User's Manual

Innovator CHV400BTD ATSC Transmitter Board Descriptions
Technical Manual, Rev. 0 52
Linear adjustments. The board has the circuitry for ALC Fault, Input Fault and
Modulation Fault monitoring and indications.
The input IF signal at J2, typically 0 dBm peak power centered at 36 or 44 MHz, is fed to a
splitter circuit Z1 which produces two equal outputs, one at Port 1 and the other at Port 2.
The output at Port 1 connects to the input and modulation fault circuitry. The output at
Port 2 connects to the pin-diode attenuator circuit.
Pin-Diode Attenuator Circuit
The output of Z1 at Port 2 connects to a pin-diode attenuator circuit that consists of CR1,
CR2 & CR3. Each of the pin diodes contains a wide intrinsic region; this makes the diodes
function as voltage-variable resistors at this intermediate frequency. The value of the
resistance is controlled by the DC bias supplied to the diode. The pin diodes are
configured in a pi-type attenuator configuration where CR1 is the first shunt element, CR3
is the series element, and CR2 is the second shunt element. The control voltage, which
can be measured at TP2, originates either from the ALC circuit when the switch S1 is in
the ALC Auto position, between pins 2 and 3, or from pot R37, MAN GAIN, when S1 is in
the Manual Gain position, between pins 1 and 2. In the pin diode attenuator circuit,
changing the amount of current through the diodes by forward biasing them changes the
IF output level of the board. By controlling the value of the voltage applied to the pin
diodes, the IF signal level is maintained at the set level.
When the IF signal passes out of the pin-diode attenuator through C7, it is applied to the
modular amplifier U1. This device contains the biasing and impedance-matching circuits
that makes it operate as a wide-band IF amplifier. The output of U1 connects through C8,
NON-LIN IN, to the Summing Port input of the splitter Z3. The splitter provides the
outputs to the Non-Linear Pre-Corrector stages. The output at Port 1 connects to the
Quadrature Pre-Corrector and the output at Port 2 connects to the In Phase Pre-
Correctors.
In Phase and Quadrature Corrector Circuits
Two of the Pre-Corrector stages are in the In Phase Amplitude pre-correction path and one
stage is in the Quadrature Phase pre-correction path. Each stage has a variable threshold
control adjustment, R67 and R69, in the In Phase path, and R89 in the Quadrature path,
which determine the point that the gain is changed in each of the stages.
The output of Z3 at Port 2 connects to J10, which is jumpered through W5 to J9. External
In-Phase Corrector circuits may be connected between these jacks. The signal from J9
connects to the first corrector stage on the board. The first corrector stage in the In
Phase path operates as follows. The In Phase IF signal is applied to the transformer T3,
which doubles the voltage swing by means of a 1:4 impedance transformation. Resistors
R75 and R78 form an L-pad that lowers the level of the signal. The input signal level,
when it reaches a set level, causes the diodes CR9 and CR11 to turn on, generating
current flow that puts them in parallel with the L-pad. When the diodes are put in parallel
with the resistors, the attenuation through the L-pad is lowered, causing stretch of the
signal.
The signal is next applied to amplifier U8 to compensate for the loss through the L-pad.
The breakpoint, or cut-in point, for the first corrector is set by controlling where CR9 and
CR11 turn on. This is accomplished by adjusting the threshold cut-in resistor R67. R67
forms a voltage-divider network from +6.8 VDC to ground. The voltage at the wiper arm