Innovator CU10BT-CU5000BT Analog Transmitter
Board Descriptions
Instruction Manual, Rev. 0
43
Circuit Descriptions of Boards in the CU10, CU60, CU100 & CU200 Systems
(A1) Analog Modulator Board, System M/N (1312264)
The analog modulator takes a 1V peak to peak video signal at J32 and applies it to a
differential amplifier intended to remove any hum caused by ground loops. It is then
applied to a group delay equalizer circuit that applies the appropriate precorrection for
the receiver equalization required by the video standard used. The video is also applied
to a sync tip clamp, which stabilizes the peak sync voltage to a constant level to track
out changes in DC level with picture content. The output of the clamp is sent to a Sync
Clip circuit to prevent sudden negative going video transients from overdriving the
transmitter. There is also a white clipper, which prevents high amplitude video signals
from reducing the video modulation past its allowed limit, which can cause buzz in the
audio of intercarrier receivers. The video signal is connected to pin 5, the I input of the
Z1 mixer.
The IF oscillator that generates the visual IF is a VCO U12 operating at 45.75 MHz and is
phase locked to an external 10 MHz signal. This visual IF connects to pin 1 the L input
of the Z1 mixer. The modulated Visual IF output of the Z1 mixer connects to a Saw
filter, FL1 and FL2 if present. The Aural IF oscillator U3 is a VCO operating below the
visual carrier at 41.25 MHz. If either of these oscillators unlock a signal is sent to the
control system to mute the RF output.
The audio circuitry can be configured to use either a 600 Ohm balanced input, or a 75
ohm balanced input using jumper W1 on J1, jumper W5 on J6 and jumper W6 on J8.
This input can also be set to have a flat frequency response using jumper W2 on J2. or
have either 50 or 75 µsec pre-emphasis applied using jumper W11 on J37. The audio
FM modulates the Aural Carrier. The loop bandwidth of the Aural PLL is set up to be
much lower than the lowest audio frequency so that it does not attempt to track out the
modulation.
The modulated Aural IF signal is combined with the modulated Visual IF signal at the
combining point of L12, L13 and R96, after the modulated visual IF has been filtered by
a SAW filter, FL1 and FL2 if present, which converts the double sideband AM modulated
signal to a vestigial sideband AM modulated signal. The aural level is adjusted, using
R103, to give a 10 dB Visual/Aural Ratio and there is frequency response correction
applied using R99, R100, C85 and C86, to fix any slope caused by the SAW filter. The
combined IF connects out of the analog modulator board at J15 that is cabled to the IF
precorrector board for processing.
