1 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 4: v2.0, 25-May-2009 (3) Technical Basics -3: Circuits Chelmsford

1Chelmsford Amateur Radio SocietyIntermediate Licence Course

Murray Niman G6JYB Slide Set 4: v2.0, 25-May-2009(3) Technical Basics -3: Circuits

Chelmsford Amateur Radio Chelmsford Amateur Radio Society Society

Intermediate CourseIntermediate Course(3) Technical Basics - (3) Technical Basics -

33CircuitsCircuits



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.



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

Parallel Tuned:-

Rejects Current at Resonance

L C

Series Tuned:-

Accepts Current at Resonance

L

C



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

Z of Circuit

Frequency

Current through Circuit

Frequency



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

Currentthrough Circuit

Frequency

Z of Circuit

Frequency



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 1Frequency 2



• 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

Power Supplies

Diodes rectify to give DC Conversion

TransformMains AC to lower Voltage

PrimarySecondary

Capacitors Store Energy and Smooth Waveform, but may still leave some ripple



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

Documents

1 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 4: v2.0, 25-May-2009 (3) Technical Basics -3: Circuits Chelmsford