TDA9886T データシートの表示（PDF） - NXP Semiconductors.

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TDA9886T

I2C-bus controlled single and multistandard alignment-free IF-PLL demodulators

NXP Semiconductors. 

NXP Semiconductors

TDA9885; TDA9886

I2C-bus controlled multistandard alignment-free IF-PLL demodulators

8.6 AFC and digital acquisition help

Each relaxation oscillator of the VIF-PLL and FM-PLL demodulator has a wide frequency

range. To prevent false locking of the PLLs and with respect to the catching range, the

digital acquisition help provides an individual control, until the frequency of the VCO is

within the preselected standard dependent lock-in window of the PLL.

The in-window and out-window control at the FM PLL is additionally used to mute the

audio stage (if auto mute is selected via the I2C-bus).

The working principle of the digital acquisition help is as follows. The PLL VCO output is

connected to a down counter which has a predefined start value (standard dependent).

The VCO frequency clocks the down counter for a fixed gate time. Thereafter, the down

counter stop value is analyzed. In case the stop value is higher (lower) than the expected

value range, the VCO frequency is lower (higher) than the wanted lock-in window

frequency range. A positive (negative) control current is injected into the PLL loop filter

and consequently the VCO frequency is increased (decreased) and a new counting cycle

starts.

The gate time as well as the control logic of the acquisition help circuit is dependent on the

precision of the reference signal at pin REF. Operation as a crystal oscillator is possible as

well as connecting this input via a serial capacitor to an external reference frequency, e.g.

the tuning system oscillator.

The AFC signal is derived from the corresponding down counter stop value after a

counting cycle. The last four bits are latched and can be read out via the I2C-bus

(see Table 8). Also the digital-to-analog converted value is given as current at pin AFC.

8.7 Video demodulator and amplifier

The video demodulator is realized by a multiplier which is designed for low distortion and

large bandwidth. The VIF signal is multiplied with the ‘in phase’ signal of the VIF-PLL

VCO.

The demodulator output signal is fed into the video preamplifier via a level shift stage with

integrated low-pass filter to achieve carrier harmonics attenuation.

The output signal of the preamplifier is fed to the VIF-AGC detector (see Section 8.3) and

in the sound trap mode also fed internally to the integrated sound carrier trap

(see Section 8.8). The differential trap output signal is converted to a single-ended signal

and amplified by the following post-amplifier. The video output level at pin CVBS is

2 V (p-p).

In the trap bypass mode the output signal of the preamplifier is fed directly through the

post-amplifier to pin CVBS. The output video level is 1.1 V (p-p) for using an external

sound trap with 10 % overall loss.

Noise clipping is provided in both cases.

TDA9885_TDA9886_3

Product data sheet

Rev. 03 — 16 December 2008

© NXP B.V. 2008. All rights reserved.

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