MultiDyne DVM-2200 - Manual (Page 8)

Instruction Manual, DVM-2200 FEATURES and OPERATION 8
*** Confidential *** Multidyne Video & Fiber Optic Systems
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.
DVM-2000-FRX RECEIVER SECTION
The data modulated light beam is transformed into a balanced PECL bit stream and
presented to U15, a monolithic high-speed deserializer. The internal state machine of U1 driven
asynchronously by Y1 a monolithic crystal oscillator, recombines the data into a 16 bit parallel bit
stream. Internal phase lock loops regenerate the sending end sampling clock with an edge
stability of a few pico-seconds. The first 12 bits of the reconstituted data are presented to U16,
the video analogue to digital converter that reconstitutes the original video signal. R16, a front
panel control varies the converter’s DC reference thus adjusting the output gain. LPF1 is the
reconstitution low pass filter that removes the clock components from the video. U17 provides
gain and flatness adjustments while interfacing the video to the outside world. The audio portion
transforms the three audio bit stream again into audio. The associated operational amplifiers are
2 pole low pass filters and output drivers. U1 edge detects the audio channel pulse and triggers
monostable multivibrator U3 that produces a strobe pulse occurring at the center of the digital
audio’s user bit location. The positive edge of U3’s output strobes that location recovering
external data and remote video presence indication. The whole audio system calibration relies
just on 6, one per channel, output gains.