Chelmsford Amateur Radio Society Intermediate Course 4 Receivers

1
Chelmsford Amateur Radio Society Intermediate
Course (4) Receivers
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Receivers

• Summary
• Block diagrams of receivers
• The Superhet receiver
• Detection of AM, FM, and SSB
• How a Diode AM Detector works
• Use of BFO and CIO
• AGC

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Crystal Receiver

• Block diagram
• Single tuned circuit poor selectivity
• No gain poor sensitivity
• Requires large aerial and earth only receives
  strong signals

4
Crystal Receiver Circuit

• L1, C1 - Tuned circuit selects signal
• D1 Detector diode demodulates
• C3, R1 Low-pass filter for audio

5
TRF Receiver

• Block diagram
• RF amplifier gain increases sensitivity
• One or more tuned circuits
• All the gain is at one frequency feedback is a
  problem
• AF amplifier provides more power for loudspeakers

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Superhet Receiver

• Block diagram
• Mixer changes variable RF frequency to fixed IF
  frequency
• IF amplifier provides selectivity with several
  tuned circuits
• Fixed IF can use non-tunable crystal or ceramic
  filters

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Mixer as a Converter

• Mixer may be used a frequency converter
• Changes the selected RF frequency to the IF
  frequency using a tunable LO signal.
• Mixers have spurious responses image frequency,
  half the RF
• LO can be above or below the RF
• IF can be above or below the RF

Image frequency is 123.6MHz-21.4MHz102.2MHz
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AM Envelope Detection

• Demodulation of a modulated audio signal
• Detector output follows envelope of RF
• Otherwise known as an envelope detector

Envelope
Modulated RF
Demodulated Audio
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Diode Detector Circuit

• Diode D1 rectifies AC into DC
• C1/R1 is a lowpass filter - filters out the RF
• D1 conducts on positive half-cycle only

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Diode Detector Operation

• Volts and Current in the diode detector (SPICE
  Simulation)

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CW Demodulation - BFO

• BFO Beat Frequency Oscillator
• Slightly offset BFO added to carrier to generate
  the beat note

the origins of the BFO !
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SSB Demodulation

• SSB filter selects only the wanted sideband
• Product detector mixes to baseband
• CIO is at the frequency where carrier would have
  been
• Product detector is a balanced mixer

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USB Demodulation

• SSB demodulation is essentially mixing to baseband

• Unwanted sideband may contain noise and other
  signals
• Mixing with the carrier frequency is product
  detection

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LSB Demodulation

• SSB demodulation is essentially mixing to baseband

• Typically, IF filter is not moved the local
  oscillators are offset.
• When mixed down, the LSB spectrum becomes
  inverted.

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Demodulation Summary

• CIO Carrier Insertion Oscillator
• A fixed local oscillator used to demodulate SSB
• It reinserts the carrier that was removed in the
  transmitter
• Wanted sideband is directly translated directly
  to Audio
• BFO Beat Frequency Oscillator
• A slightly variable local oscillator used to
  demodulate CW
• CW demodulated by envelope detector
• BFO/Detectors can resolve SSB. A product detector
  can resolve CW
• Two Issues
• Input Signal Levels can vary greatly
• Detection methods described so far are for
  Amplitude based modulations, not FM

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AGC

• AGC Automatic Gain Control
• AGC samples the signal strength at the detector
  and then feeds back a control signal to adjust
  the gain of the earlier stages
• AGC keeps audio volume fairly constant if signals
  level vary

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FM Receiver

• Uses FM Discriminator to detect small frequency
  deviations
• FM receivers use IF limiter amplifiers no AGC
  needed
• Squelch detects high-frequency noise and gates
  audio