Tektronix 494AP O o o o o o O o o o o o o, Theoryof Operafon 4g4A/494ApServlce,Vot, Tabte7-1

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Theoryof Operafon - 4g4A/494ApServlce,Vot. i

2ND CONVERTERSECTTON(Diagramg)

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Two 2nd ConvErtersystgmsare used in the spec_ trum analyzer. One converts207ZMHz to 110MHz and the other convertsg2g MHz to 110MHz. The con_ verterused is determinedby the frequencybandbeing conveted. The lF selectionfor each banOis snown in Table7-1alongwith the band frequencyrangeand the localoscillatorfrequencyrangE. two 2iO lFs are used by the analyzerfor the followingreasons:

olf the lst tF is inctudedin the frequencyband being converted,it is possiblefor some input sig- nalsto pass un-convertedthroughthe lst c6nvert6r to the 2nd Converter,appearingat ttre lst lF. The resultingspurioussignatwould causethe baseline level on the screento rise and obscurereal signat. Two 2nd convertersavoids the problem by usiig a 1st lF not in the bandbeingconverted.

IWithtwo lF'S,lF feedthroughin band2 andhigher orderspursin bands3 and4lan be eliminated.

. Because of the limited tuning range of the 719MHz LO, the lower lF cannoibe used above band4.

The 2072MHz 2nd Convertermixes the 2072MHz from the 1st Converterwith the outputtrom a 21g2MHz phase-locked2nd local oseillator.This localoscillator

is sweptandtunedover a 4 MHz range. The 2A72MHz
input tF signal is passed through a four-cavity
	.tsi band-
passfilt€r (FL14)to ailow onty the 2OZ2MHz		tf sig-
nal to pass through a1d pr€vent other signals,gei-
erated within the 2nd Converter,from getting back				to
the lst	-combines
	Converter. A diode mixer		the

2072MHz lF input and the local oscillatorsignatsto generatethe 110MHz lF output which then passes through a 110 MHz fow-pass fltter to reject any higher ordersignalsfrom the mixer.

The 829MHz 2nd Converteruses a phase-locked voltage controlledoscillatorto producethe Z1gMHz signal that is mixed with the g29MHz first tF signat. The swept 2182MHz2nd LocalOscillatoris usectas a referencefor the phasetockedoscillator.The 719MHz oscillatorcan be disabled upon command from the microcomputerin the tF selectionprocess. The phase lock circuit maintainsa constantrelationshipbetween the two local oscillatorsas the 719 MHz oscillatoris sweptandtunedovera 1.33MHz range. A four section coaxial band-passfilter is used before the mixer to excludeanyRF signalsotherthanthe d€slredg29MHz. Again,a diode mixer is usedto mix the g29MHz input and local oscillator signals to produce the 110MHz secondlF output.

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Tabte7-1

2NDCONVERTERIF SELECTION

Band Range 2nd LO Range lst lF

150 kHz-1.8GHz 2182r.2.25MHz 2072MHz

21.7-5.5GHz 719*0.75 MHz 829 MHz

3 3.0-7.1GHz 719 *,0.75MHz 829 MHz

4 5.4-18.GHz0 719*0.75 MHz 829 MHz

515.0-21.GHz0 2182x.2.25MHz 2072MHz

6 18.0-26GHz 2182*.2.25MH? 2072MHz

7 26-40.0GHz 2182*,2.25MH2 2072MHz

I 40.0-60.0GHz 2182*.2.25MHa 2072MHz

I 50.0-90.0GHz 2182*.2.25MH2 2O72MHz

1 0 75-140.0GHz 2182*.2.25MH2 2072MHz

1 1 110-220GHz 2182*.2.25MHz 2072MHz

1 2 170-325GHz 2182*.2.25MH2 2072MHz

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