Theory of Operation
1705A Spectrum Monitor 4-7
Sweep GeneratorDiagram 3This schematic containsthe 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 chargedby current
from the front-panelSWEEP S PEED control. The ramp integratesin the
positive directionfrom --2 volts to +2 volts. At +2 V the output of U24A trips to
the positive powersupply voltage (+5 V) and causes C40 to ramp down through
CR24 and R151. The value of R151 determinesthe amount of time required to
dischargeC40 (retrace time). The power supply for U24 is +5 and --11.8volts.
The output swing of U24A is nearlyto these supply values. The Ramp
Generatoroutput, through R142, drivesQ6 through U23B in the Z--Axis control
to provide retraceblanking. CR21 limits the output swing in the negative
direction to --0.7 volts.
The Ramp Generatoroutput (U24B, pin 7) drives the HorizontalDeflection
Amplifier andthe Gain Control (SPAN/DIV) switching, U20. Each switch in
U20 selects a different value of Rifor operational amplifier U18A. The gain is
maximum at FULL (50 MHz perdivision) 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 th eL --BAND VCO; U16A and
D drive the 70 MHz VCO. The Sweep Shapers pre-distort the VCO driving
ramps to compensatefor the inherent nonlinearity of the VCO. The shapers have
adjustable break points so that gain at each can be indi vidually adjusted. The
70 MHz shaperhas four adjustments, R48, R49, R55, R56; and the L--BAND
shaper hassix adjustments, R35, R51, R58, R75, R 81, andR82. Figure 4-1
shows the areaof control for the various adjustments on the 1705A L--Band
display,when a comb signal is applied.