Standard Communications mt930b, Service Manual

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Principles of Operation
MT930B Service Manual Page 5-3
C/N RATIO DETECTOR
Although a video signal and audio subcarrier are included in the demodulated
output, noise in the 10 MHz region is extracted, using BPFs, and then measured,
thus allowing the approximate C/N ratio to be found. Because the extracted
noise differs on the 70 MHz BPFs that have been selected, the noise level is
compensated by selecting the attenuation level, using D301, D302, and D303.
After the noise is amplified by Q602 and Q603, it is converted into a DC voltage
by D651. Finally, the noise is compacted by a compression circuit and sent to the
A/D converter built into QU02. The reason for the compression is that the noise
has a broad dynamic range and is difficult to handle otherwise.
VIDEO DE-EMPHASIS
525-line or 625-line scanning is selected as the de-emphasis network by video
switch QV01. The output of QV01 is split for use as both video and composite
output. Video switch QG01 is used to select de-emphasis on or off for the com-
posite output.
COMPOSITE LEVEL CONTROL
QG02 is a voltage-control amplifier that allows gain to be varied according to the
DC voltage applied to pin 2. Although not normally used, it is possible to reverse
the polarity of the composite output signal by switching SG01.
VIDEO LOW-PASS FILTERS
The unit is equipped with a total of five video LPFs: 4.2 MHz, 5.0 MHz, 5.5 MHz,
6.0 MHz, and one that can be added as an option. These filters are driven by
video amplifier QV02–QV05, and are the video switches used to select one of
the filters.
VIDEO LEVEL CONTROL
QV06 is a video-controlled amplifier (VCA) that allows the gain to be varied ac-
cording to the DC voltage applied to pin 2. Although not normally used, it is pos-
sible to reverse the polarity of the video output signal output by switching SV01.
VIDEO SYNC-SEPARATOR
After the chrominance signal is removed by LPF QM08, it is sent to sync-
separator QM09. The back porch output of QM08 is used as a pulse for clamp-
ing the back porch. The vertical sync output of QM08 is input to QX01 FPGA,
and used for video presence detection.
VIDEO SYNC-TIP CLAMPING
When sync-tip clamping is selected, QM01 and QM02 are turned on during the
sync-tip interval, and sync-tip clamping is performed by quickly charging CV24
and CV25.