After Class: Crystals
April 1960 Popular
All you need to do is
substitute transistors for the tubes used in these fundamental oscillator circuits to bring this article's content
up to date. Or, maybe you are the owner of a vintage vacuum tube radio and would like to learn a little about how
things were done in the olden days. Either way, as with so many aspects of electronic circuits, the basics haven't
changed much in the last 100 years. It's all still good.
of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Popular Electronics was published from October 1954 through April 1985. As time permits, I will be glad to scan articles
for you. All copyrights (if any) are hereby acknowledged.
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After Class: Crystals
Nearly everyone in electronics makes use of crystals every day - here's
why these rock-like plates play such an important role in keeping the world
By Jim Kyle, K5JKX/6
An amateur radio
operator, an airline pilot, a police radio dispatcher, a broadcast-station
announcer ... sound like a hodge-podge of job holders? Maybe so, but they
have at least one thing in common. All make daily use of the peculiar properties
of quartz crystals - thin, glass-like plates that keep the world in tune.
Crystals are found in almost all commercial and ham radio equipment,
but few of their users know how they work. Before we examine the details of
crystal operation, let's take time out to define our terms. There are three
important words involved: resonance, damping, and piezoelectricity.
Resonance means the frequency at which an object will vibrate most easily.
Every object has a resonant frequency. Musical instruments - the piano, for
instance - are based on this principle. When you strike a key, a hammer hits
the piano string, which then vibrates at its resonant note.
Damping means the suppression of an object's tendency to vibrate, The more
highly damped an object is, the less readily it will vibrate. In the piano,
for example, stepping on the loud pedal removes the damping from the strings.
Stepping on the soft pedal damps the vibrations even more than usual.
Finally, piezoelectricity is a property shared by several substances. It means
that the substance generates a small voltage across its opposite sides if
it is stretched or squeezed. In addition, applying a voltage to opposite sides
of a piezoelectric material will deform the substance as long as the voltage
Fig. 1.How a Crystal Works. Let's assume
that we have a crystal of some piezoelectric substance, and a means of making
electrical contact to its opposite sides. Since the crystal has mass, it will
have a resonant frequency. If it's jarred - mechanically excited - it will
vibrate at that frequency. And if the crystal's damping is light, it will
continue to vibrate at its resonant frequency for some time.
In a typical oscillator circuit,
turning the oscillator on develops a voltage between its cathode and grid,
and this voltage shocks the crystal into vibration at its resonant frequency.
The vibration in turn develops an alternating voltage across the crystal terminals,
which is amplified by the tube. The LC circuit is tuned near the crystal frequency
and presents a high impedance in the tube's plate circuit. Consequently, a
portion of the amplified voltage is fed back to the crystal and maintains
Pierce oscillator operates at almost any frequency,
but uses only fundamental frequency of the crystal. Plate voltage should be
as low as possible.
gives good output on harmonic as well as fundamental frequencies if LC circuit
is substituted for the 10,000-ohm resistor.
Overtone oscillator will provide output up to 60 mc. with crystals ground
for third-harmonic operation. LC circuit must be tuned to output frequency.
Structurally. the piezoelectric crystal
used in radio transmitters consists of a thin plate of Brazilian quartz about
1 1/2" square. There is a contact on either side, and the entire unit is enclosed
in a container for protection. The contacts are held in position against the
crystal by spring pressure, or, sometimes, they are plated onto the surface
of the quartz. Each contact is then connected to one of the base pins.
But remember that our crystal is piezoelectric. By definition, this means
that it will develop a voltage across its opposite sides whenever it's stretched
or squeezed. The electrical contacts we've connected to those sides will allow
us to utilize that voltage.
If the crystal's size is such that
the resonant frequency falls within the r.f. spectrum - and this is ordinarily
the case - we have a generator of r.f. energy. But our generator must be"
jarred to be put into operation, and it will operate only until the vibrations
die out. For it to be of use to us, we must make our crystal operate continuously.
The addition of a special vacuum-tube or transistor circuit
to amplify a portion of the crystal's developed voltage and then feed it back
to the crystal will keep the crystal operating indefinitely. Such a circuit
is called an oscillator and is used to "jar" the crystal electrically at the
proper instant to keep its vibrations going. Before looking at several types
of oscillator circuits, let's consider the reason for using a crystal at
Why Use Crystals? Since oscillators
can be built without using a crystal - the ordinary superheterodyne radio
receiver and the ham's VFO are good examples - the question arises, "Why bother
with crystals ?"
