Tuned circuits
3g.1 Recall that a series or parallel circuit of a capacitor and inductor together forms a tuned circuit.
A tuned circuit can be formed from a capacitor and an inductor either in parallel or in series.These are common in both receivers and transmitters.
3g.2 Recall 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.
The energy stored in the capacitor and inductor can
transfer from one to the other at a particular rate, known as the
resonant frequency. The
actual resonant frequency will depend on the value of the capacitor and
inductor. Low value inductors and capacitors have a high resonant
frequency. High value inductors and capacitors have a low resonant
frequency.
Recall how 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.
The resonant frequency can be lowered by increasing the value of inductance and / or capacitance.
The resonant frequency can be raised by decreasing the value of inductance and / or capacitance.
3g.3 Recall that at their resonant frequencies, series tuned circuits
present a low impedance, whereas parallel tuned circuits present a high
impedance. For example to prevent an IF signal from getting through from the
antenna to the IF stage a series tuned circuit can be connected between
the input and earth.
To allow a particular frequency to enter a radio a parallel circuit can be used between the antenna and earth.
The parallel circuit in the
circuit opposite has a high impedance at 7MHz. So, 7MHz signals will be
stopped from draining to earth and will pass on to other stages in the
radio. Other frequencies will tend to drain to earth as the parallel
circuit presents a low impedance to them.
The front end of a 7MHz receiver may have several of these to ensure that only 7MHz are accepted into the radio.
Transformers
3h.1 Understand that a simple transformer consists of two coils of wire
sharing the same magnetic field and may have an iron core to
concentrate the field.
3h.2 Understand that energy is transferred from one coil to the other
by changes in the field when alternating current is used, and that this
does not happen with constant direct current.
Understand that an alternating potential difference (such as the mains)
can be stepped down using fewer turns of wire on the secondary coil
than on the primary and can be stepped up by using more turns on the
secondary than on the primary.
A transformer takes electricity at one voltage and changes it to another voltage. This will only work with AC.
Basically a transformer has a primary winding of several hundred turns
and a secondary winding consisting of fewer turns. To concentrate the
magnetic field, the two coils are wound on an iron core. When AC is
applied to the primary, the magnetic field generated induces a voltage
in the secondary. This voltage depends on the ratio of turns.
If the secondary has fewer turns than the primary it is called a
stepdown transformer. For example is the primary has 200 turns
and the secondary 20 turns, the secondary will have 200/20 = one tenth
of the primary voltage. So, 240volts AC at the primary will create
24volts AC at the secondary.
If the secondary has more turns than the primary it is called a stepup
transformer. For example if the primary has 50 turns and the secondary
200 turns the ratio is 200/50 = 4, so 12V AC into the primary will
produce 48V AC at the secondary.
For the Intermediate you do not have to know about turns ratio.
