the module. Once this is done, RV2 is not adjusted again. U1 is a hybrid linear amplifier with a nominal gain of 17.5dB. The typical operating point of the amplifier in this application is well below the 1 Watt rating of the hybrid. The power to the Front-End module (+32VDC) comes in at TP2. The control voltage at TP1 comes from the AGC and VSWR cutback circuit in controller board. This voltage corresponds to the output power of the final amplifier. If the AGC circuit is enabled and the output power tries to increase, TP1 voltage increases, thus maintaining the output at the pre-set level. Similarly, if the reflected power increases past the VSWR cutback setting, TP1 increases and the output power is reduced. The higher the voltage at TP1, the higher the attenuation in the module, therefore, lower the overall output power. RF STATUS (TP3) and as mentioned above the PHASING control (RV1) and GAIN control (RV2) are other features of this module which are not applicable to this amplifier. PUB11-014 Rev 1 July 2011 11-014-2 MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY 3 DRIVER MODULE 21B2708G3 Drawing references: Figure 4 and Figure 5. The Driver Pallet is located in the front of the Power Amplifier. It is the second component in the signal flow of the Power Amplifier Module after the VSWR PC Board. The design of the Driver Module 21B2708G3 is based on the device MRF6V3090N. It is a Lateral N-channel Enhancement-Mode Power MOSFET capable of 90W (rated power). However, for this application and the desired linear performance, its output to the final stage is significantly below this. Components C1 through C6, along with the associated printed circuit traces, form the matching network to the gate of the push-pull transistor Q1. Similarly C7 through C14, along with associated stripline traces provide output matching on the drain of the device. C15, C16, C19 and C20 provide the DC blocking of the supply. The rest of the capacitors provide DC blocking of the bias voltage. DC power enters the module through a screw terminal connection and is fed to the drain of the device via L1 and L2 with bypass capacitors. Bias for the devices is provided via combination of resistors, capacitors and stripline traces. The bias circuit provides two functions; precision gate bias setting and external shutdown. The bias voltage is adjusted using R11 from a regulated source provided by U1. Thermistor R18 provides thermal stability for the bias. The source of the bias voltage is a reference IC, U1, which has its ground reference terminal connected to the collector of Q5. Q5 is biased using R10 and R16 to provide thermal compensation. When no Shutdown (5V) is applied at R7, the precision reference is active and provides 6.2VDC to the level adjustment circuit R21, R11 and R17. When R11 has been set to achieve 1.4A bias current to the pallet, a bias voltage of approximately 2.6 to 2.7VDC is applied to the gates of the FETs PUB11-014 Rev 1 July 2011 11-014-3 MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY 4 PA PALLET ASSEMBLY 21B2729G1 Drawing references: Figure 6 and Figure 7. The Power Amplifier pallet is a FET amplifier assembly containing a stripline balun system. The baluns are used for impedance matching and coupling signals at the input and output of the pallet. It is a practical implementation of the coaxial-based solution on a broadside-coupled horizontal stripline structure. The amplifier circuit is based on the Laterally Double-Diffused MOSFET Q1. Capacitors C1, C2, C3 and C4 along with the associated printed circuit traces, form the matching network to the gates of Q1. Similarly, C5 through C14 along with associated stripline traces provide output matching on the drain of the device. C22, C23, C26, C27, C28, C29 provide DC blocking of the supply. The rest of the capacitors provide DC blocking for bias voltage. DC power +50VDC enters the module through a screw terminal connection VDD. This DC voltage is fed to the drains of the device via L1 and L2 with bypass capacitors. Bias for the devices is provided via combination of resistors, capacitors and stripline traces. The bias circuit is located on the 21B2729 board at the input side of the pallet. It provides two functions; precision gate bias setting and external shutdown. The pallet's shutdown function is used in certain applications where the pallet is used as a driver amplifier. The bias voltage is adjusted using R11 from a regulated source provided by U1. Thermistor R18 provides thermal stability for the bias. The source of the bias voltage is a reference IC, U1, which has its ground reference terminal connected to the collector of Q5. Q5 is biased using R10 and R16 to provide thermal compensation. When no Shutdown (5V) is applied at R7, the precision reference is active and provides 6.2VDC to the level adjustment circuit R21, R11 and R17. When R11 has been set to achieve 1.4A bias current to the pallet, a bias voltage of approximately 2.6 to 2.7VDC is applied to the gates of the FETs. PUB11-014 Rev 1 July 2011 11-014-4 MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY 5 FIVE-WAY SPLITTER/FIVE-WAY COMBINER Drawing reference: Figure 8 to Figure 11 The splitter utilizes a combination of hybrid 4-port quadrature (90° phase difference) couplers that require terminations on the 4th port. The termination maintains isolation between the two output ports of the individual hybrids. The input section is a 2.22dB hybrid which feeds a 3dB hybrid from one output port and a 1.76dB hybrid from the other port. One output of the 1.76dB hybrid further feeds another 3dB hybrid. The