Oscillators - How They Work
December 1940 January 1941 National
Radio News Article
Radio Institute (NRI)
was founded in 1914 at the dawn of the radio age. It provided self-study
courses as well as classroom instruction on the art of electronics
and radio communications. A bi-monthly magazine titled National
Radio News was published from 1929-1953. This article explaining
how oscillators work appeared in the December 1940 edition. Although
circuits of the day used vacuum tubes, the principle of voltage
and phase relationships required to initiate and sustain oscillations
are the same as for transistor circuits. A step-by-step description
is provided from the time the power is applied until stable oscillations
Dec '40/Jan '41 National Radio News
of Contents] These articles are scanned and OCRed from old editions of the
National Radio News magazine. Here is a list of the
National Radio News articles I have already posted. All copyrights are hereby acknowledged.
Oscillators - How They Work
By J. A. Dowie
N. R. I. Chief Instructor
In every radio transmitter, in every super-heterodyne receiver and
in radio servicing equipment, we find oscillators producing the
signal. It is this oscillator that supplies the signal that is so
essential in carrying out our work. Since it is so important in
radio, let's study in greater detail how it works. That is, how
does an oscillator operate in generating the signal and how does
it continue to develop a signal after placed into operation?
J. A. Dowie, NRI Chief Instructor
Oscillator Circuits: There are a large number
of different types of oscillators in operation. There are oscillators
which maintain oscillation by the ionization of gas and by the projection
of electrons through chambers where the rate of travel of the electron
determines the frequency of oscillation. In this discussion I am
going to cover only the operation of the better known oscillator
circuits. The oscillators that are used most extensively in the
radio field. For example, the tuned grid, the tuned plate, the Armstrong,
the Meissner, the Hartley, the Colpitts, the ultraaudion, the push-push
and the push-pull types with either the tuned grid or tuned plate
or both. In my discussion I will cover these circuits and their
operating characteristics. It will be pointed out, that when you
understand the characteristics of oscillator circuits which depend
upon capacitive or inductive feed-back that you will understand
the operating characteristics of all of the conventional types of
oscillator circuits mentioned above.
Between The A.C. Grid And The A.C. Plate Voltages: It can
be stated that the primary requirement in order to sustain oscillations
in either an inductive or a capacity feed-back circuit is that the
applied grid to cathode voltage must be approximately 180 degrees
out of phase with the plate to cathode voltage. This means that
when the grid to cathode voltage is rising in a positive direction
the plate to cathode voltage must be dropping in a negative direction.
That is, the tube itself acts as an amplifier. Then, too, if the
reversal is of a sine wave character the wave form of the signal
generated will be a pure sine wave. Remember that the voltage applied
to the grid of a tube which is not overloaded controls the plate
circuit output wave form and that the triode tube is easily adopted
to the inductive or capacitive feedback types of oscillating circuits.
In an oscillatory circuit the tube does not become an oscillator
- it continues to act as an amplifier - amplifying the voltage which
is applied to its grid circuit and sending it through the circuit
coupled to its plate. When the plate circuit is properly coupled
to its respective grid circuit so that it continues to amplify the
signal it excites itself, the circuit and the tube become an oscillator.
Since the tube continues to operate as an amplifier even though
it is in an oscillatory circuit let us study some of the important
characteristics of vacuum tube amplifiers. In Fig. 1A is shown a
triode having electrode supply voltages. Its operating point being
at a on the Eg-Ip characteristic curve shown at B. It can be shown
that as the grid voltage of the tube is driven in a positive direction
by some force that this will result in a decrease in plate voltage
between P and K. This is due to the increase in plate current, the
voltage drop being in the plate load and the polarity or phase of
the voltage in this circuit being in a negative direction. Now when
the grid is driven in a negative direction in the grid circuit the
plate circuit voltage goes in a positive direction. This can be
proven by Fig. 1B. Point 1 on the grid voltage moves positive to
point 2 and the plate current increases from point 3 to point 4.
