some of the RF energy flowing
through them as heat. Loss in the
capacitor is primarily caused by the
dielectric material (such as
polystyrene or mica), while the
inductor loses energy to resistance in
the wire and in its magnetic core.
Remember that the skin effect limits
inductor current to a very thin layer
at the surface of the wire, so
resistance at RF will be a lot higher
than the resistance you measure with
a DC multimeter.
The effect of these losses
reduces the component’s — and thus
the tank circuit’s — Q, or Quality
Factor. Q can seem mysterious, but is
a measure of energy loss with Q =
Energy stored during one cycle /
Energy lost during one cycle. For a
component, Q is the ratio of
reactance to resistance.
For example, if an inductor has
500 Ω of reactance and 5 Ω of loss
resistance, it’s Q = XL / R = 500 / 5 =
100 — a typical value for inductors.
Capacitors have much higher values
of Q; several hundred and up. Higher
values of Q mean the “flywheel”
keeps turning without slowing down
much or changing frequency. (The Q
of an LC tank circuit is limited by the
Q of the lossiest component —
usually the inductor.)
Remember that the tank circuit
acts as the primary filter for our
feedback loop. The lossier the filter,
the more noise it allows to get to the
JFET, which happily amplifies
anything that appears at the gate.
Thus, in an oscillator, tank circuit Q
determines the oscillator’s spectral
purity — meaning how much the
primary desired sine wave is
accompanied by noise and distortion
of various sorts. If you want a clean
signal, use the highest quality Ls and
Cs you can.
The Quartz Crystal
Even the very best LC oscillators
are not all that stable, and most have
plenty of noise in their output signals.
Certainly, they are handy circuits, but
they are not suitable for precision
jobs like generating clock signals for
digital circuits and master oscillators
for ham radio transceivers. In those
applications, a different type of tank
circuit is used: the quartz crystal.
May 2015 21
FIGURE 3. The basic construction of a quartz crystal for oscillators is shown in A,
along with a depiction of the crystal’s thickness shear vibration. The equivalent
electrical circuit for the crystal is shown in B and described in the text.