There is an old saying: “Amplifiers are oscillators that don’t and
oscillators are amplifiers that do.” An
amplifier is at the heart of every
oscillator, as shown in the block
diagram of a basic
oscillator in Figure 1A.
Every single oscillator —
even the digital versions,
multivibrators like the
555 IC, and the ones in
the little metal cans —
has this same basic
structure: an amplifier,
some feedback, and a
frequency-determining
filter.
Figure 1B shows a pendulum
which is an example of a non-electronic oscillator.1 Given a push,
the pendulum will swing back and
forth at a constant frequency until
friction and air resistance bring it to a
halt at the rest position in the center.
The frequency-determining element
of the pendulum oscillator is its
length, L. (Interestingly, the mass of
the pendulum doesn’t matter!)
The amplifier is whatever delivers
the push — such as you. Obviously,
the amplifier has lots of gain because
you are very strong! By delivering
feedback in the form of just the right
strength push at just the right time,
you can keep the pendulum swinging
forever — or at least until dinner.
Switching back to Figure 1A,
let’s imagine an electronic circuit in
each block. The idea is for some
fraction, b, of the amplifier’s output
signal to be fed back and reinforce its
input signal. That input is then
amplified with some fed back, so that
the output eventually becomes self-sustaining; this is called oscillation.
Furthermore, to get oscillation only at
the design frequency and not just
produce random noise, the system
must include a filter to provide
selectivity; meaning that its response
is dependent on frequency. The filter
can be an LC circuit, a crystal, or a
timing circuit — something that is
time- or frequency-sensitive.
All this creates two requirements
for our general-purpose oscillator:
First, the amplifier has to have
enough gain at the oscillation
frequency to overcome losses in the
feedback circuit. Second, the filtered
signal fed back to the input has to
arrive with just the right phase so as
to reinforce and not cancel the input
signal.
These two conditions make up
the Barkhausen Stability Criterion: 2
Loop gain =|Aβ| = 1
and
Loop phase shift = ∠β = 0°, 360°,
720° ... 360° x 0, 1, 2, etc.
(The symbols | | mean
“magnitude of,” and the symbol ∠
means “phase shift of.” If you are
working with radians instead of
degrees, the loop phase shift
requirement is stated as β = 2πn, with
n being an integer value.)
So, just how does the oscillator
start up? Noise! Random noise at the
frequency for which the phase shift is
hwardsil@gmail.com
Oscillators
Every signal begins
with an oscillator —
the topic of this
column. In ham radio,
the oscillator is a key
element in generating
signals, mixing them
together, and
extracting the
information from
them. This month,
we'll make an audio
oscillator and learn
about common types
of RF oscillators.
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
March 2015 59
FIGURE 1. The block diagram (A)
describes an oscillator as three circuits:
one providing gain and the other two
feeding back a fraction of the output
signal into the input through a filter. B
shows a pendulum which is a mechanical
version of the system in A.
A.
B.