Actually, a crystal has only one major advantage
over a well-built variable oscillator - stability. A crystal's resonant frequency
is determined primarily by its physical size. This means that the frequency
of a crystal is relatively unaffected by outside influences.
are, however, two outside influences which can change a crystal's frequency.
One is widely known, the other almost ignored.
The first enemy of
crystal stability is heat. Like any substance, a crystal will change in size
slightly as it heats or cools - and its frequency is determined mainly by
its size. Thus, changes in temperature will be reflected as a drift in frequency.
The effect can be serious. In commercial applications, a crystal
is usually kept in a tiny oven, thermostat-regulated to maintain an even temperature.
Amateur and experimenters' crystals, in contrast, are designed to remain at
a fixed frequency under reasonable temperature variations. But overloading
the crystal in an effort to extract the last measure of power from the circuit
can push its temperature into the "unreasonable" region.
Even more serious than the heating problem-and far less widely known
- is the matter of shunt capacitance. In an ideal situation, the crystal will
"see" no load at all across its terminals. But such a circuit is impossible
in practice, since the crystal must feed an oscillator in order to operate.
And, with the oscillator connected, some capacitance is unavoidable.
This capacitance acts as a load on the crystal, slowing down its rate
of vibration and thus lowering its resonant frequency slightly. Depending
upon the capacitance in the circuit, a crystal will produce higher or lower
frequencies than that specified. Since this effect cannot be avoided, crystal
manufacturers assume a standard value of shunt capacitance and build their
crystals to give specified frequency when working into that load. For most
amateur crystals, the design value is 32 μμf.
can't be expected to know the capacitance involved, most of them refuse to
guarantee extremely close accuracy unless they can calibrate the crystal in
the actual circuit to be used.
There are a number of circuits that can be used as oscillators. All have two
things in common - a means of amplifying the crystal's output, and a way of
feeding some of that output back to keep the crystal oscillating. A typical
oscillator circuit (Fig. 1) makes use of the tube's grid-cathode voltage to
place the crystal in operation. Because the crystal is in the grid circuit,
the tube amplifies its output.
Some oscillator circuits are designed
to give output only at the fundamental frequency of the crystal - the frequency
of the crystal itself. Others are built to provide the fundamental and integral
multiples of the fundamental frequency as well. Such multiples are known as
harmonics and have frequencies of two, three, four, or more times the fundamental
Still a third variety of crystal oscillator is the "overtone"
circuit. A major difference between it and a harmonic oscillator is that the
overtone circuit produces only one output frequency-the third, fifth, or other
odd harmonic. A harmonic oscillator, in contrast, produces the entire gamut
the fundamental, the desired frequency, and other harmonics, too.
The Pierce circuit shown in Fig. 2 is typical of a fundamental-frequency
oscillator. It operates at the crystal's fundamental frequency, without tuning
or other adjustments.
One of the most popular harmonic-oscillator circuits is shown
in Fig. 3. This is the Colpitts or "grid-plate" circuit. As shown, it produces
useful power output up to the fourth harmonic of the crystal's fundamental
frequency, if an LC circuit in the output is tuned to the desired output frequency.
There are many types of overtone oscillator circuits. The one shown
in Fig. 4 is recommended by International Crystal engineers for use with their
overtone crystals in the 15-60 mc. range.
All three of these circuits
are stable. Crystal heating - if operating voltages are kept within the limits
shown - poses no problem. If you keep in mind that a crystal is intended to
control frequency and not to produce power, you'll have no trouble. Keep power
input to the oscillator as low as possible, and let other stages of the equipment
provide power output.
In both the Pierce and Colpitts circuits, shunt
capacitance affects the crystal's operating frequency. Values shown for the
Colpitts circuit result in about 32 μμf. across the crystal. In either
circuit, a small variable capacitor can be connected in parallel with the
crystal to adjust the output frequency slightly.
Since the overtone
circuit operates differently, additional capacitance across its crystal will
have little effect on frequency. If the capacitance is excessive, though,
it may keep the oscillator from functioning.
frequency of a radio transmitter is but one application of the quartz crystal.
Although it operates in a completely different way, a crystal is electrically
the same as an extremely efficient LC (or "tank") circuit. For this reason,
a crystal can replace a tank circuit in any low-power installation.
But the major use of quartz crystals today is in transmitter frequency control.
Here, they provide precision channels at low cost and enable efficient use
of the limited radio spectrum. In fact, these rock-like plates make radio
as we know it possible. Their purpose? Keeping the world in tune!