Nautel Vector-LP VR125 Technical Instruction Manual

Contents

Vector-LP Radio Beacon Transmitter Technical Instruction Manual Page 6-9 (6-10 Blank)Section 6 Theory of Operation Issue 1.1The B+ Sample input at P1-3 is a dc voltagedirectly proportional to the B+ voltage beingapplied to the transmitter's RF power stage.When the B+ voltage is 236 V (for VR250) or167 V (for VR125), the B+ Sample input and,in turn, the output of buffer U3D (TP4), are anominal 4.0 V (for VR250) or 2.8 V (forVR125). This voltage is applied to U7-X1.The gain of U6 and U7 and, in turn, thevoltage at U7-Y1 is determined by thefollowing formula:Unbalanced Audio * Carrier Ref / B+ RefWhen the Carrier Ref input (U6-Y1) is 4.7 Vand the B+ Sample input (U7-X1) is 4.0 V (forVR250) or 2.8 V (for VR125), the gain of U6is 1.18 (for VR250) or 1.68 (for VR125) for thedc reference applied to U6-X1. The U7-Y1current is 1.18 (for VR250) or 1.68 (forVR125) times the U6-X1 current. U3B isconnected as a less than unity amplifier andits gain is summed with U7's gain. The endresult is the total voltage gain of the circuit,relative to the voltage at the output of U2D(1.4 V), is a nominal 1.11 (for VR250) or 1.58(for VR125) when GAIN TRIM potentiometerR31 is set to the centre of its range. Theoutput of U3B (TP8) is 1.55 (for VR250; 1.4 Vx 1.11) or 2.21 (for VR125; 1.4 V x 1.58).The gain of U7 will change in direct proportionto changes in the Carrier Ref voltage. If theCarrier Ref voltage is set to 0 V, or it isclamped to ground because Q1 is turned on,U7's gain is minimum (zero). In turn, the dcreference's multiplication factor is minimum(zero). The transmitter's RF output is turnedoff.The gain of U7 will change in inverseproportion to changes in the B+ Sample input.This feature eliminates the need forsophisticated filtering of the transmitter's B+power supply and maintains the transmitter'sRF output at the original level for B+ voltagevariations of ±10%.GAIN TRIM potentiometer R31 provides anominal 10% adjustment in the carrier levelref output of U3B. In dual exciter applications,it is adjusted to compensate for tolerancedifferences in the PDM generators of excitersA and B. When it is set properly, thetransmitter's RF output is the same wheneither exciter is selected, provided the sameCarrier Ref is being applied.6.4.3.2 PDM DIVIDERThe PDM divider circuit divides the 2PDMinput frequency (P1-7) by two. The resultingPDM output (P1-10) is a 0 V to 15 V squarewave. Unless otherwise established duringthe transmitter's manufacture, it should be afixed frequency, nominally 130 kHz.6.4.3.3 LINEAR INTEGRATORThe linear integrator circuit converts the PDMsquare wave input to a triangular waveform.The triangular waveform has negative andpositive voltage excursions of equalamplitude and duration. The long R/C timeconstant formed by C26/R10 ensures a linearrise and fall time. Since the R/C time constantis fixed, the waveform amplitude varies overthe frequency range of PDM. The charge/discharge time and waveform amplitude aremaximum at the lowest frequency.6.4.3.4 INTEGRATOR PEAK DETECTORThe integrator peak detector circuit detectsthe positive going parts of the linearintegrator's triangular waveform. A portion ofthe resulting positive dc voltage (nominally1.8 V) at U2C’s output, is applied to theinverting input of differential amplifier U3A.The carrier level ref output of U3A is offset bythis voltage, which is proportional to thetriangular waveform voltage peaks. Thisoffset effectively sets the carrier level ref’szero power reference to the triangularwaveform's peak voltage. This ensures no RFoutput is produced when the Carrier Ref input(P1-2) is 0 V.