Tektronix 494AP service manual HARMONICMIXERDiagram36

Theory of Operation- 4g4A/4g4ApService,Vol. l

puter tunes the l st LO frequencythrough the following sequence;up 750kHz,downi.5 MHz,up 1.5MHz,ani down 750 kHz.Duringoneof these yirmwaresearches,' the lst LO frequencypassesthrough the strobe har- monicfrequencyandth€ loopacquireslock.

Any frequencydifferencebetweenthe strobs signal and the 1st LO will generatea low frequencyconection voltage. This correction voltage is filtered by the F(s) amplifier,thenusedto drivethe oscillatorFM ioilto puti the oscillatorfrequencyback to the strobe position. tf th€ lst LO drifts beyondthe error voltage range of the F(s)amplifier,comparatorson the Error Amplifierboard, that monitor the error voltage, wilt interrupt the micro- computerand indicatethe directionof drlft. The micro- computerthen tunesth€ center Frequencycontrol cir- cuits to null out any FM coil current in thL phasetock loop.

Frequency Control

The 21-bitcounterand its associatedcontrolcircui- try, on the Counterboard, plusthe HarmonicMixerand Auxiliary Synthesizer,form the frequency control hardware nucleus for the spectrum analyzer. A firmware-basedcontrol loop, that uses data from the counter as feedbackon the oscillatorfrequency,con- trols both the lst LO and the 2nd LO frequenciel. tne l0MHz lF is also countedby the Counterto d€termine the inputsignalfrequencyto th€ analyzer.

Amix down countingschemeis used to count the lst LO frequ€ncy,which varies between 2 GHz and 6 GHz. 'fhe 2O0-220MHz output from the Auxiliary Synthesizeris positionedso one of the signalharmon- ics is approximately45 MHz above the lst LO fre_ quency. This output drivesthe LO input to the Har- monicMixer,the 1st LO drivesthe RF input.Oneof the lF outputsfrom th€ HarmonicMixer is withinthe 10 to

80 MHz range(approximatety45 MHz). This tF signalis passed througha 10-80MHz band-passfilt€r, divided by 100,thencountedby the Counter. Sincethe proces- sor knows the Synthesizerfrequency,the l st LO fre- quencycan be calculatedif the processorknowswhich harmonic of the Synthesizerfrequencywas used to generateth€ lF frequencybeingcounted. The harmonic of the Synthesizerfrequencyis calculatedfrom the 1st LO tuningDAC (digital-to-analogconverter)code,sinceit indicatesthe 1st LO frequencyto within approxi- mately*10 MHz.

Countingthe znd LO frequency is much simpler. The controllable16-20MHz VCO in the 2nd LO assem- bly determinesthe frequencyof the 2nd LO; therefore, the 2nd LO frequencyis calculatedby direcilycounting the 16-20MHz signat. The 2nd LO frequenty is then calculatedfromthis frequency.

Confolling fte O3clllator Frequency. The fre- quency control loop is only closed between swesps. After the completion of each sweep, the processor switchesthe span/divto zero and then countsthe lst

LO and the 2nd Lo frequencies.tf they ar€ not at the frequencyrequiredto generatethe displayedcEntertre- quency,they ar6 set to the correcl frequencyby repeat- ing the process(i.e.,the DACsare changedto tunethe LO, the LO is counted,€tc.).

In the singleswsep mode,the oscillatorfrequencies ar€ correctedafter each single-swe€pactuataonand. beforeth€ sweep starts. In the manualsweep mode,or other non-rscurring sw€eps, the oscillators ars correctedat p€riodic intervals.

Counting the lF. In addition to countingthE fre- quencyof the 1st and 2nd LO,the 10 MHz lF is counted

when the Countermode is actuated;thus,the incoming signal frequencycan be calculatedfrom the frequency conversionequationfor the analyzer. The 1st LO is actually phase locked before the 2nd LO and lF are counted,in orderto reduceFMingin the lF signal. This allows very accurate signal counting, even in wide spans.

HARMONICMIXER(Diagram36)

The HarmonicMixer combinesa portion of the 2- 6 GHz 1st LO signal with harmonics of the 200- 220 MHzrefersncesignalfrom the AuxiliarySynthesizer to provide an output signal in the 10-80MHz range. This signal is amplified and returnedto the Auxiliary Synthesizerwhere it is countedto get an exact compu- tation of the oscillatorfrequency.The HarmonicMixer consistsof a directionalcoupler,an input amplifier,the mixer, and an output amplifier.all on a hybrid circuit. Figure7-27 is a functionalblock diagramof the Har- monicMixer.

Input signal level, from the lst LO to dirgctional couplerA2541,is a about+10 dBm. The couplingratio is 1OdB, th€refore,the couplerwill delaverabout l mW (0 dBm) to the RF input of the harmonicmixer. The through-portcontributesabout 0.5 dB of loss for the 2- 6 GHzsignal.

The 200-220MHz reference signal, at a level of about 10 mW from the Auxiliary Synthesizer, is amplifiedto a level of about 100 mW (+20dBm) by a difier€ntialamplitierQl and Q2. ResistorR27 couples the emitterstogetherand the currentis set by R13 and Rl4. Outputis transformercoupledto the input of the mixer. Input signal level to the amplifieris +7 dBm minimum.

7-78