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Theory of Operation
3-82
2715 Spectrum Analyzer Service Manual
The video monitor mode is generated by using the vertical sync outputs of the
sync separator and HCLK to control an auxiliary sweep generator formed by
U560B and its associated components. This ramp generator (U560B) provides
the vertical sweep while the normal 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 normal 5 s/div sweep to approximately 6.3 s/div.
This has the effect of stretching out the TV picture over the entire length of the
screen. When a vertical scan 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 sweep capacitors C 564 and C563 are
discharged. This resets the vertical sweep to 0 V, which corresponds to the top of
the screen. The signal VLVL is used to speed up or slow down the vertical sweep
to allow the vertical interval to 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 switches U443A or U443D, depending on the desired
polarity.
The video monitor mode has two input sources. First, in the AM mode, the input
source is from the video amplifiers located 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 the board (to U760) is
picked off just prior to the 110 MHz bandpass filter, located on the RF Mother
board. The output of the FM demodulator is then fed to operational amplifier
U690, which inverts the signal and adds an offset. Analog switches U260A and
U260C are used to select between broadcast (AM) video and satellite (FM)
video.
The selective line trigger is selected by programming a line number in the
counter made up of U510, U620, and U630. This counter is then decremented 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 delayed for one HCLK to remove
any glitches that would produce a false trigger. The delayed terminal count
appears at pin 17 (HTRIG) of U670. This signal is used to trigger the sweep. The
line trigger counter is preset during field one by the signal at pin 7 of U670. This
preset signal is generated in U670 by sensing whether timer U225A has timed
out at the beginning of the vertical interval. The vertical output of U320 always
starts at the beginning of the major serration during the vertical interval. This
serration starts just prior to an HCLK during one field. During the next field, the
serration starts on an equalization pulse halfway between HCLK. This difference
is used to determine the odd/even field. When the odd/even field flip flop inside
U670 switches to the high state, a positive edge transition detector generates the
preset enable signal for the line counter. The proper number to preset the counter
with is determined by adding an offset of 6 for NTSC and 3 for PAL.
The graticule lights are controlled by switching Q707 on, which then turns on
Q706.