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Theory ot Operation- 494A/4g4ApServlce,Vot. 1

COUPLING HATCOUPLINGLOOP FREOUENCYDETERMININGGAP

FILTE R TOP

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Figure7-1. Ctpsr section of a four-cavltyfiltee

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Selectionbetweenthe two 2ndlF signalsalsotakes place within the 829 MHz convertersystem. A diode switchingnetworkconnectsthe activej10 MHz 2nd lF signalto the outputto drivethe grd Converter.

2472MHz2ND CONVERTER(Diagram12)

The 2072MHz 2nd Converter converts the 2O72MHz signal output from the 1st Convener to 110MHz for eventualapplicationto the grd Converter. The assemblyconsistsof a four-cavityfilter connected to a narrow band mixer throughan externalcable, a 110MHz low-passfilter,anda mixer-biasingcircuit.

Four-Cavity Filter

The four-cavityfilter (FL14)is a low-loss narrow- band fifterthat only pass€s lhe 2072MHz IF signalto the mixer. Any other frequenciesare reflectedback to the lst Converterand terminated.In addition.the filter preventsthe converter LO and mixer products from enteringthe 1st Converter.

This filter has a 1 dB bandwidthof 15 MHz and an insertionloss of 1.2 dB. Eachend resonatorls capacity coupled to external circuits through a coupling hat pluggedinto a 3 millimeterconnector.Intercavitycou- pling is provided by coupling loops that protrude from the machinedfilter top. The resonantfrequencyof each cavityis determinedprimarilyby the depth of a gap in the undersideof the filter top, and is fine tuned with a tuningscrewon the side of eachcavity. All of the tight machiningtolerancesare confinedto the top. Thus,the main cavity millingneed not be a high precisionpart. When properly tuned, using a network analyzer,the filter return loss is greaterthan 25 dB from eitherend (in a 50 ohms system). Figure7-1 shows a cross sec- tional view of the filter, and Figure 7-2 shows the equivalentelectricalcircuit.

Mixer Circuit

The mixer circuit consists of a single-balanced, two-diodemixer, a bias circuit for th€ mixer, a delay line,anda 110MHzlow-passfilter.

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7-8

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