Bredhurst Receiving and Transmitting Society

3. Technical Basics

3g Tuned circuits

3g.1 Recall that a series or parallel circuit of a capacitor and inductor together forms a tuned circuit.

Tuned circuit is formed from a capacitor and inductor

Students are not being able to recall this simple series tuned circuits and understand what it is.

Inductors and capacitors can be used in circuits in two configurations to make :- Series tuned circuits and Parallel tuned circuit.

Series tuned circuit

	In this circuit the inductor and the capacitor are linked in series with each other making a series tuned circuit. Tuned circuits only work where the signal is an AC one....either Audio or Radio frequency. The most notable feature of a series tuned circuit is that it presents a very low impedance across its' terminal at resonance. Quite the opposite of the parallel tuned circuit. The effectiveness of a tuned circuit is given the term " Q "....so the better the goodness of a tuned circuit the higher the Q. Series tuned circuits have many uses e.g they can control the frequency of a VFO or control the selectivity of a receiver to name just two. A series tuned circuit will pass AC to a greater or lesser degree depending on the frequency...however it will never pass DC because the capacitor will not allow the flow of DC current!
Parallel tuned circuit Students are not being able to recall this simple parallel tuned circuits and understand what it is.
	In this circuit the inductor and capacitor are linked in parallel with each other making a parallel tuned circuit. The most notable feature of a parallel tuned circuit is that it presents a very high impedance across its' terminals at resonance. Therefore it will not pass AC at the resonant frequency...but it will be a dead short to DC across its' terminals....this is because the coil has no resistance to the flow of DC. (The parallel tuned circuit will pass AC to a greater or lesser degree depending of course on the frequency!) The effectiveness of a tuned circuit is given the term " Q " So the better the goodness of a tuned circuit the higher the Q of the tuned circuit. A tuned circuit with a Q of 120 to 150 is a very good one and difficult to achieve in practice. The inductance of the coil versus the capacity of the capacitor affect the tuned circuit performance. Parallel tuned circuits have many uses e.g they can control the frequency of a VFO or control the selectivity of a receiver to name just two. Differing values of inductance can all be made to resonate at the same frequency just by varying the amount of capacity used either in parallel or series. The ratio of the inductance of the coil versus the capacity of the capacitor affects the tuned circuit performance...the Q.

3g.2 Recall and understand that the energy stored in the capacitor and inductor can transfer from one to the other at a particular rate, known as the resonant frequency.

Recall that the resonant frequency depends on the value of capacitance and inductance.

Note that candidates must know that increasing L or C reduces the resonant frequency and vice-versa. Knowledge of the resonant frequency formula is not required.

Earlier it was explained that a capacitor could store electric energy, and it stores it in the plates of the capacitor in the form of a charge. Also it was explained that an inductor can store energy in the form of a magnetic field generated by a previous flow of current.

The diagram below shows a capacitor that can be switched across an inductor.

How it all works !

If a charge were built up on the capacitor by another part of the circuit then when the switch is closed the potential difference due to the charge on the capacitor would cause electrons to flow through the inductor. As the current (electrons) flow from the capacitor it will slowly looses it charge, or as it is called "discharges", whilst at the same time a magnetic field builds up around the inductor.

When the capacitor is fully discharged the current can no longer flow but then the magnetic field around the inductor starts to collapse and induces a potential difference, opposite to that was originally on the capacitor which causes a current to flow which recharges the capacitor.

Thus once the magnetic field around the inductor has completely collapsed (reduced to nothing) the potential difference is zero - no current flows and the capacitor is then able to start to discharge again but this time in the other direction, until it is discharged at which point the inductor's magnetic field collapses again and the induced potential difference causes the current to flow to recharge the capacitor to the state it was when the switch was first closed.

In theory the charge and discharge could continue forever (so long as the switch remains closed) but nothing is so perfect as there are losses in the circuit. Thus the charge and discharge will slowly diminish and eventually stop.

The rate at which the charging and discharge cycle takes is determined by the value of capacitor its capacitance and the value of the inductor its inductance. The larger each of these values is the slower the charge and discharge cycle occurs.

The frequency or rate at which the charge and discharge cycle take place is called the resonant frequency and then the tuned circuit is described as being resonant. Change the L or C and the resonant frequency will change.

and here is some more detail but not required for the course.

3g.3 Recall that at their resonant frequencies, series tuned circuits present a low impedance, whereas parallel tuned circuits present a high impedance.

Whilst tuned circuits are often in oscillators you need to know that:-
	a parallel tuned circuit can be used to reject current at its resonant frequency such as when it is used as an antenna trap. NB the current is AC ( as is the current in RF ) as it does not work with DC.
	a series tuned circuit can be used to accept current at its resonant frequency. NB the current is AC ( as is the current in RF ) as it does not work with DC.

SO - Be aware that the circuits have many uses in radio frequency technology - in relation to AC / RF circuits as the principle does not apply to Direct Current as DC has no frequency of oscillation (it is constant all the time).