Philips DVDR985 Technical Training Manual page 89

Tuner 1705
The Tuner is capable of receiving 125 channels,
and is cable ready. Refer to Figure 45. The RF
connections on the back of the unit provide an
RF loop through. There is no RF Modulator, as
seen in VCRs. The Tuner/Demodulator receives
two supply voltages, 33Vdc and SW5Vdc. The
channel selection information is communicated
via the I 2 C lines.
The IF signal, from the Tuner, is processed by
the demodulator, IC7703. This unit is unique in
that it has two SAW Filters. 1701 is used for the
Video IF, and 1702 is used for the Sound IF. The
AFC coil 5703 is adjusted so that when a fre-
quency of 45.75 MHz is supplied to the IF output
of the Tuner, the AFC voltage on pin 17 of 7703
is 2.5V. The AGC is set using 3707 so that, with
a sufficiently large antenna input signal (74 dBV),
the voltage at the IF output of the Tuner, 1705
Pin 11 is 500 mVp-p. This adjustment must be
performed with the audio carrier switched off.
The demodulated Video signal appears on Pin
16 of 7703. The Demodulator AGC voltage at
Pin 4 is used to determine the antenna signal
strength. The FM-PLL demodulator function of
7703 is not used and is deactivated by 3726.
SIF1 is generated for demodulation in the Sound
processor, 7600.
The final stages in the demodulation process fil-
ter and amplify the Video. The signal is buffered
by 7705, AGC_MUTE. In the opposite direction,
this line may be used to mute the demodulator to
avoid crosstalk when the Tuner signal is not
needed. In this case, a High is sent via
AGC_MUTE. The Video trap 1703 reduces adja-
cent channel video and any sound carrier left in
the Video. The demodulated Video signal VFV is
available after the buffer and limiter stage. The
Limiter,7706, filters noise peaks.
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