Tektronix 494AP service manual O o a o a, Tracemodes, Malntenance- 4S4Al4g4ApServtceVol

Malntenance- 4S4Al4g4ApServtceVol. 1

TRACEMODES

Trace Mode provides information on how the fre- quency control system is working. lt is acc€ssed by pressing <Blue-SHtFT> PULSE STRETCHERmenu. item #7, then selectinglst LO (menuitem#1),2nd LO (menuitem #2} MARKER(menuitem#3), or CORREG_ TION TIMER (menu item #4), gRD tF dOUrur (menu item #5), or DISPLAYRESULTS(menuitem#6).

Trace Mode 1, starts tracing the l st LO control actions. Trace Mode 2, starts tracing the 2nd LO con_ trol actions. Trace Mode g, starts tracingsignalcounts. Trace Mode4 starts tracing markerconectioncycles.

Informationfrom these four trace modesis stored in RAM and can be disptayed by setecting DtSpLAy RESULTS(menuitem 6) from the TRACEMbDE menu. This mode disptaysup to 16 lines of data gatheredby the trace modes.

Informationused is obtainedonlywh€n the internalfrequencycorrectionoccurs.

Thihs correction is related to the drift rate of the spectrum analyzer. The time between corrections can be as long as 30 s.

To assure informationis available,change th€ FREO SpAN/Dtv one position then retum. This forces the correction cycle to occur.

For modes1 and 2, the flrst fieldof the displayindi- cates which mode was active at the time the informa- tion was gathered. The secondfieldof th€ displayindi- cates which attempt at tuning or correctingthe oscilla_

tor the data is for. The next lield containsthe tuning DAG settings before a tune or conection took ptace. The first three digits are the upper DAG settings, the next threedagitsthe lowErDAC s€ttings. The next field containsthe DAc settingsafter the tuningor correction was attempt€d. Again, the first three digits are the upper DAC, and the next three digits the lower DAC settings. The next field indicates the time delay betweensettingthe DACSto the new valuesand read- ing the resultingfrequency,in unitsof millisecond.The final field contains the frequencyof th€ oscillator in question,afterthe tune or correctionattempt. Actually, the displayedfrequencyis the beat notefrequencyfrom the auxiliarymixer for the l st LO (in KHz), and the 16-20 MHzoscillatorfrequencyfor the 2nd LO.

For mode 3, the resultingtrace displayconsistsof sevencolumns. The first columnis alwaysg. indicating that the marker is being traced. The second column givesthe iterationnumberof the correctioncyclewhich the displayedline describes. The third columnconsists of two leftersand a number. The first letteris p if the

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data applies to the primary marker and S if the data applies to the secondarymarker. The second letter is

cif th€ oscillators are being counted at the marker positionto determinethe markerfrequency,or S if the markerpositionis beingsynthesizedto maintaina con- stantmarkerfrequency.The numberin this columnis 1 if the 1st LO is beingcountedat the markerand 2 if the secondLO is being counted. The fourth columnis the hexadecimal setting of the marker DAC. The fifth column is the decimal digital storage location of the mark€r. The sixth columnis the oscillatorsettlingtime in ms before the count. The last column is the har- monic mixer output frequencyin KHz for a 1st LO count,or the 16-20 MHz VCOfr€quencyfor a 2nd LO count at the rnarkerposition.

The sequencE<Blue-SHIFT>PULSESTRETCHER, #7, #0, terminatestrace actionsand erasesthe RAM of alldata.

Alternate Frequency Display

The AlternateFrequencyDisplay mode selectsan alternatefrequencydisplayinstEadof the normalGenter Frequencydisplay. These alternate frequenciesare selected by pressing <Blue-SHtFT> PULSE STRETCHER,selecling menu item #0, and selecting #0, #1, or #2 as indicat€dby the menu.

The normalCenterFrequencyis displayedwhen#0 is selected.

The frequencyof the 1st LO is displayedwhen #1 is selected.This displayis updat€deach time ths 1st LO frequencyis counted.

The frequencyof the 2nd LO is displayedwhen #2 is selected. This displayis updatedeach time the tre- quencyof the 2nd Lo is counted.

Auxiliary Synthesizer Control

The AuxiliarySynthesizerControlcan be turnedon continuously,or turnedon only duringcorrectionfor the 1st LO tunes. This modeis toggled(turnedon continu- ouslyor during1st LO corrections)by pressing<Blue- SHIFT> PULSESTRETCHER,and selectingmenuitem #4, A messagewill come on screen indicatingwhich modethe AuxiliarySynthesizeris in.

Correction Disable/Enable

Correctionof the 1st and 2nd LO frequenciescan be disabledor enabledby pressing <Blue-SHIFT> 10 dB/DlVor <Blue-SHIFT>PULSESTRETCHER,and selectingmenu item #6. When corrections are dis- abled. the oscillator frequenciesare counted but no furtheractionis taken. This modecan be usedto moni- tor the drift of the oscillatorsby activatingthe respec- tive trace mode. When correctionsare disabled.the 1st LO cannotbe phaselocked!

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