Detector Circuits

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Detector Circuits

• Lecture 5
• Dr. Yossef Ben-Ezra

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Envelope detector

• An ideal envelope detector is a circuit that
  produces to the real envelope R(t) of its
  bandpass input which may be represented by
  where
• The output of the ideal envelope detector is
  
• where K is the proportionality constant

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A diode envelope detector
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Operation principles

• The current pulses charge the capacitor to
  produce the output voltage waveform.
• The RC time constant is chosen so that the output
  signal will follow the real envelope R(t) of the
  input signal.
• The cutoff frequency of the low pass filter needs
  to be much smaller than the carrier frequency fc
  and much larger than the bandwidth of the
  (detected) modulation waveform B. That, is

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Typical uses

• Envelope detector is typically used to detect the
  modulation AM signals
• In case vin(t) has the complex envelope
• If , then

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Typical values

• For the case of audio (not video) modulation
  typical values for the components of the envelope
  detector are
• This combination of values provides a LPF
  cutoff frequency (3dB down) of

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Product detector

• Product detector is the mixer circuit which
  down-converts the input (bandpass signal plus
  noise) to baseband.

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Operation principle

• The output of the multiplexer is
• The LPF passes only the down-conversion term, so
  that the output is

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Product detector is sensitive to AM and PM

• If the input contains no angle modulation, so
  that and the reference
  phase is set to zero (i.e. )
  , then
• If an angle modulated signal
• Is present at the input and the
  product detector output is
• In this case, the product detector acts like a
  phase detector with a sinusoidal characteristic

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Frequency modulation detector

• An ideal frequency modulation detector is a
  device that produces an output that is
  proportional to the instantaneous frequency of
  the input.
• That as, if the bandpass input is represented,
  the output of the ideal FM
  detector is
• Usually, the FM detector is balanced. This means
  the dc voltage related to the first term does not
  appear on the output. In this case output is

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FM detectors

• There are many ways to build FM detectors, but
  almost all of them are based on one of three
  principles
• FM to AM conversion
• Phase-shift or quadrature detection
• Zero-crossing detection

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Slope detector

• A slope detector is one example of the FM-AM
  conversion principles.

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Slope detection using a single-tuned circuit for
a frequency to amplitude conversion