User's Manual

Innovator CHV400BTD ATSC Transmitter Board Descriptions
Technical Manual, Rev. 0 57
gain pot R62. When the switch is in either ALC or manual, the voltage in the unused
circuit is preset low by the circuitry connected to pins 4-6 on SW1. This allows the RF
power to ramp up slowly to full power when the switch changes positions. CR8, C33 and
associated components control the ramp up speed of the manual gain circuit. CR9, C42
and their associated circuits do the same thing for the ALC circuit. The practical effect of
this is to preset the RF drive power to near zero output power when enabling and
disabling the ALC, followed by a slow controlled ramp up of power.
The ALC circuit normally attempts to hold the tray output power constant, but there are
four faults that can override this. These faults are Input Fault, VSWR Cutback Fault,
VSWR Shutdown Fault and Overdrive Fault.
The Input Fault is generated by comparator U7C and presets the PIN attenuator and ALC
circuit to maximum attenuation whenever the input signal drops below about -7 dBm.
Test point TP2 allows the user to measure the detected input voltage.
The VSWR cutback circuit is set so that the ALC circuit will start reducing RF drive once
the Reflected power reaches a level of about 6% and will keep reducing the drive to
maintain that level. The cutback is generated by U8A, U8B and their associated
components that diode-or the metering voltages. The forward power is scaled to
2V = 100 % and the reflected power is scaled to 2V = 25%. The Reflected metering
voltage is doubled again by U8B so that when the voltage of U8B exceeds the voltage at
the output of U8A, the reflected power takes over the ALC circuit. Once the U8B voltage
drops below the forward power at U8A, the forward power takes over again.
The VSWR shutdown circuit will shut the tray down if the Reflected power increases to
15% or higher, which can happen if the tray sees reflected power when the ALC is in
manual.
The Overdrive protection looks at a sample of the RF signal that is applied to J1 of the
board. The peak level of this signal is detected and can be measured on TP1. This
voltage is applied to a comparator with the threshold set by R38. If this threshold is
exceeded, the ALC circuit mutes then ramps up to try again. This circuit also works in
manual gain as well.
(A6) Amplifier Assembly (1313959) – Used in the CHV20B Tray
The (A6) Amplifier Assembly (1313959) is made up of (A6-A1) the VHF HB Pre-Driver
Assembly (1313899) and (A6-A2) the 100 Watt Amplifier Pallet, Italmec (1313484).
The ALC Board (1308570) is also part of this assembly. The assembly has
approximately 36 dB of gain.
(A6-A1) VHF HB Pre-Driver Assembly (1313899)
The VHF HB Pre-Driver Assembly (1313899) consists of a driver stage and a parallel
connected final amplifier stage, that have a total gain of approximately 23 dB.
The input RF at J1 connects through a matching network consisting of R11-R13 to a
splitter IC Z1. The split outputs connect to parallel-connected push-pull 1 Watt high
linearity amplifier ICs (U1& U4) operating in class AB each with approximately 17 dB of
gain. The board uses a power supply voltage of +42VDC that connects to J6. The
+42VDC is filtered on the board and connected to the step down transformer T1 which
produces a +12VDC output that is used by the two amplifier ICs (U1 & U4). The two
amplified outputs are connected to a combiner IC Z2. The combined output connects