Powerwave Technologies 5JS0042 Linear Feed-Forward Power Amplifier User Manual Part 6

Powerwave Technologies Inc Linear Feed-Forward Power Amplifier Part 6

User Manual Part 6

044-05055 Rev. A 4-1PRINCIPLES OF OPERATION4-1. INTRODUCTIONThis section contains a functional description of the Multicarrier Cellular Amplifier System.4-2. RF INPUT SIGNALThe maximum input power for all carrier frequencies should not exceed the limits specified intable 1-2. For proper amplifier loop balance, the out of band components of the input signalsshould not exceed -60 dBc. The input VSWR should be 2:1 maximum (or better).4-3. RF OUTPUT LOADThe load impedance should be as good as possible (1.5:1 or better) in the working band for goodpower transfer to the load. If the amplifier is operated into a filter, it will maintain its distortioncharacteristics outside the signal band even if the VSWR is infinite, provided the reflected powerdoes not exceed one watt. A parasitic signal of less than one watt incident on the output will notcause distortion at a higher level than the normal forward distortion (i.e. -60 dBc).4-4. SYSTEM FUNCTIONAL DESCRIPTIONThe amplifier system is comprised of an MCR20XX Series (NTL107AC) subrack and one or twoG3X-800 Series (NTL107AA) plug-in power amplifier modules. The G3H-800 amplifier is a linear,feed-forward power amplifier that operates in the 25 MHz frequency band from 869 to 894 MHz.A typical two module system is shown in figure 4-1. Power output specifications for a one or twomodule system are listed in table 1-2. Each amplifier is a self-contained plug-in module and isfunctionally independent of other amplifier modules. The amplifier modules are designed forparallel operation to achieve high peak power output, and for redundancy in unmanned remotelocations. The subrack houses a two-way splitter and active power combiner, control assembly,true RMS detector, and back-plane/in-rush current assembly. The rear panel of the subrack hasI/O connectors that interface with the host system, RF signal source, system antenna, and thesystem DC power source. The amplifier system can simultaneously transmit multiple carrierfrequencies, at an average total power output of 110 watts (one amplifier module in a subrackunit) to 200 watts (two amplifier modules), with -68 dBc third order intermodulation distortion(IMD).A composite RF signal from the base station radios is applied to the RF input (J9) of the subrack.From there the signal passes through a voltage variable attenuator (VVA), then a two-way splitter.Each leg of the splitter passes through an isolator, then the blind-mate connector to interface withthe MCPA. The signal then returns to the subrack via the blind-mate connector after beingamplified by the MCPA modules. The two high-power signals are combined by the active powercombiner. The active power combiner has the capability of switching MCPA channels off-line bythe use of RF switches. If an MCPA is not present, turned off, or faulted, the switch will open inthat channel and physically disconnect that MCPA. The combiner maintains its low insertioncharacteristics when used in the single path configuration. Note that the splitter is not switched,therefore the power is automatically reduced by 3 dB, thus eliminating an output overdrivecondition. The output of the combiner is fed through a coupler, then a receive-band filter. Theamplified RF signal is available for use at the output of the receive-band filter (J2). The coupler isused to sample the output power to the true RMS detector. The true RMS detector will supply themicrocontroller with an accurate average power regardless of the signal modulation type. TheSection4
044-05055 Rev. A 4-2dynamic range is 25 dB. The power reading is used during the gain initialization phase whendeploying the system or monitoring to detect excessive output power. In both cases the VVA willbe adjusted accordingly. Two non-RF features of the subrack system are inrush current limitingand alarm status/serial interface ports. The inrush current limiting circuitry is used to minimize theinstantaneous current demand when the MCPA is first DC powered-up. This is due to the highcapacitance on the MCPA’s DC input. The circuit is placed in series on the DC source before theMCPA. Voltage for the subrack is derived prior to the in-rush current limit circuitry. The circuitry isin a high impedance state upon DC power-up. When an MCPA is enabled the impedance isslowly brought down to nearly 0 ohms. This will allow the capacitors to charge over a longerperiod of time, thus reducing the high current drain on the power supply. The three ports on therear of the subrack are for Form-C alarms (J1), RS485/Addressing serial communication (J7) andRS485 and RS232 serial communication (J8). The J1, J7, and J8 connectors are detailed inchapter 2 (see figures 2-2, 2-3, and 2-4, and tables 2-2, 2-3, and 2-4). The serial interface allowsthe user to acquire MCPA internal voltages and status, exercise MCPA and VVA control, upgradeMCPA or subrack firmware, and obtain true RMS power readings.Figure 4-1. MCR20XX Series (NTL107AC) Two Module Amplifier System4-5. MCR20XX Series (NTL107AC) SUBRACKThe MCR20XX Series (NTL107AC) subrack (see block diagram figure 4-1) is not field repairable.The subrack functions are described in section 4-4.RF IN J9 VVA COUPLER BPFALC_LEDMOD PORTBDM PORTRF OUT J2J8RS485/ADD.RS485/RS232RMS DET.500-001090J5/J6J3/J4INTERFACEASSY.500-1107POWER SPLITTERFORM C ALARMSJ7SIGNAL DISTRIBUTION1717ACTIVE POWER COMBINERMCA 1MCA 2IN-RUSHIN-RUSHDC_CH1DC_CH2SPLITTER &SIGNAL DIST.ASSY.500-00109580A MAX80A MAXDB9DB9J1DB92X32X32124COMBINERASSY500-0010945 5CONTROLASSEMBLY500-001105100 30
