Hitachi CL2554AN, Руководство по обслуживанию

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Service manual CL2554AN / CL2854AN
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5-2 IF
The TDA936x has an alignment free IF PLL demodulator. The fully integrated oscillator is
automatically calibrated, using the 12 MHz crystal as a frequency reference. The IF frequency is simply
set in TV-Processor by I2C bus.
The AFC information is available via I2C bus from the TV-Processor status bytes. The controlling
software uses this information for tuner frequency tracking ( automatic following ). The AFC windows
is typically 125Khz wide. The minimum frequency step of the tuner is 62.5 kHz.
This AFC function is disabled when a program is tuned using the direct frequency entry or after fine
tuning adjustment. Therefore it is recommended to tune channel with the TV search function ( manual
or ATSS ) or using the direct channel entry to enable the Automatic Frequency Control.
SAW filters
Ref. Standard Features
K3953M B/G - D/K - I - L/L’ - IF filter for video application
- TV IF filter with Nyquist slopes at 33.9 MHz and
38.9 MHz
- Constant group delay
K9650M B/G - D/K - I - L/L’ - IF filter for audio application
- TV IF audio filter with two channels
- Channel 1 (L’) with one pass band for sound
carrier at 40.40 MHz
- Channel 2 ( L, D/K, I, B/G) with one pass band for
sound carriers between 32.40 MHz and 33.40 MHz
For SECAM L and L’ the TDA936x is switched to positive modulation via I2C bus. SECAM L’ only
occur in VHF band I and have their picture and sound carrier interchanged, compared to SECAM L and
PAL B/G channels. For SECAM L’ the picture carrier is situated at 33.9 MHz and the AM sound
carrier at 40.40 MHz. The IF PLL reference is tuned from 38.9 to 33.9 MHz, this is done via I2C Bus
and the SIF filter is switched from channel 2 to channel 1 ; this is done by pin 4 of TDA 936x. The
tuner AGC time constant is slower than for negative modulation, because the TDA936x reduces its
AGC current. To even slower the AGC time constant an extra series resistor R103 is added. To prevent
IF overload when jumping from a very strong transmitter to a weak transmitter a diode D101 has been
added
The SAW filter ( SF1 ) has a double Nyquist slope at 38.9 MHz and 33.9 MHz needed for this
multistandard application. The disadvantage of this choice is that a 5.5 MHz trap filter ( Z501 ) is
needed to suppress the residual sound carrier in the video for B/G signals.