Theory of Operation
1705A Spectrum Monitor 4-3
bright-up signal, centeredaround the center frequency, is generatedfor the
Z--AxisControl circuit by the Marker Generator.
The Z--AxisControl circuit provides the driving signal for the Z--AxisAmplifier
(Diagram8). Included in theseunblanking signals are the sweep unblanking,
readout unblanking, andthe intensified marker. Thiscircuit controls both
display and readoutintensity and adding the intensified marker in FULL
SPAN/DIV.
The output signal fromthe Log Detector (Diagram 2) is buffered and switched in
and out, for time sharingwith the readout signal, prior to driving the Vertical
Output Amplifier. The output amplifier normalizes gain and matches the crt
deflectionplate input impedance.
The ramp signalfrom the Sweep Generator (Diagram 3) is buffered, inverted,
and has its gain setprior to being used to drive the Horizontal Output Amplifier.
Like the VerticalOutput Amplifier, the readout signal is time shared with active
signal. Thehorizontal amplifier has approximatelythree times the gain of the
verticalamplifier to meet the crt gain requirements.
The microprocessor, along with the EP ROM that contains the measurement and
diagnostic routines, is the controlling elementof the 1705A. A Non-Volatile
Random Access Memory (NVRAM) provides a meth od to store the current
front-panelswitch settings, at power down, so that the 1705A can come back up
with the last front-panel setup.
CENTER FREQUENCY readout data is convertedto digital data by an
Analog-to-DigitalC onverterand then input into the microprocessor. Readout
data and the Readout Enable (/RO--EN) are output from the microprocessor. The
readout datais converted back to an analog signal by the Digital-to-Analog
Converter.
All of the switching andexternal control for the 1705A is shown on this
diagram. Control voltages arelimited to 11.8 V or less. All switching is done in
conjunction with the microprocessor(Diagram 5). Indicator lights, that are
controlled by themicroprocessor, are also included on this diagram.
The +18 V supply thatis designed to drive an external Block Down Converter is
also shown on this schematic. The On/Off switch for the +18 V supply is
located on the1705A rear panel.
The Low VoltagePower Supply converts the mains ac line voltage to 4 dc output
voltages (--15 V,+5 V, +15 V,and +40 V). The +40 V is used to power the
VerticalDeflection Amplifier (Diagram 4). The + and --15V supplies are post
Deflection Amplifiers(Diagram 4)Microprocessor(Diagram 5)Front Panel (Diagram 6)Low VoltagePower Supply(Diagram 7)