VARIOUS TUNING CAPACITORS
Look into how they work! good AM radios will use hederodyning to take the am and actually mix it up to a HIGHER
intermediate or IF frequency where the effective fractional bandwidth is lower (bandwidth is on the order of 30kHz
to 50kHz I think) and then demodulate it with a 50kHz notch filter followed up by a simple diode peak detector to
do envelope detection. The variable capacitor will change the local oscillator feeding into the input to the mixer.
FM there is a bit more involved, the RF is usually mixed down to a more manageable 10.7MHz IF frequency and
filtering and demodulation happens there. W2EAW made a recent video showing how to demodulates FM using a
cool circuit implementing analog integrator or charge pump. Phase Locked Loops can also be used.
knowing how they work you can see what values can be adjusted to affect your circuit. values are relative to other
components at the end of the day 🙂
Capacitor when operated as tunning capacitor is connected to the two ends of the oscillating coil of the resonant circuit to play
the role of selecting the oscillating frequency.
VARIABLE CAPACITORS: TUNING AND ADJUSTING
1 YEAR AGO | BLOGS | BY: ELECTRONOOBS
Introduction
Learn about variable capacitors, essential parts of many electronic devices.
Adjustable capacitance makes these capacitors essential for fine-
tuning electronic circuits. In electronic applications like radios and
oscillators, their ability to adjust capacitance by changing surface area, plate
spacing, or dielectric material allows for precise control. Anyone interested
in electronics must understand these components' operation and
maintenance, whether they are electronically or mechanically adjusted.
Understanding Variable Capacitors
In order to adjust capacitance, a variable capacitor modifies the surface area
of its overlapping plates. A variable capacitor, sometimes referred to as a
tuning capacitor, is a kind of capacitor in which the capacitance can be
mechanically or electrically altered on a regular basis. Altering the physical
parameters that dictate capacitance, such as the conductor plates' surface
area (A), spacing between them (d), and permittivity (ε) of the dielectric
material between them, can produce this shift in capacitance.
The adjustment of the distance (d) between the plates is another feature of
certain variable capacitors. Capacitance exhibits a negative correlation with
increasing distance and a positive correlation with decreasing distance. This
is due to the fact that capacitance and plate distance have an inverse
relationship.
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Tech Notes - Variable capacitors
A Variable Capacitor is one whose capacitance may be intentionally and repeatedly
changed
mechanically. Variable capacitors are often used in
L/C circuits
to set the resonance frequency, for
example, to tune a radio (therefore they are sometimes called
tuning capacitors
), or as a variable
reactance
for
impedance matching
in
antenna tuners
. Can also known as a "variable air” capacitors.
2
The old name for a capacitor was condensor.
Tuning capacitors:
A typical variable capacitor consists of two sets of metal plates arranged so that the rotor
plates
move between the stator plates. Air is often the dielectric. As the position of the rotor is
changed,
the capacitance value is likewise changed. This type of capacitor is used for tuning most
radio
receivers. A typical physical construction is shown below.
The capacitors pictured above, from left, include our 365 pF dual-gang variable, a 365 pf
variable
with 8:1 shaft reduction drive built into the shaft, a 365 pF with shaft extended with nylon
parts to
isolate the capacitor from the user, and a 365 pF attached to a 6:1 external planetary
reduction drive.
Variable capacitors for tuning radio equipment are made in many formats and can be single
gang
type (the second and fourth ones above) but can have two three or four sections “ganged” in
the
Tech Notes - Variable capacitors
same shaft. The fourth one above has a five to one reduction drive connected between the
capacitor
and the tuning knob to make tuning easier. Simple radio receivers will have a “dial drum”
connected to a small diameter shaft to achieve the same aim.
The most common form of mechanically controlled variable capacitors, the amount of plate
surface
area which overlaps, is altered by the control shaft. They consist of a series of semicircular
metal
plates on a rotary axis
(“rotor”)
that are positioned in the gaps between a set of stationary plates
(“stator”)
so that the area of overlap can be changed by rotating the axis. Air or plastic foils can be
used as
dielectric
material. By choosing the shape of the rotary plates, various functions of
capacitance vs. angle can be created, e.g. to obtain a linear frequency scale. Various forms of
reduction
gear
mechanisms are often used to achieve finer tuning control, i.e. to spread the variation
of capacity over a larger angle, often several turns.
