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| 型號(hào)： | LA7577N |
| 元件分類(lèi)： | 接收器 |
| 英文描述： | FM, AUDIO SINGLE CHIP RECEIVER, PDIP24 |
| 封裝： | DIP-24 |
| 文件頁(yè)數(shù)： | 14/16頁(yè) |
| 文件大小： | 354K |
| 代理商： | LA7577N |
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LA7577N
No. 4037—7/16
Buzz Canceler
Phase-locked loop (PLL) detectors feature lower harmonic
distortion in the video stage, higher IF phase differential
suppression and much lower audio buzz than conventional
quasi-synchronous detectors. However, voltage-controlled
oscillators (VCO) in PLL detectors, generally, are highly
susceptible to interference from yback pulses. This inter-
ference can affect the frequency of the VCO, resulting in
added output noise components and audio buzz. This
interference is minimized by VCO supply voltage regula-
tion.
The PLL detector is shown in Figure 1. The automatic
phase control (APC) circuit multiplies the IF signal by the
VCO output signal, which is phase shifted by 90
°, to sup-
press the AM component. The APC output is passed
through a low-pass lter to form the VCO control signal.
This results in a signal with a good carrier-to-noise ratio
(C/N).
A simple PLL detector, however, can cause other audio
problems, because the broadcast signal is transmitted
using vestigial sideband modulation. In this case, the RF
signal is converted to an IF signal by the Nyquist slope of
the SAW lter. Since the sidebands in the vicinity of the
Figure 1. PLL detector
picture carrier are attenuated, the magnitudes of the upper
and lower sideband vectors are different. The result is a
phase distortion component,
θ,in the composite vector as
shown in Figure 2.
Figure 2. Phase noise component
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