Theory of Operation
1705A Spectrum Monitor 4-9
conditions the output goes toward the positive supply. If the input is between +
and --0.7V,neither diode conducts and the amplifier has no input and, conse-
quently,an output of 0 V. When the output of U14D is near 0 V, a pulse that
produces thecrt bright zone is generated by the Z--AxisControl circuit.
The Z--AxisControl circuit is comprised of a transistor array,U13, and discrete
transistors Q4 and Q5. The output is the common collector line of the three
differentialamplifiers (Q5, and pins 5 and 8 of U13). The output line drives the
summing junction of the Z--Axis Amplifier on Diagram 8. The front-panel
INTENSITY control drives the currentsource (pins 12, 13, and 14 of U13). All
of the currentfrom this transistor can be directed to the Z--Axis Amplifier,
through the differentialamplifiers or shunted away if Q6 is turned on. Q6 is on
during sweep retrace, to blank the crt, and during crt readout time, when
/RO--ENis active (low). With Q6 off, the current from the source is split
between R54 and R79. The current throughR 54 is thecollector current on pin 5.
The currentthrough R79 will appear as collector current on pin 8 if its base
(pin 9) is low,which occurs at marker time. The result is that when the
instrument is in FULL SPAN, the traceis brightened for the marker. In any
other SPAN/DIVsetting, the trace is of uniform brightness when the output of
U14D is near 0 V,a pulse that produces the crt bright zone is generated by the
Z--AxisControl circuit.
The readoutintensity is controlled separately by Q4 and Q5. If the RO--BLANK
control line is active(low), Q4 is turned off and Q5 is turned on. When Q5 is
on, the currentthrough R89 and R91 is sent to the Z--Axis Amplifier. When the
control line goeshigh, Q4 turns on and the emittercurrent from the readout
intensity control is directedinto the +5 volt supply instead of into the Z--Axis
Amplifier.
Deflection AmplifiersDiagram 4Circuitry on this schematicnormalizes gains, and drives the crt deflection plates.
The verticalsignal from the Log Detector is buffered by U27. U27 drives the
VerticalOutput Amplifier, whose input can be filteredto reduce the effects of
noise. The Video Filter is a 15 kHz, 3-pole, low-pass filter whose output is
switched in by U30A. The enable, for U30A, is the /VFILTERsignal from the
microprocessor (Diagram 5).
U27 is a switchablegain amplifier. When high gain (2 dB/Div) is selected with
the front-panelpush button, the signal at pin 12 of the microprocessor(U2) goes
high, and the switch in U21C grounds R135 through pins 3 and 4. This
increasesthe gain of U27 by approximately a factor of five. U21A also
switches, connectingpins 13 and 14, putting a portion of the Vertical Position