Chelmsford Amateur Radio Society Intermediate Course (3) Technical Basics - 3 Circuits

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Chelmsford Amateur Radio Society Intermediate
Course(3) Technical Basics - 3Circuits
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Tuned Circuits

• Recall that the Reactance of Inductors and
  Capacitors relates to their reaction time to
  store/release energy when AC is applied
• Radios depend on the concept of tuned circuits.
• Tuned circuits are built from combinations of
  Inductors and Capacitors which have a
  self-resonant frequency
• At resonance their combined Impedance is either
  extremely high or low depending on the circuit
  configuration
• Tuned circuits are thus able to selectively pass
  or block frequencies in transmitters, receivers
  and antennas
• They are the basis of tuners, filters,
  oscillators, traps, ATUs etc.

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Tuned Circuits - 2

• Tuned circuits are based on combinations of
  Inductors and capacitors that have a resonant
  frequency
• There are Two Basic Combinations Series and
  Parallel
• Note that increasing the values of L or C reduce
  the resonant frequency and vice-versa

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Parallel Rejector Circuit

• At DC and low frequencies, current flows through
  the inductor (whilst the high reactance
  capacitor blocks current)
• At high frequencies the Inductor reactance will
  become high, but the capacitor will then be low
  reactance and let current flow
• At Resonant Frequency they react in time against
  each other and block current/power flow with High
  Impedance, Z

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Series Acceptor Circuit

• At DC and low frequencies the capacitor blocks
  current due to its high reactance
• At high frequencies the Inductors reactance will
  be high and block current, despite the capacitor
  having low reactance
• At Resonant Frequency they mutually react in time
  with each other and permit current to flow with
  Low Impedance, Z

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Application of a Rejector

• Use of a pair of high-impedance rejectors, also
  known as Traps can enable a Dipole antenna to
  operate at two frequencies
• Termed a Trap Dipole
• At Frequency-2 the high impedance of the traps
  isolates the end of the dipole, effectively
  shortening it

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Power Supplies

• In a Power Supply fewer turns on the Secondary
  coil steps down AC Mains to a lower level (or
  steps it up if it had more turns)
• Diodes convert this to DC, which is smoothed by
  Capacitors
• NB Full-wave rectifiers use more diodes to ease
  smoothing

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Power Efficiency

• Circuits consume more power than they output.
  Nothing is 100 efficient, and the waste is
  dissipated as heat.
• Example The output power of a linear RF Power
  Amplifier is less than the DC input power due to
  such inefficiency it may only be 20-30
  efficient.
• Thus a 50W linear PA may also generate 100W of
  heat, and will need a total of 150W from a power
  supply
• Ambient and Internal Temperatures and Sound
  Mechanical Construction can therefore be key
  factors in Circuit Reliability and Stability