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LA7577N

Super-split PLL-IIVIF and SIF IF Signal Processor for TV/VTRs

SANYO -> Panasonic 

LA7577N

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 flyback 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 filter to form the VCO control signal.

This results in a signal with a good carrier-to-noise ratio

(C/N).

Figure 1. PLL detector

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 filter. Since the sidebands in the vicinity of the

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

No. 4037—7/16

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