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LA7578N Datasheet PDF : 18 Pages
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AFT characteristics
AFT characteristics
Fig. 19
LA7578N
Pins 11 and 12 (AFT coil)
Pins 11 and 12 are the AFT pins.
In the quadrature detection system, a signal that has been
phase-shifted 90 deg is generated by an external AFT tank, and
phase detection is done by the multiplier. The IF signal has the
band characteristics shown in Fig. (A) because of the SAW
filter. Accordingly, even if the band characteristics of the AFT
tank change, the AFT characteristics are extended in the low
band as shown in Fig. C, which makes misoperation more
likely. As a result, band restrictions are placed on the low band
frequencies as shown in Fig. D by inserting C2 in series with
the AFT tank. The ratio between C1 and C2 needed in order to
suppress misoperation should be about 5:1 (C1:C2). In this
case, insert either a resistor or an L in parallel with C2 so that
the DC balance of the AFT circuit is not disrupted.
(1) AFT DEFEAT
To implement AFT defeat, connect these pins to GND
through resistor R1. The resistance used in this case is 20
kΩ or less.
Freq
Fig. 20
Carrier filter
Also serves to protect IC.
Fig. 18
AFT loop time constant
Pin 14 (AFT output)
This is the AFT output pin.
This pin controls AFT of the tuner. The AFT voltage is generated
by an external bleeder resistor. The AFT loop time constant is
determined by inserting a resistor (R3) and a capacitor as shown
in the diagram between this output pin and the AFT input pin of
the tuner. This resistor also serves to protect the IC.
When the system of the selected station is the PLL system or the
voltage synthesizer system, the variation of the no signal voltage
of the AFT output becomes a problem. In this type of case,
connecting a series resistor (R4) to the IC output as shown in Fig.
22 narrows the range over which AFT varies, making it possible
to reduce the variation of the no signal voltage of the AFT output.
No.4037-13/18
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