Circuit Description; Dm-152 Board - Sony HXCU-TX70 Service Manual

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Circuit Description

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DM-152 Board

The DM-152 board has two blocks: one is for digital triax transmission and the other is for video signal processing.
Digital Triax Transmission Circuit
The transmission circuit consists of an FM signal transmitter/receiver for the tone/sync signals, and an OFDM signal
transmitter/receiver for the video/audio/trunk signals and commands.
This circuit has a function to monitor the receiver operation status for the OFDM signal transmitter/receiver control and
a function to detect the length of the connected triax cable.
• In the FM receiver, the FM signal (carrier frequency 1.4 MHz) is extracted from the RF signal sent from the camera
by the MPX filter and BPF, and is demodulated by IC1516. The demodulated signal is separated into tone signal
(16.38 kHz) by the HPF (14 kHz). The tone signal is used to control the main power supply (180 VDC) to the
camera.
In the FM transmitter, the sync signal is FM-modulated with a carrier frequency of 1.0 MHz and is then sent to the
CCU while the main power is turned on.
• The OFDM signal receiver circuit consists of two blocks: one is for the F1/F2/F3 low bands and the other is for
F4/F5/F6 mid bands to support six waves F1 to F6 for transmission of video/Skin gate/MIC/INCOM/command
signals.
The low-band receiver circuit removes low-frequency noise from F1 to F3 signals that are output from the LPF
side of the MPX filter by the HPF (0.9 MHz), and amplifies the signals by the low-noise amplifier, and selects them
for each band by the BPF for individual processing.
The levels of the selected F2 and F3 signals are regulated by the step gain amplifier (that automatically switches
amplifier gain to four steps according to input signal level) and the AGC amplifier. Then the signals are converted
to signals with a center frequency of 36 MHz by the mixer circuit.
Since the AGC amplifier does not function in the F1 band, the F1 signal processing circuit configuration is different
from that for F2/F3 signals. In the F1 signal processing circuit, the F1 signal is input to the step gain amplifier
(IC3402) and then to the mixer circuit. The F1 signal is converted to a signal with a center frequency of 36 MHz,
and the signal level is regulated by the AGC amplifier.
The converted OFDM signals F1, F2, and F3 (36 MHz center frequency) are converted to TS data by the demodulator
ICs (IC3404, IC3504, and IC3604), and the serial TS data is output to the video signal processing circuit.
The mid-band receiver circuit amplifies the F4 to F6 signals that are output from the BPF side of the MPX filter
by the low-noise amplifier, and selects them for each band by the BPF for individual processing. Since attenuation
in the cable transmission becomes large in the mid signal band, this circuit has a function to switch the low-noise
amplifier gain to two steps according to the transmission distance.
The selected F4 to F6 signals are processed in the same circuit configuration as for the F2 and F3 signals.
• The OFDM signal transmitter circuit consists of two blocks: one is for F7 + F9 bands and the other is for F8 band
to support three waves F7 to F9 for transmission of return video/prompter video/INCOM/PGM/command signals.
The video signal processing circuit generates OFDM signals with D/A conversion. However, because the clock
frequency is 183 MHz, this circuit generates OFDM signals with low frequencies (26 + 46 MHz and 36 MHz) but
not with direct (F7 + F9) and F8 frequencies, and then converts them to signals with (F7 + F9) and F8 frequencies
by the mixer circuit. Then the converted OFDM signals are mixed by the respective output drivers and are output
to the camera. Since attenuation in the cable transmission becomes large in the F7 to F9 signal bands, this circuit
has a function to switch the output signal level to four levels by the gain control amplifier and ATT according to
the transmission distance.
This digital triax transmission circuit is provided with a transmission cable length detection function for transmission
distance-based control, a mixer circuit, and a 4MHz clock generator for the OFDM signal demodulation IC.
HXCU-TX70
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