Theory of Operation
4-18 1705A Spectrum Monitor
CR4 is the high voltage rectifier. Filter capacitors C3, C4, and C8 work with
CR4 to provide --2530V to the crt cathode. U1 is a four-timesmultiplier
providing +11kV to the crt anode.
Q1 and Q2 form anoperational amplifier that sets the voltage at the bottom of
the focus divider. The front-panel FOCUS pot determines the voltage at the
bottom of the focus divider. The Center Focus control, R11, is set for optimum
beam focus, as viewed on the crt, with the front-pan el FOCUS control set to mid
range. Oncethe Center Focus adjustment has been set, adjusting the front-panel
FOCUS control changes the voltage at the bottom end of the divider and,
consequently,the voltage on the crt focus anode.
The cathodeof the crt is at a --2530 V potentialwith the grid coupled to the
Z–Axis Amplifier by thegrid drive circuit. The grid is approximately 75 V
negative with respectto the cathode. The 200 V p-p sine wave present at the
cathode of CR8 is input to the Grid Drive circuit where it is clipped for use as
the crt control grid bias.
The sine wave from the cathodeof CR 8 is coupled through R47 to a clipping
circuit consistingof CR 5 andC R6. Clipping level forthe positive excursion of
the sine wave is set by the CRT Bias adjustment, R58. The negative clipping
level is set by thefront-panel INTENSITY control through the Z-Axis Amplifier.
The clipped sinewave is coupled through C11 to a rectifier made up of CR1 and
CR3. The rectified, clipped sine waveis thecrt control grid bias voltage. C9
couples the blankingsignal from the Z-Axis Amplifier to the crt control grid.
DS1 and DS2 limit the crt grid to cathodevoltage at instrument turn on or off.
DS3 limits the crt heaterto cathode voltage.
This is an inverting amplifierwith negative feedback. R22 is the feedback
resistor whileR 7, R20, and R33 act to maintainthe summing junction at +5 V.
Withoutany Z���Axis input current, the amplifier output is approximately +10 V.
Negative Z–Axis input currentwill cause the output to go positive.
Power SupplyOutput sFocus AmplifierGrid Drive CircuitZ-Axis Amplifier