Tektronix 1705A - Manual (Page 83)

Theory of Operation
4- 10 1705A Spectrum Monitor
control into the summing junction of U27 (pin 2). The Vertical Position control
is attenuated by resistors R152 and R140 (2 dB POS RANGE). This positions
the video signal at pin 6 of U27, providing greater positioning range while in
High Gain mode.
The horizontal ramp signal from the Sweep Generator (Diagram 3) is approxi-
mately 4 V in amplitude. Typical sweep length is 12 divisions. The variable
resistor, R168, is one-half of the input resistance (R
i
) for U26B, an inverting
operational amplifier. The amplifier feedback resistance (R
f
) consists of R180
and R177. Reducing the R
f
of the operational amplifier while holding R
i
constant reduces the gain, and in this case the sweep length. Strapping across
W11 and W12 provides a convenient method of shortening the sweep to adjust
the Horizontal Gain (R168) against the 10 divisions of the crt graticule. When
the strap is removed the 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
alphanumeric readout. The readout signal components are from the microproces-
sor through a Digital-to-Analog Converter (DAC) (Diagram 5). The vertical
positioning signal, from the front-panel VERTICAL POSITION control, is
applied to the readout in order to common mode out positioning effect when the
trace is repositioned.
The vertical output signal, or the Y--Axis readout, drives the base of Q26, one
side of a differential input amplifier. The other side, Q28, is driven by the
positioning voltage from the front-panel VERTICAL POSITION control. The
signal from the collectors of Q26 and Q28 drive Q27 and Q29, common base
amplifier stages. The gain of the Vertical Deflection Amplifier is approximately
7. The gain normalized output voltage, to drive the crt deflection plates, is
developed across R229 and R230, the load resistors. Q22 and Q23 are a
temperature-compensated current source from the --11.8 volt supply.
Sweep ramp or X--Axis readout 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-panel HORIZONTAL POSITION control (Diagram 5). The signal
from the collectors of Q17 and Q16 drive Q14 and Q15, common base amplifier
stages. The gain of the Horizontal Deflection Amplifier is approximately 20.
The gain normalized output voltage, to drive the crt deflection plates, is
developed across load resistors R178 and R179. Q18 and Q19 are a temperature-
compensated current source from the --11.8 volt supply.
Vertical Deflection
Amplifier
Horizontal Deflection
Amplifier