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Theory of Operation
2715 Spectrum Analyzer Service Manual
3-81
When this voltage reaches approximately 1 V, an RS flip flop inside U460 resets and
the holdoff cycle is finished. The sweep gate logic circuit is then enabled to accept
another trigger and, if in the free run mode, another sweep is then started.
Transistor Q574 is used to abort the sweep and initiate another holdoff.
Transistor Q255 is used to choose a long or short holdoff time. A logic 1 at
pin 13 of U312 turns on the transistor, diverting the available current to charge
C252. This has the effect of making the holdoff capacitor look larger.
Multiplexer U170 is used to determine the timing current to be used while U160
chooses the timing capacitor.
The trigger signal is selected by multiplexer U452. The output on pin 3 (TRIG)
goes to the input of comparator U470A. This is wired as a Schmitt trigger with
the threshold set by the voltage appearing at pin 4 of U470A. This threshold
voltage is always positive regardless of the value of TLVL. The circuit composed
of CR373 and U374B provides an absolute value generator. When the value of
TLVL is negative, operational amplifier U374B is in equilibrium and the output
is positive. When the value of TLVL is positive, the output of U374B goes as far
negative as possible, effectively turning off CR373 (the operational amplifier is
open loop). With CR373 off, the value of TLVL appears at pin 4 of U470A.
When U374B is open loop, Q351 is turned on and pin 3 of U460 goes to a
logic 0. This is the slope input that is EXCLUSIVE ORed with the trigger input
at pin 2. Thus, the opposite slope is selected.
When TLVL is not being used to set the trigger level of the sweep, it can be used
for the manual scan as these two functions are entirely mutually exclusive.
During a manual scan, pin 7 of sweep logic IC U460 is set low. This sets the
sweep gate flip flop in U460. Also, multiplexer U160 is set to select R162, the
100 kΩ resistor. Multiplexer U170 is set to select current input from R175 that
ultimately originates at TLVL. Miller integrator U270 is then used as an
amplifier and the output voltage is then proportional to the value of TLVL.
The source for the continuous TV line trigger and TV field trigger is from the
sync separator, U320. This chip provides a vertical interval and composite sync.
The composite sync contains the elements of the equalization pulses that occur
during the vertical interval. This composite sync is fed to the two monostable
multivibrators that are used to strip off the double pulses that occur during the
vertical interval. One of the timers, U225A, is set for a pulse width of 3/4 of a
horizontal scan. This output is used to inhibit another trigger from occurring on
either monostable multivibrator until it has timed out. The other timer, U225B,
provides an approximately 10 s pulse that is used as a general horizontal
blanking pulse (HCLK).
Composite video source for the sync separator is from the output of operational
amplifiers U244 ( for positive sync) or U343 (for negative sync). U244 also
forms the source of the internal trigger.