Theory of Operation
3-82 2715 Spectrum Analyzer Service Manual
The video monitor mode is generatedby using the vertical sync outputs of the
sync separator and HCLK to controlan auxiliary sweep generator formed by
U560B and its associated components. This ramp generator(U560B) provides
the vertical sweep while the no rmal sweep circuit forms the horizontal sweep .
When the video monitor mode is selected,Q522 is turned off, which lowers the
sweep current, and slows the normal5 s/div sweep to approximately 6.3 s/div.
This has the effect of stretchingout the TV picture over the entire length of the
screen. When a verticalscan is nearly complete, the output of comparator U540A
goes high, and on the next HCLK the signal at pin 14 of U670 goes high. At this
time, Q664 turns on Q663 and the vertical sweepcapacitors C564 and C563 are
discharged. This resetsthe vertical sweep to 0 V, which correspondsto the top of
the screen. The signal VLVLis used to speed up or slow down the vertical sweep
to allow the vertical intervalto be observed in detail and to allow for various
vertical scan rates.When the video monitor is enabled, the Z axis signal is
selected through analog switchesU443A or U443D, depending on the desired
polarity.
The video monitor mode has two input sources. First, in the AM mode, theinput
source is from the video amplifierslocated on the Log board. Second, in the FM
(satellite) mode, the video source is from the FM demodulator (U770 on the
Sweep board). The input of the FM demodulator is from amplifier U760, which
provides approximately 16 dB of gain. The input to theboard (to U760) is
picked off just priorto the 110 MHz bandpass filter, located on the RF Mother
board. The output of the FM demodulator is then fedto operational amplifier
U690, which inverts the signal and adds an offset.Analog switches U260A and
U260C are used to select betweenbroadcast (AM) video and satellite (FM)
video.
The selective line triggeris selected by programming a line number in the
counter made up of U510, U620, and U630. This counter is thendecremented by
HCLK. When the counter is at a minimum count (0),the signal at pin 15 of
U630 goes to a low state. This signal is then delayedfor one HCLK to remove
any glitches that would produce a false trigger.The delayed terminal count
appears at pin 17 (HTRIG) ofU670. This signal is used to trigger the sweep. The
line trigger counter is presetduring field one by the signal at pin 7 of U670. This
preset signal is generatedin U670 by sensing whether timer U225A has timed
out at the beginning of the verticalinterval. The vertical output of U320 always
starts at the beginning of the major serration during the verticalinterval. This
serration starts just priorto an HCLK during one field. During the next field, the
serration starts on an equalizationpulse halfway between HCLK. This difference
is used to determine the odd/evenfield. When the odd/even field flip flop inside
U670 switches to the high state, a positive edgetransition detector generates the
preset enable signal for the line counter.The proper number to preset the counter
with is determined by adding an offsetof 6 for NTSC and 3 for PAL.
The graticule lights arecontrolled by switching Q707 on, which then turns on
Q706.