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Theory of Operation
1705A Spectrum Monitor
4-7
Sweep Generator
Diagram 3
This schematic contains the free--running Ramp Generator, Switchable Gain
Control (SPAN/DIV), Sweep Shapers, Marker Generator, and the Z--Axis
(Brightness) control.
U24 is the Ramp Generator; U24B is configured as an integrator and U24A is
used as a comparator. C40 is the integration capacitor and is charged by current
from the front-panel SWEEP SPEED control. The ramp integrates in the
positive direction from --2 volts to +2 volts. At +2 V the output of U24A trips to
the positive power supply voltage (+5 V) and causes C40 to ramp down through
CR24 and R151. The value of R151 determines the amount of time required to
discharge C40 (retrace time). The power supply for U24 is +5 and --11.8 volts.
The output swing of U24A is nearly to these supply values. The Ramp
Generator output, through R142, drives Q6 through U23B in the Z--Axis control
to provide retrace blanking. CR21 limits the output swing in the negative
direction to --0.7 volts.
The Ramp Generator output (U24B, pin 7) drives the Horizontal Deflection
Amplifier and the Gain Control (SPAN/DIV) switching, U20. Each switch in
U20 selects a different value of R
i
for operational amplifier U18A. The gain is
maximum at FULL (50 MHz per division) and minimum at 100 kHz per
division. The output of U18A is a negative-going ramp that drives the Sweep
Shapers.
U16B and C form the Sweep Shaper that drives the L--BAND VCO; U16A and
D drive the 70 MHz VCO. The Sweep Shapers pre-distort the VCO driving
ramps to compensate for the inherent nonlinearity of the VCO. The shapers have
adjustable break points so that gain at each can be individually adjusted. The
70 MHz shaper has four adjustments, R48, R49, R55, R56; and the L--BAND
shaper has six adjustments, R35, R51, R58, R75, R81, and R82. Figure 4-1
shows the area of control for the various adjustments on the 1705A L--Band
display, when a comb signal is applied.
Ramp Generator
Gain Control (SPAN/DIV)
Sweep Shapers