User's Manual

Innovator CU5-1800BTD/BRD ATSC Transmitter/ Board Descriptions
Regenerative Translator
Instruction Manual, Rev. 0 45
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
of R67 is buffered by the unity-gain amplifier U5A. This reference voltage is then applied
to R68, R71, and C33 through L11 to the CR9 diode. C33 keeps the reference from
sagging during the vertical interval. The .9 VDC reference voltage is applied to the unity-
gain amplifier U5B. The reference voltage is then connected to diode CR11 through choke
L12. The two chokes L11 and L12 form a high impedance for IF that serves to isolate the
op-amp ICs from the IF.
After the signal is amplified by U8, it is applied to the second corrector stage in the In
Phase path through T4. The second In Phase Stage and the stage in the Quadrature path
operate in the same fashion as the first. All three corrector stages are independent and
do not interact with each other. The In Phase Correctors can be disabled by moving the
jumper W4 on J8 to the Disable position, between pins 1 & 2. This moves all of the
breakpoints past the signal peaks so that they will have no affect. The pre-distorted IF
signal, in the In Phase path, connects to the op amp U9 whose output level is controlled
by R88, the in phase amplifier adjustment. The pre-distorted In Phase IF signal
connects to Port 1 on the combiner Z4.