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Theory of Operation- 4g4ful4g4ApService,Vot. 1

couNTERandPHASELocKsEciloN (Diagramg)

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FUNCTIONALDESCRIPTION

This section consists of a Counter, phase Lock assembly,phase Gate, HarmonicMixer, and Auxiliary Synthesizer.The Counter,HarmonicMixer,and Auxili- ary Synthesizer,form the nucleusof the frequencycon- trol hardwarefor the instrument.Both the lst LO and 2nd LO frequenciesare controlledvia a firmwarebased controlloop that uses data from the Counteras feed- back to controloscillatorfrequency.The 10 MHz lF is alsocountedto accuratelycalculatesignalfrequency.

The phase Lock assemblystabilizesthe t st LO fre- guency. lt consistsof an outerand innerloop.

_--l!g-inner loop uses the subharmonic of the 100MHz reference frequency,from the 3rd Converter, to mix with the outputfrom a 2i.Og2to25.095VCOand comparesthis lF differencewith a +N number(between 32 kHz 3nd 94 kHz) set by the processor. Any devia- tion is detectedby a phase/frequency

detectorwhose outputenor voltageis used to pull the VCO frequency andphaseinto lock with the innerloop reference.

_ . The_outertoop consistsof the innerloop, a Strob€ Driver,Phase Gate Detector,Error Amplifier,and th€ 1st LO. The frequencyof the innerloopVCO is divided down and appliedas a strobe putseto the phase Gate Detector.This strobe.pulsecontainsenergyat frequen- cies equallyspaced throughoutthe specirum. One of thesefrequencieswiil be within2.5 Mi{z of th€ l st LO lrequencyat the otherinputto the phaseGateDetector. The PhaseGate Detectoroutputsan error signal pro- portionalto the differencebetweenthe neareststrobe and th€ lst LO frequency.This errorsignalis amplified and filteredby the ErrorAmplifierand aiptieOto the FM coil of the 1st LO to pull it into trequencyand phase lockwithth€ strobe.

The HarmonicMixer mixes the 1st LO freguency

and'signal a harmonicof a synthesized 200-2ZAMHz

from the.AuxiliarySynthesizer.The exact frequencyof the synthesizersignal is a functionof the +N factor from the processor. The HarmonicMixer output is a signal within the 10 to g0 MHz range. This signat is divided in the AuxitiarySynthesizeiand sent to the Counter. The microcomputerlooks at the resultant count and decides which way to move the 1st LO to bringit to ths correctfrequency.

Phase Lock Assembly

. As previouslystated, the phase lock system con_ sistsof two frequencyservoloops,calledthe outerloop and inn€r loop. In the inner loop operation, the

100MHz referencesignal from th€ grd Converter,is divideddown to 25 MHz,on the Synthesizerboard,and applied as the referencesignal to the mixer on the OffsetMixer board. The 25 MHz signal is also applied

'Syn-

as a clock signal to +N countercircuits,on the thesizerboard,whichoutputa frequency(dependingon the +N number from th€ processor) tieiween g2lHz and 94 kHz. This signat is apptiect to the phase/frequency

detector on the Offset Mixer board. where it is compar€d to tho lF output (difierence betweenth€ 25 MHz referenceand the autput trom the VCO (voltagecontrolledoscillator)and any difierenceis outputas an €rror voltageto the Enor Amplifier.

The VCO operates between 25.032MHz and 25.094MHz, depending on th€ driv€ from the Error Amplifier. This signalis apptiedto thg RF inputof the mixer on th€ Offset Mixer board, where it mixes with the 25 MHz referencefrequency. The differencefre- quency,whichis between32kHz and 94 kHz,is applied t1 thj phase/frequency

detector and comparedto the +N frequency. lf the two signals are edge and fre_ qugngycoincident,phase lock occurs. lf they do not coincide,an error signalis generated,passedthrough the Error Amplifier,and apptiedto the VCOto shiftthe oscillatorfrequencyuntilit is phaselocked. Thisevolu- tion typicallylasts for only a few milliseconds,so the innerloop phaselockis essentiallyinstantaneous.

The_outer loop, which includesthe inner loop cir- cuits (OffsetMixer, Error Amplifier,and VCO)consists of the Strobe Driver,phase GatE,Error Amplifier,and 1st LO. (fhe HarmonicMixer, Auxiliary Synthesizer, and Counter, are a part of the operation, but are not considereda part of the foop.)

The signal between 2S.0gZMHz and 25.094MHz from the VCOis appliedto the StrobeDriver whereit is divided by five, filtered,and sent to the phase Gate Detector as a strobe signal between 5.006MHz and 5.019 MHz. This strobegeneratesline spectrathat are egually spaced approximatelyS MHz over the spec- trum. At about the 400th line, which correspondsto

2 GHz, assumingthat the l st LO is tuned to a fre- quency near 2 GHz, one of these lines (at about the 400th line) will be within 2.5MHz of the 1st LO fre- quency. The PhaseGate Detectorwill then outputan enor signal that is proportional to the dilference betweenthe lst LO frequencyand that of the nearest strobe line, if that differencefrequencyis less than approximately1 MHz.

For phase-lockacquisition,the microcomputercal- culates the strobe frequencyrequiredfor the desired l st LO frequency.The strobe is set to this lrequency and the lst LO is set to the requiredharmonicof the strobe. The outer loop is closed, and the microcom-

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