Miniature Plastic Tuning Gangs
The tuning capacitors (tuning gangs) pictured above are too large for most modern radios
and so
much smaller capacitors with plastic insulation material between the plates are now very
common.
Very cheap variable capacitors are constructed from layered aluminium and plastic foils that
are
variably pressed together using screws.
The one pictured here has only two sections and is used in AM radio receivers but there are
others
available with four or more sections for AM / FM type radios. The FM sections have a much
lower
capacity than the AM sections because of the much higher frequencies tuned.
All tuned circuits need to have some way of presetting the capacitance value so the circuits
are
tuned to the correct frequencies at each end of the band being received.
Trimmer capacitors
are
used for this and the two small holes in the right hand photo above allow access to two built
in
trimmer capacitors.
Multiple sections (referred to as Tuning Gangs)
Very often, multiple stator/rotor sections are arranged behind one another on the same axis,
allowing for several tuned circuits to be adjusted using the same control, For example an RF
Stage
(preselector), a mixer stage and the corresponding oscillator in a high performance receiver
circuit,
three sections in total. Typical capacitance ranges for the gang in an AM radios is
somewhere in the
Tech Notes - Variable capacitors
range of 330pF to 400pF and 25pF to 45pF for the FM section of a receiver.
The sections can have identical or different nominal capacitances.
The tuning gang pictured below is typical of those used in AM radios before the introduction
of FM
broadcasts. The local oscillator generates an RF signal 455 KHz above that being received
and this
has caused some difficulties in design over the years as the RF and oscillator must track each
other
across the band, staying 455 Khz apart. Up to the 1960's identical sections were used in all
tuned
circuits of an AM radio and a
padder capacitor
was used to lower the maximum capacitance of the
oscillator section so that circuit would track the Antenna input tuned circuits. As radios were
made
smaller tuning gangs with different sections were used. One such example is pictured below
and all
the miniature plastic tuning gangs are guilt this way.
Butterfly
A
butterfly capacitor
is a form of rotary variable capacitor with two independent sets of stator plates
opposing each other, and a butterfly-shaped rotor arranged so that turning the rotor will vary
the
capacitances between the rotor and either stator equally.
Butterfly capacitors are used in symmetrical tuned circuits, e.g.
RF
power
amplifier
stages in
pushpull
configuration or symmetrical
antenna tuners
where the rotor needs to be “cold”, i.e. connected
to RF (but not necessarily
DC
)
ground
potential. Since the peak RF current normally flows from
one stator to the other without going through wiper contacts, butterfly capacitors can handle
large
resonance RF currents, e.g. in
magnetic loop antennas
.
In a butterfly capacitor, the stators and each half of the rotor can only cover a maximum
angle of
90° since there must be a position without rotor/stator overlap corresponding to minimum
capacity,
therefore a turn of only 90° covers the entire capacitance range.
Split stator
The closely related
split stator variable capacitor
does not have the limitation of 90° angle since it
uses two separate packs of rotor electrodes arranged axially behind one another. Unlike in a
capacitor with several sections, the rotor plates in a split stator capacitor are mounted on
opposite
sides of the rotor axis. While the split stator capacitor benefits from larger electrodes
compared to
the butterfly capacitor, as well as a rotation angle of up to 180°, the separation of rotor plates
incurs
some losses since RF current has to pass the rotor axis instead of flowing straight through
each rotor
vane.
Differential Variable Capacitor
Differential variable capacitors
also have two independent stators, but unlike in the butterfly
capacitor where capacities on both sides increase equally as the rotor is turned, in a
differential
variable capacitor one section's capacity will increase while the other section's decreases,
keeping
Tech Notes - Variable capacitors
the stator-to-stator capacitance constant. Differential variable capacitors can therefore be
used in
capacitive
potentiometric
circuits.
Vacuum variable capacitor
A vacuum variable capacitor uses a set of plates made from concentric cylinders that can be
slid in
or out of an opposing set of cylinders (sleeve and plunger). These plates are then sealed
inside of a
non-conductive envelope such as glass or ceramic and placed under a high vacuum
.
