Tektronix 494AP service manual A o o o o o O o o

Theory ol Operation- 494A/494ApSewice,Vot. 1

time. Whena filter is selected,the seriesdiodes are biased on and the shunt diodes are biasedoff. The diode conditionsare opposite for the filters that are not selected. Sincethe switchingoperationis th€ samefor all ftlters,the followingdescriptionfor the 100kHz filter selectionappliesto all filters.

When the 100kHz filter is sel€cted, line 2 from U4035will be low. This tums on switchingtransistors Q3015and Q3055. With inputswitch03015turnedon, the current path is through R4012, R4010, L4010, CR3013,L3015,R3015,and e3015. This currentis determinedthe resistorsin this seriescircuit. The vol- tage drop across the resistors is enoughto turn the seraesdiode on and reversebias shuntdiodeCR3012. The same case exists for the lilter output switch. 03055. SEriesresistors establishthe currentto for- ward biasCR3063and reversebiasCR3062.

Thereforethe signal from the VR Input circuit,via jumper B, is appliedthrough the sel€ctedtilter to the 10 dB Gain Steps circuit via jumper K. Nominalloss through the lilter circuit is approximately6 dB, with slightvariationsamongthe filt€rs. The 1st FilterSelect output level is nominally-25dBm. Any differencein gain betweenthe filters is compensatedfor later in the 2nd FilterSelectcircuits.

In the non-selectedfilter sections,the inputand out- put switchtransistorsare tumed off by the highoutputs from decimaldecoderU4035. The collectorsare pulled toward -15Vthrough the resistorsthat forward bias th€ shunt diodes in the input and output. Since one filter is always selected,the voltagedrop across the common input and output resistors back biases the seriesdiodes.

Afilteris not usedin the 3 MHzsectionbecausethe wide bandwidthfiltering takes ptace in the 110MHz filters betweenthe znd and 3rd ConvertErs.Insteadof a filter, a 6 dB attenuatoris containedin the 3 MHz selectioncircuit. This attenuatorhelpsmatchthe lEvels of the various bandwidthsby simulatingthe insertion loss of the othersections.

The 1 MHz filter sectionconsistsof a pi attenuator and an LC band-passfilter. The attenuatoradjuststo matchthe levelof the 3 MHz section. Gainis adjusted for bothbandwidthsin the 2nd FilterSelectcircuit.

The 100kHz filter is a doubte-tunedLC circuit designedfor a good time-domainresponseshape. The filter is tun€d with compositevariablecapacitorscon- sisting of small air variablesparalleledwith switched fixed capacitors. A third variable capacitormay be adjustedto establishthe desiredbandwidth.For Option

07instruments,a similar 300 kHz filter replac€s the

100kHzfilter.

7-24

The 10 kHz filter uses a pair of two-polemonolithic crystal filters that ar€ interconnectedby variableshunt capacitor C2030. Input and outprrt impedancesare matched with broadband transformers T2025 and T3040. A 3 dB pi attenuator is included at the filter inputto helpmatchthe loss of the othersections.

The 1 kHz resolutionfilter consistsof a singletwo- pole monolithiccrystal filter, matched to the 50 ohm impedance with broadband transformers T2035 and T2040. A 2 dB attenuatoris also includedat the filter inputto h€lp matchthe loss of the oth€r filters.

The loHzfiAA Hz filter, A69, is contained in a separateassemblywith switchingdone on this board. One set of switchingtransistorsenablesthE filter path when eitherthe 100Hz or 10 Hz bandwidthis selected. Another switch selects betweenthe two bandwidths. Decimal decoder U4035 selects 100Hz bandwidthby pulling output 5 (pin 6) low, and selects 10 Hz bandwidth with output 6 (pin 4. The filter path is selectedwheneitheroutput5 or 6 are low.

Diode pair CR1030turns on 01025 at the input to forurrardbias diode CR1011. Diode pair CR4055tums on Q4050at the outputto fonilard bias diodeCR4061. Diodespairsin CR1012and CR1020providelimiterand clamp action at the filter input to removeRF excursions causedby the dc switching.

The filter has a bandwidthof 100Hz whenits input port is low (-15V) and 10Hz when high (+15\4. TransistorQ1027doesthe switching.When100H2is selected.output 5 (pin 6) of decoderU4035is low and output 6 is high. This turns transistorQ1025on, for- ward biasing CR1011 and reverse biasing CR101O. This appliesthe lF signalto the filter input. This also biases 01027 off, placing-15V at the filter input to selectthe 100Hz bandwidth.(04040also switchesthe output, but is not effectivein this instrumentsinceA69 only requiresswitchingat the input).

Whenoutput6 of U4035is low and output5 is high, Q1027 and Q4040 are on in addition to Q1025and 04050. This selectsthe filter pathand applies*15 V to the input and output ports, switchingthe tilter to the 10 Hz mode.

100 Hz and 10 Hz Bandpass Filter (Diagram19A)

The 100 Hz and10 Hz bandwidthsare providedby a dual-bandwidthfilteringassembly(A69). The signalis convertedfrom 10 MHzdown to the 250 kHz centerfre- quency of the filter. Filtering at 250kHz makes the bandwidtha muchhigherpercentageof the filtercenter frequencythan if filteredat 10 MHz. The ltlteredsignal is then convertedbackup to 10 MHz.

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