Tektronix 494AP A o o a, 110MHztF AMpLtFtERDiagram17, Theory of Operation 4g4Al4g4ApService,Vol

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Theory of Operation_ 4g4Al4g4ApService,Vol. 1

3RDCONVERTER(Diagram4)

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The 1t0MHz tF Amptifier(A32)and 3rd Converter (A34)down convertsthe ttO MHz ouiput signatfrom the 2ndConverterto 10 MHzfor the VariaUieResolution circuits. A 100MHz crystat controlledoscillator pro- vides.thethird LO signal.Thisoscillatoris phaselocked to either a precise internal10MHz referenceor an external1, Z, 5, or 10 MHz reference.The 100MHz LO signaris appliedto the mixerand is oistriouteothrough output amplifiersto many other circuitsthroughoutt6e instrumentas a referencesignal. The 100MFz signal and its harmonicsare also availablefor externaluse at the front-panelCAL OUT connectorfor irequencyanO 100MHzamplitudecalibration.

. The 110MHz signal is amplifiedin a three_stage gain btockand apptiedthrougha band_paisfitterand a low-passfitter.From the toiv_passfil6r, the signal is appliedto the converter,whichconsistsof a mixer,an oscillator,andvariousoutputamplifiers.

110MHztF AMpLtFtER(Diagram17)

-lnitial gain for the analyzer is provided by the 110MHz lF Amplifier. Thi; gain compensatesfor conversionlossesin the thre€ mixers. Typicalgain for

the amptifieris 2t dB. The amplifie, of three stagesof amplificationand an attenuator"onjirts.

The first two mixers in the RF syst€m ofier no high_frequencygain; therefore,it is importantthat this ariptifiei exhibit low noise characteristics.lt must atso be relativelyfree from third-orderintermodulation

distortion.

.	Signalinputis appliedthroughan impedance
							rnatch-
ing b.and-passfitter(L2044anOCSZS)to'aparailel
circuit. The signalis injected							tuned
				intothe paraltet-tuned
cuit througha tap in the inductor							cir_
						and taken out at the
high impedanceside throughvariablecapacitorf;2A47.
Inductive input			provides	for			to high
impedancewithin					"onu"riion
			the tuned circuit;the extra capacltor
on the outputprovidesfor conversion
		pr'.m."ry.				backto 50 ohms
l.Iil_"]:_I!:
			tuning capacitor,C325, adjusts
tn€ resonant point; the
adjust€din combination output capacitor, C20C7,is
			with bSZSt6r good impedance
match at 110MHz. This is done wiih a return loss
bridge. The nominalreturnloss is						35 dB. The e of the
inputfilteris approximately
			20.

- Fromthe input fitter,the signatis appliedto e4053, which is the first stage of impliRcaiion. This is a broad-bandfeedback amplifierto provide good input and output impedanceand controliedgain. All feed- 919! is through reactive componenti (transformer T3054),not resistivecornponents.Thus,the impedance and gain can be controlledwithout significantnoise problems.

.. The second amplifierstage, e40g7, is essentially the same as the first, with oniy minor bias differences. Gain through each of these itages is approximately 9 dB. The output is appliedthrougha 3 dB attenuator, to..preservethe impedancefigure, to the bridged .T; adjustableattenuator.The 3 dB attenuatorconsistsof resistorsR2039,R2038,and R2043.

From the 3 dB attenuator,the signalis capacitively goupledthrough C2Ogl to the aOyustabteattenuator. This attenuator uses two ptN diodes, CR3030 and CR1029,in the modewhenthe resistanceto RF signal lolv is c_ontrolledby the current through the diodes. Referto FigureT-7 to aid in understandirigtt e foflowing description.

ffresistorR1 in Figure7-T wereset to infiniteresis_ tance and resistor R2 were set to zero resistance,the RF signalpath would be throughR2 to ground,to pro- duce infinitesignalattenuation.lf resistorRl were set to zero resistanceand resistor R2 were set to infinite resistance,the RF signalpath would be throughRl to th€ load,to producealmostno attenuation.This,basi- cally,is.howthe adjustableattenuatoroperates,except that resistorsR1 and RZ are actuallyplN diodes and the RF path resistancethrough theie diodes is con- trolled by the currentthroughthe diodes in an inverse proportion(highercurrentresults in less resistanceto RF).

Figute 7-7. Bridged 'T'

attenuator equlvalent clrcuit

Resistors R3035 and R20gO on the detail€d schematic diagram establish a constant current of approximately2 mA fromthe 15 V supplyto the diodes. This currentis dividedaccordingto the bias on the diodes.The bias,in turn,is establishedby gainadjust- mentR1015,fromthe+15V suppty.lf Rl015is setlow (nearground),diode CR30g0is reversebiasedandthe

2mA flows throughdiodeCR1029. This routesthe RF signafthroughresistorsR2092and R3029and capaci_ tor C2029, with the impedance characteristicsof

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