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Chelmsford Amateur Radio Society Intermediate Course (4) Receivers. 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. Tuned circuit. Diode detector. Earphone. Crystal Receiver.

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Presentation Transcript
receivers
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
crystal receiver

Tuned

circuit

Diode

detector

Earphone

Crystal Receiver
  • Block diagram
    • Single tuned circuit; poor selectivity
    • No gain; poor sensitivity
    • Requires large aerial and earth; only receives strong signals
crystal receiver circuit
Crystal Receiver Circuit
  • L1, C1 - Tuned circuit – selects signal
  • D1 – Detector diode – demodulates
  • C3, R1 – Low-pass filter for audio
trf receiver

RF

amplifier

Demodulator

AF

amplifier

BFO

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
superhet receiver

Mixer

IF

amplifier

Demodulator

AF

amplifier

Local

oscillator

BFO

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
mixer as a converter

~

Mixer

145MHz–123.6MHz=21.4MHz

IF frequency

RF 145MHz

~

LO 123.6MHz

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-21.4MHz=102.2MHz

diode detector circuit

Output

Voltage

Time

Input

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
diode detector operation

2

D1 cathode output

1

Diode Voltage / V

0

D1 anode input

-1

-2

800

600

Diode conducts on peaks only

Diode Current / µA

400

D1 current

200

0

Time/µSecs

0

0.5

1

1.5

2

2.5

3

3.5

4

Diode Detector Operation
  • Accurate SPICE simulation of diode detector
envelope detection
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

cw demodulation bfo

+

Carrier Wave

Envelope

Detector

BFO

Beat Note

CW Demodulation - BFO
  • BFO = Beat Frequency Oscillator
    • Slightly offset BFO added to carrier to generate the “beat note”

…the origins of the “BFO” !

ssb demodulation

IF

Sideband

Filter

IF

Amp

Product

Detector

AF

Amp

~

CIO = Carrier Insertion Oscillator

CIO

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
usb demodulation

Amplitude

Lower

Sideband

Upper

Sideband

Frequency MHz

IF Filter

Mixer

Amplitude

Amplitude

Upper

Sideband

Upper

Sideband

Frequency MHz

Frequency

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
lsb demodulation

Amplitude

Lower

Sideband

Upper

Sideband

Frequency MHz

IF Filter

Mixer

Amplitude

Amplitude

Lower

Sideband

Lower

Sideband

Frequency MHz

Frequency

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.
demodulation summary
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
slide16

RF

Amp

Mixer

IF

Amp

Demod

AF

Amp

LO

AGC

Circuit

AGC
  • AGC = Automatic Gain Control
  • AGC adjusts the gain to keep carrier level constant
  • Signal level varies widely, but audio volume stays constant
fm receiver

Mixer

IF

amplifier

Discriminator

AF

amplifier

Local

Oscillator

Squelch

circuit

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