Tektronix 494AP O o o o O o a o o o o O a o O a o o, 2182MHz PHASELOCKED2nd LO Diagrams13

Theory of Operation- 494A/494ApServtce,Vot. 1

Fllter to Mlxer RF Input Cable. Several products and harmonicsof the local oscillatorand RF inputfre- quencieswill exit the mixervia the RF input port of th€ mixer. The image(RF inputminusthe 2nd LO)and the sum (RF input plus the 2nd LO)are two significantpro_ ducts. There is enoughenergyin these two signalsto warrantefforts to r€coverthat energy.

Onfy the RF signal at 2072MHz can pass through the four-cavity filter. Thus, any other signal frequeniy that is appliedto th€ filter (that is, signils exitingthi mixer via the RF port) is reflectedback to the mixlr by the filter. lf the cablebetweEnthe filter and the mixeris the correctlength,the mostslgnificantreflectedsignals (i.e., the image and th€ sum) can be returnedto the mixer in phaseand convert€dinto additional€nergyat the intermediatefrequency. This techniqueis called "image enhancementmixing,,and typically improves conversionloss by approximately3 dB at the design frequencies.

The image frequency,in this instance,is very near the RF frequency.A very sharpcut-offfilter is required to pass the RF, yet refl€ctthe image. The four_cavity filterperformsthis function.

2nd LO lo Mlxer LO Input Cable. The imageand sum products are also presentat the LO port of the mixer. These signals leavethe mixer via the cable to the 2nd LO and are reflectEdback to the mixer by the LO .The oscillatorsresonatorappearshighlyreflective to the imageand sum signalsbecauseit is tunedto the LO frequency. Again,the lengthof the cablefrom the LO to th€ mixer LO port is adjustedso the lmageand sum signals are rEflectedback to the mixer, in the proper phase, for re-conversionto suppty additional energyat the lF frequency.

2182MHz PHASELOCKED2nd LO (Diagrams13 and i4)

The 2182MHz phaselocked2nd LO assemblycon_ tains a tunable microwaveoscillator,frequencyrefer_ ence, and phase lock circuitry. A two-sectionhousing containsthe circuitry. Microwavecircuitryis packaged within the machinedaluminumportion of the housing. Low frequencyphase lock circuitry is within the mu- metalcompartment,

In the microwaveor LO portionof the assembly,the 2182MHz MicrostripOsciilatorg€nerates2Ig2 MHz tor the 2nd convertersand the 2nd LO internalreference circuitry. The 2200MHz Referencecircuit receivesa 100MHz drive signal from the grd convert€r crystal oscillatorand produces100MHz harmonics. The i2nd harmonicor 2200MHz is mixed with 21g2MHz from the microstrip oscillator in the 2Z0OMHz Reference

7-10

Mixer circuit. The differencefrequencyof 18 MHz is thenfed to the phaselock side of the module.

Aphase/frequencydetector, on the 16-20MHz Phase Lock circuit board, compares the 18 MHz differencefrequencywith a signal from a linearized varactor tuned, 18MHz voltage controlled oscillator. The det€ctor output tunes the 2182MHz Microstrip Oscillator such that the differencefrequencyexactly matchesthe frequencyof the 18 MHz referenceVCO.

Sweep and tune signals from the Span Attenuator and Cent€rFrequencyControlcircuitstunethe 18 MHz VCO. The output voltage from the phase/frequency detector forces the Microstrip Oscillatorto tune the sameamount.

2182 MHz Microstrip Osciltator (Diagram 14)

This oscillatorconsistsof a printed1/2 wavelength resonator driven by a common-emitt€r feedback amplifier(01021). The base of Q1021is capacitivety tapped into the resonator. The resonatorserves as a tuned phaseinv€rterand impedancetransformer,con- nectedbetweenthe base and coll€ctorof Q1021. Part ot the base feedback capacitanceis provided by a bendabletab (C'1021)This. allows line adjustmentof the total feedback. This feedback RF signal is detected,by the base- emitter junction of Q1021,to producea changein bias voltagethat is relatedto the amount of feedback. The base voltage can be moni- tored at TP1015 with a high impedance voltmeter withoutsignificantlydisturbingthe oscillator.

The dc collectorvoltage and current for Q1021 is regulated by an activ€ feedback circuit containing transistorQ2021. Voltageat the junctionof R2023and L2023is a functionof Ql021 collectorcurrent. This vol- tage is sensedby Q2021,which altersthe basecurrent to Q1021 thereby regulatingthe collectorcurrent and maintaining+10 Vdc on the resonator. Decouplingand control of bias loop dynamicsare providedby C2104. Resistor R2016 swamps the negativebase resistance of Q102'lto providestabilization.ResistorR2015pro- tects th€ base- emitterjunctionof Q1021trom exc€s- sivereversebiasin the eventthe +12 V supplyfails.

The oscillatoris tuned by varactor diode CR1028, connectedto one end of the resonator.Decouplingfor the varactoris providedby the low-passelementsin the tune fine. Bendabletab C1022can be usedto finetune the oscillatorcenterfrequency.

Three output taps are coupled to the resonator through printed capacitors under the resonator. One output supplies2182MHz througha 6 dB attenuatorto the HarmonicMixer in the 829 MHz 2nd Converter.The other two output taps coupl€ LO power through 6 dB attenuatorsto bufferamplifiersQl031 andQl0'll. The

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