Instruction Manual, DVM-2000
FEATURES and OPERATION
7
DVM-2000-FTX TRANSMITTER DESCRIPTION
U22 is an AD8052 dual wide band operational amplifier. The first half is the input video
buffer. It interfaces video to the anti-aliasing low pass filter after some processing. Diodes D10 to
D13 suppress transients that could be harmful to the buffer. R60 adjusts the common mode
rejection of the buffer. Capacitors C89 to C92 together with resistors R65 to R68 form a 4 pole
variable cable equalizer that restores the phase and amplitudes of certain video frequency
components. LPF1 is an elliptical 11 pole low pass filter. It attenuates out of band signals that
would produce aliasing products at the output of U23, the video analogue to digital converter.
The second half of U22 is the video post amplifier that overcomes the filter losses and, through
the adjustment of C68 trims the high frequency response of the input video channel. R48 is
adjusted to 1VPP terminated in 75
ω
. Q1 is the video sync separator. Composite sync is present
at Q1’s collector. Q2 is a “box car” pulse former that generates a positive going pulse beginning
with the second edge of horizontal sync and ending approximately with the end of color burst.
The positive pulse is AC coupled to the gate of Q3 a junction NFET that abruptly conducts during
burst time and dumps the average DC value plus any low frequency component into C57. U29, a
very high gain amplifier, applies the DC error in phase opposition to the input buffer thus
canceling its effects and clamping the video’s back porch to the level set by R7. U20 is a high
precision push-pull video driver. The analogue to digital converter wants to see video 4 volts (2
volts on each side) of push pull video centered on its +2V reference, U20 does just that. The
video channel is adjusted using full level modulated stair step as follows. Adjust the video front
panel controls for a well-equalized 1VPP at J9 then while monitoring video at the receiver’s
output adjust R70 for maximum gain and R7 to center the DC bias for not clipping of the chroma
or sync. It is very helpful to monitor U23 pin 14 for no pulses during this adjustment. U23 makes
a 12 bit measurement of the video at every tick of the clock supplied by a monolithic crystal
oscillator Y1. U24 is the high-speed monolithic serializer. It accepts 16 bits plus system clock
and serializes them into a single differential PECL bit stream that drives the laser or LED
module.
The audio section interfaces a well balanced audio signal to the audio analogue to digital
converters. U1, U5 and U9 are dual audio buffers. They convert balanced audio to single ended.
U2, U3, U6, U7, U10 and U11 convert single ended audio to accurately balanced audio ready to
be digitized by U4, U8 and U12 stereo analogue to digital converters. U4 is the master that
supplies drive signals to U8 and U11. LEDs 1, 2 and 3 indicate overflow. U18 strobes the
registers where this overflow signals are being stored. They glow when the input audio goes over
+19bB. All measurements are performed at input level of +18dBm. U15 is a quadruple digital
multiplexer. It inserts external asynchronous data into the digital audio user bit location.
Normally only 2 channels are used for external data while the third one carries video presence
indication from the transmitter to the receiver. Alarms are provided at the transmitter end for
laser failure and for video loss. At the receiver end there are alarms for remote video loss and
loss of optical input. The optical failure alarms on both sides are dry relay contacts grounding the
external connection, the video loss failure are on both sides open collectors sinking 40mA
positive DC to ground. U20 is an automatic reset generator.
