FM demodulation PLL, FM demodulation PLL, Frequency synthesis PLL

1
Lecture 9

• FM demodulation (PLL), FM demodulation (PLL),
  Frequency synthesis (PLL)

2
FM demodulation (PLL)

• With a FM input signal the PD output contains a
  component proportional to the information signal,
  which can be selected by filtering. When there is
  a loop bandwidth large enough to pass the
  information signal spectrum, the VCO tracks the
  changing phase of the carrier to maintain phase
  lock.

3

• When a FM signal is present at the PLL input,
  the PD output is proportional to the information
  signal.
• If is present at the PLL input then the PD will
  produce an error signal with a d.c. component
• In the FM signal must be
• If the loop filter passes, i.e.
  , then
• , which is proportional to
  .
• Thus the PD output is proportional to the
  information signal

4
FM demodulation (PLL)

• With a FM input signal the PD output is
  proportional to the integral of the information
  signal.
• With a loop bandwidth large enough to pass the
  information signal spectrum, the VCO tracks the
  changing frequency of the carrier to maintain
  phase lock.
• As the VCO must produce a FM output to match the
  FM input, the VCO command signal must contain a
  good replica of the information signal.

5

• When a FM signal is present at the PLL input,
  the PD output produces a form of the info
  signal.
• If is present at the PLL input then the PD will
  produce an error signal with a d.c. component
• In the FM signal, must be
• If the loop filter passes , i.e.
  , then
• , which is proportional to .

6

• This is a form of the information signal,
  actually the integral of the information signal
  .
• The loop filter can function as a
  differentiator. For a simple low-pass filter

7
Frequency synthesis (PLL)

• A VCO is a natural FM modulator since its
  frequency tracks the amplitude of the command
  signal.
• However, a VCO may drift in frequency making it
  unacceptable as a FM communications carrier.
• A FM carrier can be generated through frequency
  synthesis (transformation of the output of a
  stable oscillator to a signal of desired
  frequency) by applying PLL techniques to phase
  lock the output signal to a stable reference such
  as a crystal-controlled oscillator.

8

• Frequency synthesis is the process of generating
  a signal, which is a multiple of a reference
  frequency.
• A PLL can function as a frequency synthesizer by
  phase locking the VCO output to a stable
  reference.
• With a 1/N frequency divider (delivers an output
  wave whose frequency is a submultiple of the
  input frequency) in the control loop, the VCO
  locks at a frequency N times the stable
  reference

9
FM modulators (PLL)

• A frequency synthesizer can be FM modulated by
  simply making the information signal part of the
  VCO command signal.
• The loop filter blocks the information signal
  spectrum, preventing the modulation at the VCO
  output from affecting the VCO command signal by
  feedback.
• This allows the VCO to produce a stable FM
  carrier by maintaining phase lock with the
  reference oscillator.

10
PLL benefits

• Modulators which use PLL techniques generate
  very precise carriers as they use frequency
  synthesis.
• Demodulators which use PLL techniques can
  slightly extend their improvement threshold and
  are relatively unaffected by amplitude-related
  noise, eliminating the need of a limiter.