Nautel Vector-LP VR125 Technical Instruction Manual

Contents

Vector-LP Radio Beacon Transmitter Technical Instruction Manual Page 6-29Section 6 Theory of Operation Issue 1.16.6.2 RF Filter PWBSee Figure SD-4. The RF filter PWB (A14) isa band pass filter that attenuates theharmonics of the square wave output appliedfrom the selected RF power module before itis applied to the RF combiner/probe. The filterhas a flat response characteristic over theoperating bandwidth [two PWB options;standard frequency band (190-535 kHz) andextended frequency band (536-1250 kHz and1600-1800 kHz)]. It has a nominal input andoutput impedance of 50 and a loaded Q of2. The band-pass is selected using frequencydependent link connections (E1 through E36;see Tables 2-7a and 2-7b).6.6.3 RF Power Probe PWBSee Figure SD-4. The RF power probe PWB(A15) contains various circuits that monitorthe RF output and provide RF voltage, RFcurrent, forward power and reflected powermonitoring samples to the control/monitorstage.6.6.3.1 FORWARD/REFLECTED POWERPROBEThe forward/reflected power probe circuitconsists of 40:1 RF current transformer T1,80:4 RF voltage transformer T3 andassociated components. These transformersform the current and voltage arms of aforward/reflected power bridge, whichsamples the RF output.The current flowing into the RF output passesthrough transformer T1’s primary. The currentin T1’s secondary develops a voltage acrossresistors R33, R34, R41, R42, R43 that isproportional to the RF output current. Theanti-phase voltage across the secondary ofRF voltage transformer T3 is applied(summed) to the centre-tap of T1’ssecondary. When the RF output impedance isprecisely 50 , the RF current waveform is in-phase and of equal amplitude to the RFvoltage waveform on one half of T1’s center-tapped secondary and equal amplitude, but180 out-of-phase on the other half.The in-phase voltages are summed, rectifiedby CR35, low-pass filtered by L9/C53/L11,resulting in a dc voltage being applied to theFwd Pwr Sample output (J1-1). This voltageis proportional to the RF output's forwardpower level.The out-of-phase voltages are summed,rectified by CR36, low-pass filtered byL8/C54/L10, resulting in a dc voltage beingapplied to the Refld Pwr Sample output(J1-5). This voltage is proportional to the RFoutput's reflected power level.T3’s secondary voltage, which is a truesample of the RF output’s voltage waveform,is also applied to the RF Volts Monitor output(J1-9). This output is intended for monitoringby a modulation monitor and for testequipment during maintenance.6.6.3.2 RF CURRENT PROBEThe RF current probe circuit consists of 40:1RF current transformer T2 and associatedcomponents. The current flowing into the RFoutput passes through the primary oftransformer T2. The current in T2’ssecondary winding develops a voltage acrossresistors R37 and R40 that is proportional tothe primary (RF output) current. This voltageis applied to the RF Current Sample output(J1-17), which is used by the control/displayPWB's high RF current detector andmetering circuit. This high RF currentdetector produces a high RF current alarmand – via the control/display PWB - causesthe transmitter's RF output to shut back (turnoff) when the RF current exceeds athreshold that represents the maximumstress current for the RF power modules.The current sample voltage is also applied tothe RF Current Monitor output (J1-13), whichis used for external monitoring purposes.