| 摘要 |
<p>A tuner demodulator alignment system adjusts the centre-frequency of the PLL-demodulator VCO to achieve alignment with the centre-frequency of the IF entering the demodulator by: injecting into the PLL stage a signal having a frequency equal to the nominal centre-frequency of the IF; electronically adjusting, e.g. by means of a digital-to-analogue converter (DAC) controlled from a tuner micro-controller, the VCO frequency until this frequency is aligned with the injection frequency, then removing the injection signal and reverting to normal tuner operation. Alignment with the injection frequency may be signalled by a change in logical state of an AFC flag signal provided by the PLL stage and employed during normal use of the tuner. This may be repeated on an ad-hoc basis, e.g. whenever channels are changed by the user, or more periodically as controlled by the tuner software, or may be performed on a one-off basis only on the production bench.</p> |
