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| 型號(hào)： | LA7577N |
| 元件分類： | 接收器 |
| 英文描述： | FM, AUDIO SINGLE CHIP RECEIVER, PDIP24 |
| 封裝： | DIP-24 |
| 文件頁(yè)數(shù)： | 3/16頁(yè) |
| 文件大小： | 354K |
| 代理商： | LA7577N |
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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 lter 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
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