Axcera CHV400BTD, Техническое руководство

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Innovator CHV400BTD ATSC Transmitter Board Descriptions
Technical Manual, Rev. 0 53
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.
The Port 1 output of Z3 connects from J11 through the W6 jumper to J12. The IF is
connected to T5, the 1:4 impedance transformer input to the Quadrature circuit.
External Quadrature Corrector stages may be connected between jacks J11 and J12.
The pre-distorted IF signal, in the Quadrature Phase path, connects to the op amp U11
whose output gain is set by R102, which provides a means of balancing the level of the
Quad Phase pre-distorted IF signal that connects to Port 2 on the combiner Z4.
The Quadrature and In Phase pre-distorted IF signals are combined by Z4, amplified by
U10 and connected through C57 to the S Port of the splitter Z2. Z2 provides two outputs
of the combined Quadrature and In Phase pre-distorted IF signals.
Frequency Response Corrector Circuit
The output of Z2 at Port 2 connects to the first corrector stage of the three-stage
frequency-response corrector circuit. The three stages are adjusted as needed to attain
the best response across the bandwidth. The frequency-response corrector circuit
operates as follows. Variable resistors R24, R25 and R26 are used to adjust the depth
and gain of the notches and variable caps C14, C15 and C16 are used to adjust the
frequency position of the notches. These are adjusted as needed to compensate for
frequency response problems. The jumpers W1 on J4, W2 on J5 and W3 on J6 are
moveable to set the frequency response of the circuits for 44 MHz, which is between
pins 2 & 3 or between 1 & 2 for 36 MHz.
The Non-Linear and Frequency Response pre-corrected IF is connected to the op-amp
U2. After amplification, the IF is split with one path connected through a divider
network to J1 the IF output jack on the board, -12 dBm. The other path is fed through a
divider network to J3 the IF Sample Jack, –18dBm.
ALC Circuit
The other non-linear pre-corrector output of Z2 at Port 1 connects to the input of the ALC
circuit. The IF signal is applied to the transformer T1, which doubles the voltage swing by
means of a 1:4 impedance transformation. It is connected to the ALC detector circuit,
consisting of C11, CR4 and R21. The detected ALC level output is amplified by U3A and
wired to U3B, pin 6, where it is summed with the power control setting of R40 the ALC
Adjust pot.