Nautel Vector-LP VR125 Technical Instruction Manual

Contents

Vector-LP Radio Beacon Transmitter Technical Instruction Manual Page 6-15Section 6 Theory of Operation Issue 1.16.4.4 Exciter Monitor/GeneratorPWBsSee Figures SD-19 and SD-20. The exciter/monitor/generator PWBs [A7 (exciter A) andA10 (exciter B, if used)] perform thefollowing functions: Use the press-to-talk input to selectbetween beacon and voice modes. Monitor and control modulation depth Selects the parameter, RF current or RFvoltage, to be applied to the remoteinterface PWB for external monitoring. Generate the appropriate tone frequency Monitors tone and keyer operation Monitors the forward power and controlschangeover and inhibit functions.6.4.4.1 MICROCONTROLLERMicrocontroller U11 uses the SPI bus tocommunicate with shift register U16 whichshifts out digital data used to control varioussignals including changeover, shutdown, RFoff, and alarm. Watchdog timer IC U2 andassociated components generate a Keying/Misc Alarm (J1-4) if the microcontroller fallsinto an unknown state.The clock for the microcontroller and the toneoscillator is generated using Y1, U4:E and theassociated components.The microcontroller (U11) continuallymonitors its internal Flash and EEPROMmemory using checksums. If for somereason, the checksum returns as invalid, themicrocontroller will detect the error and raisea flag, resulting in the output power beinginhibit. U2 acts as a watchdog timer, if thewatchdog times out, the monitor fault will alsobe raised, causing the output power to beinhibited (via the control/display PWB). Thiswill guarantee that the U11 is alwaysoperating as intended.6.4.4.2 PRESS-TO-TALKThe input to U11:11 indicates to the excitermonitor/generator PWB which mode ofoperation it should be operating in. It willreact by adjusting the U16:1 that controls theMUX U1:X.6.4.4.3 MODULATION DEPTHFUNCTIONSModulation depth of the beacon, voice andbeacon, and voice are controlled using thedigital pots 1, 2 and 3 of U10 respectively.The levels of the pots in U10 are adjusted bythe microcontroller U11 over the SPI bus.A selection between RF Monitor Voltage andRF Monitor Current samples is made usingthe MUX U14:C. CR9, U9:B and theirassociated components RF detect theselected RF monitor signal. The RF monitorsignal is then used to calculate themodulation depth. The microcontroller (U11)compares the average and peak levels ofthe RF signal. The average value is detectedusing U9:A and its associated components.The peak level is detected using U9:C, CR7,and their associated components.6.4.4.4 TONE GENERATIONThe keyer tone is generated using the DDSIC U5. The microcontroller U11 sends anSPI signal to U5 indicating the frequency ofthe tone to generate. The tone is thenamplified using U8:C and its associatedcomponents. The microcontroller (U11) thencontrols keying by toggling the MUX U1:Z.Correct operation of the keyer is monitoredby U8:A, U8:B, CR11, and CR12 and thereassociated components. The analogsamples at U11:27 and U11:28 arecompared to the low threshold levels (set upusing the GUI) to determine if the keyinggate is opened or closed. If at any time thekeying gate is opened when it should beclosed or closed when it should be opened,and alarm is flagged and output power isinhibited (via the control/display PWB).