3DQDVRQLF
18
1.1.7 AFC - Tuning - ATS
IF VCO tuned circuit
The IF VCO tuned circuit is applied to pin 3 and 4 of the TDA8844. The resonance frequency is two
times the IF-frequency suitable for IF standard of 38.9 MHz. The VCO frequency can be adjusted by
I
2
C bus, so there is no need to readjust the external coil.
Catching range
The VCO is automatically tracked to the incoming IF frequency when the PLL is in-lock. Within the PLL
catching range (2MHz) the performance of the demodulator is independent of the incoming IF-
frequency (is one of advantages of a PLL-demodulator). Correct VCO adjustment is only required
when the AFC is used.
SECAM L’
For SECAM L’ a frequency shift is made of typical -5.5MHz by means of I
2
C bus; a capacitor of 10pF
is added to the VCO tuned circuit.
AFC output
The AFC output is available by I
2
C bus.
The analogue AFC characteristic is derived from the PLL loop-filter pin 5. The figure 10 below gives
the relation with the incoming IF frequency. The digital signals AFA anti AFB are derived form the AFC
slope via comparators
The microcontroller uses these digital AFC signals to :
•
drive the main IC for VCO reference frequency adjustment.
•
catch the tuning voltage of each program during channel search.
•
compensate for IF frequency drift daring normal operation .
2.7 V
3.4 V
2 V
in window
outside window
RF too low
outside window
RF too low
above reference
below reference
f
IF
AFB
AFA
Vpin5
Fig. 10 : AFC Slope and I
2
C output bits
