Tektronix 494AP O o a o a o a o, O o a o o o o o, U5031OUTPUTLINES, Span Volts Span Volts

Theory of Operaton - 4g4A/4g4ApServtce,Vol. 1

oThe trackingadjustmentcircuit,whichcompensates for differentpreselectorsensitivities,compensat€s any pres€lectoroffset, and compensatesfor non- linear operationcausedby magneticsaturationof the Preselector.

rThe final driver stage, which changesthe applied signalvoltageinto a currentdrivefor the preselec- tor coil.

eThe preselectorswitchdriver,whichdrivesthe filter sElect switch€s, shown on Diagram 12. The switches require a positive putse to select the Low-passFilter and a n€gativgpulseto select the PresElector.

Digital Control Circuits

The microcomputerinterface circuits, which exer- cis€ digital control of the preselectorDriver circuits, consist of address decoder US0g6 and latch U5O3l. Both the write address(27)and the read address(F7) are decodedby U5036.

Data is latchedinto u5031 on the trailingedge of the DATA VALID signal for address 77. This event coincideswith the rising edge of th€ pulse on pin 3 of U5036.Table7-17tistsoutputtinesfrom U5Og1.

When address F7 is specified,US0g6pin 7 goes low. This pulls data line DB3 low, informingthe micro- computerthat a Preselectoris used.

Oscillator Voltage Processor

The oscillator voltage processor consists of Ul011A, U2028, and retated components. The PreselectorDrive signal from the 1st LO Driver is appliedto a voltage divider and scaling network con- sisting of Rl022, Rl023. R1024, and lnput Offset adjustmentR1031. The input vottageis *10V. This voltage is the summationof the sweep and tune vol- tages,with appropriatescaling. The outputof the vol- tage processor is about 1 V at 2.A72GHz to about g V at 6-35GHz, which correspondsto a scale factor of

2.1GHz/V. The voltageis directlyproportionalto fre- quency; thus the offset is such that if the oscillator could operate to 0 Hz, the voltage processor output wouldbe at 0 V.

Sincethe preselectordrive inputis not swept by the 1st LO Driverwhen FM Coil spansare used,tfre SpnN VOLTSfrom the Span Attenuatormust be summedby this stage. The DB3 inputto u5031 goes low when FM coil spans are selected,turning 01011 on. This switchesthe FET01022 on so the SpanVoltssignalis now appliedto the invertinginputof U1011A,whereit is invertedand appliedto the inputol lJ21Zg.

7,70

OperationalamplifierU2028,hasa gainof 1 or 3, as directedby the microcomputer.Theoutputsignalin the X3 gain mode represents the effective oscillator fre- quencyswingfor bands4 and 5 whenthe 3rd harmonic of the LO is used. When DBo at U5031goes tow, the respective output of quad comparator u5022 is also low, which holds FET c.2024cut ofi. V2028 is now a unity-gain,non-invertingamplifier. When DBO goes high, O2024 switches on and the gain of U2028 increasesto three. The X3 Gain adjustment,R1052, setsthe gainto preciselythree.

Table7-17

U5031OUTPUTLINES

Input

liqnal	High		Low
DBO	Selects	x1	Selects	x3
	gain	for	gain	for
	u2028		u2028
DB1	Not used		Not used
DB2	Gonnects		Disconnects
	tracking		tracking
	adjustment		adjustment
	output to final		output	from
	driverstage		final driver
			stage;
			(Preselector
			currerit	goes
			to zero

DB3 Connects Disconnect

SPAN VOLTS SPAN VOLTS

signal to fromUl011A Ul011A input

for FM coil spans

D84 Selects Low- Selects pass Filter Preselector

(Band1) (Bands2-5)

DB5 Disconnects Adds output output filter- filtering

ing

DB6 Connects Disconnects

-829 MHz -829 MHz

offsetoffset

DB7 Connects Disconnects

+829 MHz +829 MHz

offsetoffset

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