Radio Receivers.

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Radio Receivers

• CEC

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Contents

• Crystal Receiver.
• Tuned Radio Frequency Receiver.
• Superheterodyne Receiver (AM)
• Superheterodyne Receiver (FM).
• Comparison.

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Carrier Frequencies

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Crystal Receiver

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Crystal Receiver

• No active parts.
• Powered only by the radio signal itself, whose
  detected power feeds headphones in order to be
  audible at all .
• Large antenna used.
• Relies on detection using germanium diode.
• Simple and can be easy to make or even improvise.

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Crystal Receiver

• Needs a strong RF signal and a long antenna to
  operate.
• No amplification.
• Displays poor selectivity since it only has one
  tuned circuit.

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Demodulation/Detection

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Tuned Radio Frequency Receiver

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Tuned Radio Frequency Receiver

• One or more stages of radio frequency amplifier
  all tuned to the desired reception frequency.
• Comprises of an envelope detector using a diode,
  followed by audio amplification.
• Used in cheaper radios till 1960s.

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Superheterodyne Receiver
Heterodyning to produce Intermediate Frequency
(IF)

Radio Frequencies (RF), µV range
Audio Frequencies
IF 455 KHz
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Superheterodyne Receiver

• One or more stages of radio frequency
  amplification to improve the receiver's noise
  figure.
• RF signal enters a mixer along with the output of
  the local oscillator to produce intermediate
  frequency signal.
• Local oscillator tuned to higher (or lower)
  frequency than the intended reception frequency.
• IF signal frequency is 455 KHz in AM Receivers
  and 10.7 MHz in FM Receivers.

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Superheterodyne Receiver

• Tuning the receiver involves changing the
  frequency of the local oscillator.
• Intermediate Frequency less than the broadcast
  range and will not produce interference.
• Demodulation to extract envelope of the modulated
  signal.
• Both envelopes of the modulated signal contain
  the same intelligence.

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Heterodyning
f1 gt f2
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FM Receiver

AGC Automatic Gain Control for gain control of
preceding amplifier stages.
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Limiter

• In FM, carrier frequency varied, amplitude
  remains constant after modulation.
• Atmospheric noise picked up by transmitted
  modulated radio signals may rise above the
  amplitude level of modulated signals.
• This may produce signal distortion at the
  receiver and spurious sounds at the loudspeaker.
• Limiter to limit amplitude variations of the
  received signal and improve SNR.

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Preemphasis and Deemphasis

• Preemphasis done at the transmitter.
• Boosting high frequency signals
• Pre-emphasis refers to boosting the relative
  amplitudes of the modulating voltage for higher
  audio frequencies from 2 KHz to approximately 15
  KHz.
• Deemphasis done at the receiver.
• Attenuating high frequency signals by the amount
  by which they are boosted.
• Done to improve Signal to Noise Ratio.

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R1C time constant t 75 µS
Pre-emphasis at the transmitter side.
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R
t 75 µS
C
De-emphasis at the receiver.
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Receiver Characteristics

• Sensitivity.
• Selectivity.
• Fidelity.
• Stability.

SSFS
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Receiver Stability

• Frequency stability the receiver must stay
  "tuned" to the incoming radio signal and must not
  "drift" with time or temperature.
• Great magnitude of gain generated must be
  carefully controlled so that spurious emissions
  are not produced within the receiver.
• Lack of Stability would lead to distortion of the
  recovered information, or, at worst, may radiate
  signals that interfere with other receivers.

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m gt 1
Germanium diodes deduct their cut in voltage (
0.2 V).
Add 0.2 V dc to negate the effect of diode cut
in voltage.
This portion contains intelligence.
Intelligence may lose continuity.
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Thank You