Transcript Technical Basics - 2 - Chelmsford Amateur Radio Society, G0MWT

Chelmsford Amateur Radio Society
Intermediate Course
(3) Technical Basics - 3
Circuits
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
1
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.
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
2
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
C
Parallel Tuned:Rejects
Current at
Resonance
L
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Intermediate Licence Course
C
L
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Series
Tuned:Accepts
Current at
Resonance
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
3
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
Parallel
Tuned
Current
through
Circuit
Z of
Circuit
Frequency
Frequency
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
4
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
Series
Tuned
Current
through
Circuit
Z of
Circuit
Frequency
Frequency
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
5
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
Frequency 2
Frequency 1
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
6
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
Transform
Mains AC
to lower Voltage
Primary
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Intermediate Licence Course
Secondary
Diodes rectify to
give DC Conversion
Murray Niman G6JYB
Capacitors Store
Energy and Smooth
Waveform, but may
still leave some ripple
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
7
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 4: v2.0, 25-May-2009
(3) Technical Basics -3: Circuits
8