Theory of Operation
4-4 1705A Spectrum Monitor
regulatedto become the + and --11.8V supplies on the Main circuit board
(regulatorcircuits are on Diagram 4).
The Power Supply schematic also contains the 11.8V Post R egulators,whose
outputs are usedby most of the circuits on the Main circuit board. The Post
Regulators do not appearon the block diagram.
The unblanking signal from the Z--AxisControl drives the Z--AxisAmplifier.
The Focus Amplifier,controlled by the front-panel control, provides a voltage to
the crt focus ring. The crt is of the Post Acceleration type,which requiresa
relativelyhigh potential difference between the cathode and post anode. The
boost in 2nd anode voltageis provided by an encapsulated 4X Multiplier.
The High Voltagesupply also provides the +100 V,required by the Horizontal
DeflectionAmplifier (Diagram 4), to drive the crt horizontal deflection plates.
RF InputDiagram 1This diagram hasboth R F inputs forthe Spectrum Monitor. The L--Bandand
70 MHz inputs are both off-boardsubassemblies. The 70 MHz input consists of
discretecomponents on a small circuit board that is totally shielded. The
L--BandTuner is also self contained, totallyshielded, and contains no user-ser-
viceable parts.
The L--Bandsignal (900 --1750 MHz) is input directly to a tuner subassembly
that contains theTuned RF Amplifier, mixer, and VoltageControlled Oscillator
(VCO) requiredto produce the first Intermediate Frequency (IF) of 489.9 MHz.
The 489.9 MHz 1st IF signal is cabled to the Mainboard.
The tuner subassemblyis powered by the +11.8 volts, which is controlled by the
Input Switching. (The+11.8 V to the L--BandTuner is disabled when the
70 MHz input is selected.) In addition, the Block Down Converter +18 V supply
(from Diagram 5) is fed to the tuner for out put on its F-type connector. A
recessed,rear-panel slide switch connects the +18 V supply to the L--BandTuner.
A pre-corrected ramp (from Diagram 3) to drive the VCO is also supplied to the
tuner subassembly.
The /LBAND control line, from the microprocessor,is low when the L--Band is
selected. When/LBAND goes high (indicating that the 70 MHz input has been
selected)Q31 turns on and Q30 turns off, causing Q32 to turn off and disconnect
the +11.8V from the tuner.
The input to the 70 MHz tuner has a 75Ω, 10 dB pad, which allows it to accept
signals up to --20dBm. The 70 MHz signal (45 -- 100 MHz) is preconditioned