splitter provides five equal amplitude signals to the pallet amplifier inputs with the proper phase relationship so that the combiner (a reverse image of the splitter) will provide a single output at the desired connector. Any phase and / or amplitude differences will result in increased dissipation in the combiner isolation terminations and a corresponding reduction in output power. The five-way splitter and five-way combiner cover the entire UHF television band (470-860MHz). The combiner can have in excess of 1.5dB unbalance at some frequencies and still combine the signals with less than 0.1dB loss to the reject loads. The splitter and combiner are constructed using a stacked arrangement of three separate PCBs. All of the stripline traces are found on the middle PCB and each hybrid coupler is formed using broadside coupled lines on opposite sides of the middle board. The two outside PCBs support the ground planes required above and below the stripline. Cutouts are made in the upper and lower ground plane PCBs to allow access to the connection points on the middle layer. Direct connections are made using short lengths of bus wire soldered between the combiner inputs and each amplifier pallet output. These short connections minimize the phase errors and reflections that can occur with many cable interfaces. PUB11-014 Rev 1 July 2011 11-014-5 MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY 6 DIRECTIONAL COUPLER 21B2746G1 Drawing reference: Figure 12 and Figure 13 At the rear of the heatsink and fed by the output of the combiner is a directional coupler which is constructed from edge coupled transmission lines. This is also a 4 port device similar to the 3dB couplers, except the coupling is much weaker (-40dB nominal in this case). The input and output port of the directional coupler is the main output 50Ω transmission line, while the other two ports are the ends of the coupled line. The coupled line is simply a second 50Ω transmission line laying parallel and spaced some distance away from the main line. The coupled port closest to the amplifier output connector is terminated by a 51Ω resistor and the other coupled port is directly connected to the centre pin of the ‘RF-TP’ (RF Test Point) BNC connector located on the rear panel of the amplifier. The primary function of this directional coupler is to provide a low power RF sample of the forward power appearing at the amplifier combined output. The nominal coupling is given as -40dB, but the coupling does vary with frequency from about -43dB at 470MHz to about -38dB at 860MHz. PUB11-014 Rev 1 July 2011 11-014-6 MXi501/1002U Power Amplifier Heatsink Assembly
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY PUB11-014 Rev 1 July 2011 11-014-7 MXi501/1002U Power Amplifier Heatsink Assembly 7 7.1 F7.2 DBENCH TEST PROCEDURES The following procedures are test instructions for the amplifier modules comprising the MXi amplifier. RONT-END MODULE, 21B1473G7, BENCH TEST PROCEDURE • Connect +28V to TP2. Limit the power supply current 0.5A. Check that the current consumption is about 0.30A. • Apply a 0dBm (1mW) RF input to the amplifier. • Turn RV2 fully clockwise. The Front-End module should have a gain of about 11dB. Adjusting RV2 from one extreme to the other should vary the gain by 20dB. Set RV2 fully clockwise after checking the range (maximum Gain) • Set phasing adjustment, RV1, fully clockwise (Phasing not required in this model). • RF Mute Check: Connect a variable power supply to TP1. Gradually increase the voltage until the gain drops by 20dB or more. The applied voltage should be about 7.0 volts in this condition. • Telemetry (not used on all models): Monitor TP3. Note that the voltage on TP3 decreases to 0.0V when RF output has been muted. RIVER PALLET, 21B2708G3, BENCH TEST PROCEDURE • BIAS SETTING: Connect a 50-Ohm load to the output of the Driver pallet. • Before applying +50V to the module, adjust R11 fully counter-clockwise (CCW). Limit the power supply current to 1.0A. • Apply +50V to the B+ terminal. Monitor the Drain current and adjust R11 clockwise (CW) for a current of 0.35A + 0.05A. • Proceed to the next step if a network analyzer or similar equipment is available. • RF SWEEP: Check that the gain of the Driver pallet, in the frequency range of 470MHz to 806 MHz. is between 20 to 22dB. 7.3 PA PALLET 21B2729G1 BENCH TEST PROCEDURE • BIAS SETTING: Connect a 50-Ohm load to the output of the PA pallet. • Before applying +50V to the module, adjust R11 fully counter-clockwise (CCW). Limit the power supply current to 2.0A. • Apply +50V to the B+ terminal. Monitor the Drain current and adjust R11 clockwise (CW) for a current of 1.45A + 0.05A. • Proceed to the next step if a network analyzer or similar equipment is available • RF SWEEP: Check that the Gain of the PA pallet, in the frequency range of 470MHz to 806MHz, is between 20 to 22 dB.
Figure 2 21B1473 MXi Front End UHF DTV Driver Assembly
Figure 3 11A1354 MXi Front End UHF DTV Driver Schematic
Figure 4 21B2708S Driver Pallet Assembly
Figure 5 21B2708S Driver Pallet Schematic
Figure 6 21B2729A1 Pallet Circuit Assembly
Figure 7 21B2729S1 Pallet Circuit Schematic
Figure 8 21B2444A1 5 Way Splitter Assembly
Figure 9 21B2444S1 5 Way Splitter Schematic
Figure 10 21B2445A1 5 Way Combiner Assembly
Figure 11 21B2445S1 5 Way Combiner Schematic
Figure 12 21B2446S1 Directional Coupler Assembly
Figure 13 21B2446S1 Directional Coupler Schematic
MXi501/1002U POWER AMPLIFIER HEATSINK ASSEMBLY NOTES PUB11-014 Rev 1 July 2011 MXi501/1002U Power Amplifier Heatsink Assembly

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