Tektronix 494AP Theory of Opera0on- 4g4\l4g4Ap Servlce,Vot, TabteT-18, Preselector Freouencybands

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Theory of Opera0on- 4g4\l4g4Ap Servlce,Vot. I

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lF Ofiset

The -10 V reference,from the oscillatordriver,fur_ nishes the precise reference vo,tage for the lF offset circuit. Sincethe offsetvoltageis pioportionat

to the lF mings 2.072GHz, no ottset id requireo for the +2.072GHz tF. FET 02094 adds th€ *829 MHz net- work into the circuit and e2036 adds the _829 MHz nehi/ork.Databits DB6 and DB7throughtwo compara- tors in U5022,control the two FET swirchese203i and 02036. One,but not both, transistorbare switchedon to providethe offset voltage to the invertinginput of U2045. An output voltage of _-9V from the amplifier correspondsto -2901MHzor (_0esnHzy_zot2MHzl.

_ The signaton the FINETUNEVOLTStine,from the CenterFrequencyControlboard,which is used to tune the 2nd Locat Oscillator, is appli€d to the input of U2O47.Sinceit is apptied here,-iiis independentof the voltagetriplingaction in the voltageprocessorcircuit. T!:.lulilg voltageis atso appliedto the input networks of U2045through R9044,O2Og4,and R1037,02096. By varyingthe magnitudeof signalin the invertingpath comparedto the direct path, the propermagnitudl and polarityof finetuneoffsetfor eachlF is provided.Table 7-18liststhe offsetvoltagerequiredfor Lachband.

Summing Amplifier

The effective oscillator frequency voltage, from U2028, and the offset tF voltage, tiom U2b+5, are appfied to the inverting input of U2C/T. This stage drivesthe trackingadjustmentsstage and furnishesa signal for external preselectordrivl circuits as well. The externaldrive line has its own r€turn to reduce groundloops.

Tracking and Shaper Circuits

This stage consists of gain-setting,offset,and shaping circuits. preselector Sensitiviiy adjustment R1065 compensatesfor sensitivityvariationsbetwe€n preselectors. Preselector Offset adjustment R10O4 compensatesfor the offsst in the presel€ctor. This adjustmentsets th€ preselectorfrequencyto 2OTZMHz whenthe output ot LJaO4Tis at OV.

The four oth€r adiustmentsR1054,R1056,R1061, and Rl 063, are part of a shapernetwork. The network compensatesfor magneticsaturationin the pr€selEctor, which would cause non-linearityat frequenciesabove

GHz. Eachshapernetworkis switchedin by a resis-

.1.4

tive divider that, at a given frequency, provides forward-biasto the diode in the shap€r to shapetne currentoutput.

The front-panelpEAKINGcontrol applies a small offset through R5065to the input of the current driver stage. This corrects for nonjinearityor temperature drift in the l st LO andpreselector.

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TabteT-18

	PRESELECTOR
		FREOUENCYBANDS
Band	FrequencyRange		I F	Harmonlc	B Approximate
					VoltageOffset
2	1.7-5.5GHz	-829 MHz		1 s t	9.0v
3	3.0-7.1GHz	+829MHz		1 s t	3.9V
4	5.4-18.GHz0	-829	MHz	3rd	9.0v
5	15.0-21.0
	GHz	+2.472GHz		3rd	0 v

Ia	7-71
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