7
SECTION IV
SECTION IV
SECTION IV
SECTION IV
ARTS
ARTS
ARTS
ARTS----201B
201B
201B
201B
Circuit description
Circuit description
Circuit description
Circuit description
Video Path
Video Path
Video Path
Video Path:
Only one signal path, A, will be described since both A and B paths are the
same. The signal comes from the input BNC on the motherboard and onto the
main board where it is terminated in 75 ohms. The video is then fed into the
differential amplifier U16. This amplifier is a true differential amp, and will reject
common mode signals such as AC line hum due to ground loops or AC cabling
running in parallel with the video cable. The input stage’s gain is set with a
variable pot R71 that can be adjusted to compensate for any gain inaccuracies in
the remainder of the path to the output. The next stage has an adjustable
equalization circuit to compensate for cable runs of up to 100 meters of Belden
8281 cable, or equivalent. This essentially operates as a unity gain stage when
the R6 pot is adjusted to the end closest to the amplifier’s output. As the pot is
adjusted to the opposite end, the equalization network attached to the wiper
connection of R6 begins to have more effect. The equalization network, by rolling
off the higher frequencies, causes the gain of the amplifier to rise at higher
frequencies. Resistor R11 sets the DC gain and low frequency gain of the
amplifier, which compensates for the cable loss at DC from the cable resistance.
There are three different destinations for the video signal after the gain stage.
One is to the sync separator U19, which is used to determine video presence. A
second destination is to the monitor amplifier U18, which has a gain of two to
drive a source terminated output. U18’s input is also shared by the output
amplifier/mux, U21, which does the actual selection of either the A or B input. A
resistor-capacitor divider network is shared by both the U18 and U21 amplifiers,
which is used to adjust the overall frequency response of both the monitor and
main output of the RTU. The output amplifier drives two video output BNC
connectors through the 75 ohm source resistors R23 and R24.
A dual form C relay has been inserted into the video path. One section, K4B, is
located at the A Video Input and selects whether the input signal passes to U16,
or whether it bypasses the entire 2x1 switching system. The other section, K4C,
is located after the 2x1 switch, U21. During normal operation the relay coil is
energized and K4B passes the input signal through the input amplifier U16,
equalizer U17 and 2x1 switch U21 to K4C to the output BNC connectors. Should
there be a power failure within the unit, the relay drops to the relaxed mode, and
the video input signal is routed directly to PGM output #1. When power is
restored, the relay coil is re-energized and the signal is again routed to the 2x1
switch output via U16, U17 and U21. The relay coil is powered from the -15 volt
regulator U37.
