LA7577N

No. 4037—11/16

AFT Tank (Pins 11 and 12)

The automatic frequency tuner (AFT) tank connected to

pins 11 and 12 generates the 90

° phase shift required for

quadrature detection. The band-pass frequency character-

istics of the IF SAW filter and the AFT tank are shown in

Figure 17(A) and 17(B), respectively. The combined

response is shown in Figure 17(C). The resulting extended

low-frequency response, which increases susceptibility to

incorrect operation, can be reduced by connecting capaci-

tor C2 in series with the AFT tank as shown in Figure 18.

The resultant frequency response is shown in Figure

17(D).

Capacitors C1 and C2 should have a ratio of approxi-

mately 5 to 1. An inductor or resistor should also be con-

nected in parallel with C2 to maintain the DC balance of

the AFT tank.

The AFT can be defeated by connecting pin 11 to ground

through resistor R1, which should be 20k

Ω or lower.

Figure 17. AFT tank characteristics

AFT Output (Pin 14)

An external bleeder resistor is required to generate the

AFT voltage. The AFT loop time constant is formed by

external resistor R3 and capacitor C2, as shown in Figure

19. The resistor also provides overvoltage protection.

Fluctuations in the AFT quiescent output voltage, if

present in station selector systems using PLLs or voltage

synthesizers, can be reduced by connecting series resistor

R4 as shown in Figure 20. Note, however, that this also

reduces the AFT range.

Figure 18. AFT tank

Figure 19. AFT loop time constant