Explain application of PLL as frequency synthesizer.

Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 5 Marks

Year: Dec 2015

  1. In most countries, the frequency spectrum of 87.5 MHz to 108.0 MHz is used and each radio station is required to fit into one of 101 channels, numbered from 200 (87.9 MHz) to 300 (107.9MHz) in increments of 200 kHz.
  2. For successful signal reception, the synthesized frequency needs to be accurate to within 10 parts per million (ppm), which is to say that the accuracy of a crystal oscillator is required.
  3. Considering that the crystal itself is one of the bulkiest pans that is used in modern radio electronics, having 101 crystals in parallel would be a very impractical radio indeed. Instead, a single crystal reference is used to provide a stable input reference to PLL that includes a "programmable divide' in its feedback path.
  4. Operation of the frequency synthesizer is based on a single 10 MHz crystal reference whose frequency is divided by 50, hence the 200 kHz reference is perceived by the PD.
  5. A tunable VCO is designed to generate a wave in the 87.9 MHz to 107.9 MHz frequency range as its output, which is then taken back through the feedback path into the programmable divider whose division ratio is N = 439.5 to 539.5.
  6. Consequently, any of the 101 discrete frequencies can be divided down to a 200 kHz wave, which is then compared with the 200 kHz reference wave.
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