044-05055 Rev. A 4-34-6. G3X-800 SERIES (NTL107AA) AMPLIFIER MODULEThe amplifier module, figure 4-2, has an average power output of 110 watts with intermodulationproducts suppressed to better than -65 dBc below carrier levels. The amplifier provides anamplified output signal with constant gain and phase by adding approximately 30 dB of distortioncancellation on the output signal. Constant gain and phase is maintained by continuouslycomparing active paths with passive references, and correcting for small variations through theRF feedback controls. All gain and phase variations, for example those due to temperature, arereduced to the passive reference variations. The amplifier module is comprised of:Delay linesCouplersPreamplifiersMain amplifierError amplifierTwo feed-forward loops with phase-shift and gain controlsDC/DC power regulatorAlarm monitoring, control and display panelFigure 4-2. G3X-800 Series (NTL107AA) Power Amplifier Module Functional Block DiagramThe main amplifier employs class AB amplification for maximum efficiency. The error amplifierand feed forward loops are employed to correct signal nonlinearities introduced by the class ABmain amplifier. The error amplifier operates in class AB mode. The RF input signals are coupledand amplified by a preamp, then fed to an attenuator and phase shifter in the first feedforwardloop. The main signal is phase shifted by 180 degrees and amplified in the premain amplifier.The output from the driver amplifier is fed to the class AB main amplifier. The output from themain amplifier is typically 160 watts. The signal is output to several couplers and a delay line.The signal output from the main amplifier is sampled using a coupler, and the sample signal iscombined with the main input signal and input to the second feed-forward loop. The error signal isattenuated, phase shifted 180 degrees, then fed to the error amplifier where it is amplified to alevel identical to the sampled output from the main amplifier. The output from the error amplifier
044-05055 Rev. A 4-4is then coupled back and added to the output from the main amplifier. The control loopscontinuously make adjustments to cancel out any distortion in the final output signals.The primary function of the first loop is to provide an error signal for the second loop. The primaryfunction of the second loop is to amplify the error signal to cancel out spurious productsdeveloped in the main amplifier. The input signal is fed to a coupler and delay line. The signalfrom the coupler is amplified by a preamplifier and fed to the attenuator and phase shifter in thefirst loop. The first loop control section phase shifts the main input signals by 180 degrees andconstantly monitors the output for correct phase and gain.The second loop control section obtains a sample of the distortion added to the output signals bythe main amplifier, phase shifts the signals by 180 degrees, then feeds it to the error amplifier. Itis then amplified to the same power level as the input sample and coupled onto the main outputsignal. The final output is monitored by the second loop and adjusted to ensure that the signaldistortion and IMD on the final output is canceled out.4-6.1. MAIN AMPLIFIERThe input and output of the amplifier employ two-stage, class AB amplifiers which provideapproximately 25 dB of gain in the 25 MHz frequency band from 869 to 894 MHz. The amplifieroperates on +27 Vdc, and a bias voltage of +5 Vdc, and is mounted directly on a heat sink whichis temperature monitored by a thermostat. If the heat sink temperature exceeds 85 °C, thethermostat opens and a high temperature fault occurs. The alarm logic controls the +5 Vdc biasvoltage which shuts down the amplifier.4-6.2. ERROR AMPLIFIERThe main function of the error amplifier is to sample and amplify the signal distortion levelgenerated by the main amplifier, to a level that cancels out the distortion and IMD when the errorsignal is coupled onto the main signal at the amplifier output. The error amplifier is a balancedmultistage, class AB amplifier, has 51 dB of gain and produces an 80-watt output. The amplifieroperates on 27 Vdc and a bias voltage of +5 Vdc, and is mounted directly on a heat sink.4-6.3. AMPLIFIER MONITORINGIn the main and error amplifier modules, all normal variations are automatically compensated forby the feedforward loop control. However, when large variations occur beyond the adjustmentrange of the loop control, a loop fault will occur. The alarms are displayed on the front panelindicators and output via a 21-pin connector on the rear of the module to the subrack summaryboard for subsequent remote monitoring via the ALARMS connector. Refer to paragraph 2-5 aswell as figure 2-2 and table 2-2 for a description of the ALARMS connector.4-6.4. AMPLIFIER MODULE COOLINGAlthough each amplifier module contains its own heat sink, it is cooled with forced air. Four fansare used for forced air cooling and redundancy. The fans, located on the front and rear of theamplifier module, draw air in through the front of the amplifier and exhaust hot air out the back ofthe module. The fans are field replaceable.4-7. POWER DISTRIBUTIONPrimary DC power for the system is provided by the host system to the MCR20XX Series(NTL107AC) subrack. The subrack supplies each amplifier module with +27 Vdc directly and viathe RF power splitter/combiner. The amplifier module has a DC/DC converter that converts the+27 Vdc to +15 Vdc, +5 Vdc and -5 Vdc.4-8. INTERMODULATIONThe G3X-800 amplifier is designed to deliver a 100-watt composite average power, multicarriersignal, occupying a bandwidth less than or equal to 25 MHz, in the bandwidth from 869 to 894MHz. The maximum average power for linear operation, and thus the amplifier efficiency, willdepend on the type of signal amplified.
044-05055 Rev. A 4-54-8.1 TWO TONE INTERMODULATIONWhen measured with two equal CW tones spaced anywhere from 30 kHz to 20 MHz apart, and atany power level up to the average power, the third order intermodulation products will be below-65 dBc4-8.2 MULTITONE INTERMODULATIONAdding more tones to the signal will lower individual intermodulation products. If the frequenciesare not equally spaced, the level of intermodulation products gets very low. When the frequenciesare equally spaced, those products fall on top of each other on the same frequency grid. Theaverage power of all intermodulation beats falling on the same frequency is called the compositeintermodulation; it is -65 dBc or better.4-9. ALARMSThe presence of several plug-in amplifier alarms can be detected at the ALARMS connector onthe subrack rear panel. Refer to table 2-2 and figure 2-2 for a description of the connector.

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