Tektronix 494AP service manual VariabfeResolution Diagrams19, 19,104Id,and, Bandwidthselection

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Theoryof Operation- 4g4ful4g4ApServlce,Vot.i

lF SECTTON(DiagramS)

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The lF sectionreceivesthe lO MHz lF signalfrom the 3rd converter, establis_hesthe system resolution, levelsthe gain across the frequencyi"ng., logarithmi_ callyarnplifiesthe signal,and Oetecisthe-signaito pro_ ducethe videooutputto the Display

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- System bandwidth resolution is selectablefrom 3 MHz to 10 Hz. This s.etectionis pe;orm€d by the

VariableResotutioncircuits and isover the

instrumentbus. Generally,two "6niiot"O

to establisheach bandwidth. A sets oi Ritersar€ used uano-pasifilteris also

includedat the circuitsoutput.

Significantgain is al.s: prgvidedby severatstagesof amplificationwithinthe VariableResoiution-circuit

Other gain steps are also providedblock. by-switchinggain blocks in or out of tle^ signal path.ffiese gain blocks

provide-10,+10, *20, or +dOdB ot aOOitionatgain whenswitchedin combination.

		amplification
..Logarithmic				of the signal
					-Tt	is requiredto
calibratethe graticulein dB/division.						i. i" performed
!y :.."y".n	stage amptifiert-hatproOuces
						Ln output pro-
portionalto the logarithmof the input. thus, tne screen
displacementcan be selectabletor ttre amount of
change per division, and can Ue projortional to the
input level		.change.		For exampl"i irilt"
					'displar"r.nt10dB/div
display mode, each division of						on the
screen represents			a	signal level change of 10 dB
regardress
	of whetherit is at the top or 50ttom
screen.						of the

The detectorfollowsthe logarithmicamplifierto pro- positive-going

9t"9 "outputsilnat that is apptieOto the displaysectionas the VDEOsignat.

VariabfeResolution (Diagrams19, 19,104;Id,'and

21)

The VariabteResotution(VR) assembty(A6g) and the 10 Hzl100Hz Bandpass Firter aiiJmOty inOSy establishthe resolutionbandwidthand provideipproxi_ Ta.tely41 dB of system gain. The Vi assembiycon- sists of two sets of filters plus gain stages. The 1.0Hl100 Hz BandpassFitterissem-Otyacts as part of the VR circu.its,but is physicaily

in a seiarateassembty. 'is

Since the input to the VR circuits nominaltyat -35-dBm and the Log Amptifierinput,"quir"" *6 dBm for full screen,the VR circuits mi.,.t prduiOethe gain difference. The VR suppties30 dB oi aOJifionat and. 10dB of gain reductionfor alt verticat lain

modes.

disptay

Physically,the VR assembly contains two sub- assembliesthat connect together and plug onto the instrumentMotherboard. The input"ir",lit-iare in one

sub-assemblyand the output circuitsand digitalinter- face are in the other. Each of the sub-assembliescon-sists of boardsthat plug onto a four_layerVR Mother boardwith a groundplane on both outsidelayers. Only pow€r supply and control voltages travel through th6 VR Motherboard. All signatconiectionsare by joaxial cable.

VR Input (Diagram 19)

The VR Input circuit receivesthe _3S dBm I0 MHz signal from the Ard Mixer throughJ6g3. This signal goesthroughan amplifierandan attenuator.

The signal is applied to broadband feedback amplifier01029, which is biased for a large output

current (approximately reduce intermodula-

50 mA) to

I_o-1I:t:ni9n. This performanceis provided primarily throughthe largecurrentcapacityby negativefeedbaci through resistor R1025 and by emittei degeneration resistorR1023.

A6 dB attenuatorat the outputof amplifierelo2g providesa clean50 ohm outputto the lst FilterSelect circuit.

1st Filter Setect (Diagram t9)

The 1st Filter Select circuit operateswith the 2nd FilterSelectcircuitthroughbanksof switchedfiltersto set the overallsystembandwidth.Databits 0, 1, and2 from the data bus are appliedto decimaldecoderlC U4035(it providesa low signalon the appropriateout- put pin to enablethe selectedfilter). Bandwidthsetec- tionsare 10 Hz to 1 MHz in decadesteps, and 3 MHz. The data bits select a bandwidthfiltei accordingto Table7€.

Table7-3

BANDWIDTHSELECTION

Bandwidth	DBO	DBl DB2
3 MHz	1	0 0
t MHz	1	1 1
100kHz	0	1 0
10 kHz	1	1 0
1 kHz	0	0	' l
100Hz	1	0	1
1 0H z	0	1	1

Filters are selected by diode switching. Series and shunt diodes are at the input and output of each filter, The instrument allows only one filter to be selected at a

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