The movable
part (plunger) is mounted on a flexible metal membrane that seals and maintains the
vacuum. A
screw shaft is attached to the plunger, when the shaft is turned the plunger moves in or out of
the
sleeve and the value of the capacitor changes. The
vacuum
not only increases the working
voltage
and
current
handling capacity of the
capacitor
it also greatly reduces the chance of
arcing
across the
plates. The most common usage for vacuum variables are in high powered
transmitters
such as
those used for
broadcasting
,
military
and
amateur
radio
as well as high powered
RF tuning
networks
.
Electronically controlled ( varactors or
varicaps)
The thickness of the depletion layer of a reverse-biased semiconductor
varies with the DC voltage
applied across the diode. Any diode exhibits this effect (including p/n junctions in
transistors), but
devices specifically sold as variable capacitance diodes are designed with a large junction
area and a
doping profile specifically designed to maximise capacitance.
Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance
would
be affected by the change of signal voltage, precluding their use in the input stages of highquality
RF communications receivers, where they would add unacceptable levels of
intermodulation
. At
VHF
/
UHF
frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by noise rather
than large signal handling requirements, and Varicaps are commonly used in the signal path.
Varicaps are used for
frequency modulation
of oscillators, and to make high-frequency
voltage
controlled oscillators
(VCOs), the core component in
phase-locked loop
(PLL)
frequency
synthesizer
s
that are ubiquitous in modern communications equipment.
Trimmer Capacitor
Trimmer capacitors come in all shapes and made in a variety of ways. The oldest and
simplest
consists of two plates separated by a sheet of mica. A screw adjustment is used to vary the
distance
between the plates, thereby changing the capacitance. These are often called compression
trimmers
and are not stable enough for very high frequency circuits or critical applications.
Pictured below is a selection of more modern trimmer capacitors:
Tech Notes - Variable capacitors
In addition to air and plastic, trimmers can also be made using a ceramic dielectric. The first
two
have plastic plates as a dielectric, the third has a ceramic material, these are very stable
devices.
Much of the older communications equipment you will come across will have the Bee Hive
trimmers and these are excellent in all respects
Fix The Damaged Variable Capacitor
It is possible to repair the variable capacitor in accordance with various
circumstances appropriately. Still, its primary faults are the collisions between the
moving and fixed pieces, leakage static induction, loose moving pieces, and
incorrect moving piece positioning.

When a sealed variable capacitor's moving and fixed
pieces collide (causing damage to the film), you can undo
the nuts on the four fixed columns, remove the damaged
film, and replace it with a good one (which can be
scrapped from another kind of variable capacitor to stop
using the film).

Sealed variable capacitance makes it simple to produce
static induction during prolonged use of the film medium;
adjustments are made in response to "Kaka" noise and
general alcohol cleaning. But because alcohol is so
volatile, it will collect static electrons on the film after
cleaning for a while, which will cause the regulation to
continue making noise. The best course of action is to take
off the plastic cover, apply clean lubricant, and mist
lubricant along the rotation's edge to prevent electrostatic
induction that could cause a prolonged noise. A thin steel
sheet is useful to modify the movable piece's deformation
when it comes into contact with the rotating medium.

After using the variable capacitor in the air medium for an
extended period, the film between the fixed and moving
pieces becomes unusable due to the accumulation of dust
and oil between the sheets or the corrosion of the moving
and fixed pieces. The surface layer also becomes
oxidized, peeling, and damp, reducing the insulating
resistance between the two pieces. This instance involves
inserting a thin steel sheet between the human slices in
order to remove the oil stain and dust and correct the
bumps.
What Is A Variable Capacitor's Accuracy?
The difference between the value of a variable capacitor as it is and the value that
it should have based on a given table is its accuracy. One can limit the accuracy
of a variable capacitor by understanding the physics of capacitors. A variable
capacitor's error is typically proportional to the difference between its rated
capacitance and physical size. Reducing the rated capacitance or making the
object larger physically will lessen this. In a circuit with a precise voltage divider or
transmission line, an electronic variable capacitor can be used as an input or
output filter to decrease error further.
Conclusion
Investigating variable capacitors' subtleties reveals their importance in electronics.
Due to their precise capacitance adjustment, they are useful in signal filtering and
radio tuning. To ensure their optimal performance, you must understand their
accuracy factors and learn repair and maintenance skills. Because we use
electronics more and more, variable capacitors are essential. Understanding
these electronic components is becoming crucial.