Digital tuning systems are fast replacing the conventional mechanical systems in AM/FM and television receivers. A simplified block diagram of a typical digitally tuned receiver is shown in the figure.
Notice this receiver could be one for AM, FM, marine radio, or television; it makes no difference. The frequency synthesizer block generates the local oscillator frequency for the receiver, just as a conventional mechanical tuner would. However, the phase-locked-loop (PLL) acts as an integral frequency multiplier of an accurate crystal controlled reference frequency while the mechanical type provides a continuously variable frequency output with no reference. Some method of controlling the value of the multiplier for channel tuning must be provided. The other RF, IF, and audio/video circuitry will be the same as in the mechanical tuning method. [Diagram source: National Semiconductor Notes].
Notice this receiver could be one for AM, FM, marine radio, or television; it makes no difference. The frequency synthesizer block generates the local oscillator frequency for the receiver, just as a conventional mechanical tuner would. However, the phase-locked-loop (PLL) acts as an integral frequency multiplier of an accurate crystal controlled reference frequency while the mechanical type provides a continuously variable frequency output with no reference. Some method of controlling the value of the multiplier for channel tuning must be provided. The other RF, IF, and audio/video circuitry will be the same as in the mechanical tuning method. [Diagram source: National Semiconductor Notes].
0 comments:
Post a Comment