Theory of Operation
4-10 1705A Spectrum Monitor
control into the summing junctionof U27 (pin 2). The VerticalPosition control
is attenuatedby resistors R152 and R140 (2 dB POS R ANGE). This positions
the video signal atpin 6 of U27, providing greater positioning range while in
High Gain mode.
The horizontalramp signal from the Sweep Generator (Diagram 3) is approxi-
mately 4V in amplitude. Typicalsweep length is 12 divisions. The variable
resistor,R168, is one-halfof the input resistance(Ri) for U26B, an inverting
operationalamplifier. The amplifier feedback resistance (Rf) consists of R180
and R177. Reducing the Rfof the operational amplifierwhile holding R i
constant reducesthe gain, and in this case the sweep length. Strapping across
W11and W12 provides a convenient method of shortening the sweep to adjust
the Horizontal Gain (R168) againstthe 10 divisions of the crt graticule. When
the strap is removedthe sweep length increases to 12 divisions.
Switches U30B and C are are enabled by the Readout Enable (/RO--EN) from the
microprocessor(Diagram 5). When enabled, the input to the Deflection
Amplifier is the X--(horizontal) and Y--Axis(vertical) components of the crt
alphanumericreadout. The readout signal components are from the microproces-
sor through a Digital-to-AnalogC onverter(DAC) (Diagram 5). The vertical
positioning signal, from the front-panelVERTICAL POSITION control, is
applied to the readoutin order to common mode out positioning effect when the
traceis repositioned.
The verticaloutput signal, or the Y--Axisreadout, drives the base of Q26, one
side of a differentialinput amplifier. The other side, Q28, is driven by the
positioning voltage from the front-panelVERTICAL POSITION control. The
signal from thecollectors of Q26 and Q28 drive Q27 and Q29, common base
amplifierstages. The gain of the VerticalDeflection Amplifier is approximately
7. The gain normalized output voltage, to drivethe crt deflection plates, is
developed acrossR229 and R230, the load resistors. Q22 and Q23 are a
temperature-compensatedcurrent source from the --11.8volt supply.
Sweep ramp or X--Axisreadout drives the base of Q17, one side of a differential
input amplifier. The other side, Q16, is driven by the positioning voltage from
the front-panelHORIZONTAL POSITION control (Diagram 5). The signal
from the collectorsof Q17 and Q16 drive Q14 and Q15, common base amplifier
stages. The gain of the HorizontalDeflection Amplifier is approximately 20.
The gain normalizedoutput voltage, to drive the crt deflection plates, is
developed acrossload resistors R178 and R179. Q18 and Q19 are a temperature-
compensatedcurrent source from the --11.8volt supply.
VerticalDeflection
Amplifier
Horizontal Deflection
Amplifier