NEC DTE-29U1TH, Service Manual

Page: 35 / 71

-34-
Service Manual CP-785F
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 Auto setup) or using the direct channel entry to enable the Automatic
Frequency Control.
SAW filters
Ref. Standard Features
K3953M B/G - IF filter for video application
- TV IF filter with Nyquist slopes at 38.9 MHz
- Constant group delay
K9650M B/G - IF filter for audio application
- TV IF audio filter with two channels
- Channel ( B/G) with one pass band for sound
carriers 33.40 MHz
The SAW filter ( SF1 ) has a double Nyquist slope at 38.9 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.