An increase in plate current means a drop in plate to cathode voltage
and from a high positive value to a less positive value with respect
to the cathode. It is therefore evident that the voltage applied
to the grid-cathode circuit must always be 180 degrees out of phase
with the change taking place in the plate circuit of the tube in
order to have the tube excite itself and thus maintain oscillation.
Figure 1A - Oscillator biasing.
Figure 1B - Operating point.
So long as the signal voltage on the grid of the tube does
not swing beyond points band c on the Eg-Ip curve there will be
no wave form distortion introduced and if the coupling between the
grid and plate circuit permits uniform wave form changes, then a
sine wave will be developed in the plate circuit and consequently
at the output of the oscillator. This condition of operation is
known as class A amplification.
Class Of Amplification:
The efficiency of an oscillator is dependent to a large extent upon
the class of operation. As in the case of the various classes of
amplifiers used, the class A, Band C, the efficiency of the oscillator
tube is the same as if it were an amplifier insofar as the tube
is concerned. Figure 2 shows the relationship between the grid bias
voltage, grid swing and plate current for the three fundamental
types of amplifiers all of which may be used in the operation of
Outstanding Amplifier Characteristics : The
outstanding operating characteristics of a properly operated class
A amplifier is the fact that the variations in excitation do not
produce a change in the average D.C. plate current. That is, the
increases in plate current are equal to the decreases and for this
reason the average current taken from the power supply does not
change. The grid excitation signal never drives the grid positive
with respect to the cathode of the tube.
the class B amplifier increases in grid excitation produce proportional
increases in the average D.C. plate current, that is, an increase
in excitation raises the output of the oscillator. The grid excitation
is sufficient to drive the grid positive but not off the straight
portion of the Eg-Ip curve. The class C amplifier is operated so
that further increases in grid excitation show no further increases
in the average plate current. This condition of operation can only
exist with the flow of grid current. The grid is driven positive
and far enough to cause plate current saturation. It can also be
stated that an oscillator employing a class C amplifier has very
high harmonic content as the plate current exceeds the saturation
point as can be seen at the upper right in Fig. 2.
How Phase Relation Is Obtained: As stated there
must always exist a 180 degree phase shift in the voltages between
the grid and plate circuits in order to use a triode as an amplifier
tube in an oscillator circuit. This required phase shift can be
obtained by means of a transformer or through the aid of a phase
shifting network or phase inverter consisting of another tube. Most
oscillatory circuits use a transformer consisting of either one
winding having a tap on it or two separate windings. For example,
Fig. 3A shows a tapped transformer, often referred to as an auto-transformer.
The position of the tap being selected to give the required excitation
voltage for the class of amplification desired. The transformer
winding gives us the desired phase shift because one end of the
winding will always be of opposite polarity with respect to the
other. No coil or winding on a transformer can have the same polarity
when the winding is wound in one direction. One end will always
be positive while the other is negative. This is the condition when
all turns are linked together by the same electro-magnetic field.
If we use an oscillator coil having two windings then the
windings must be connected so that the grid end of one winding will
be of opposite polarity with respect to the plate end of the other,
thus keeping the 180° phase shift. The connections will be as
shown in Fig. 3B.
How To Determine Coil Polarity: Oftentimes the
serviceman is required to make an oscillator coil replacement and
he is confronted with the job of connecting the unmarked leads of
an oscillator coil to produce oscillations. In order to connect
the coils of an oscillator so the phase will be correct, refer to
Fig. 3B. Note that a and d are at opposite ends of the oscillator
coil. If lead c is connected to the plate coupling condenser Ope
then to insure proper polarity lead b must be connected to the grid
coupling condenser Cgc. It isn't difficult to remember this requirement.
I always say that when the grid is at one end of a coil form having
two windings then the plate must be at the other end of the coil
form when the two coils are wound in the same direction. That is,
these two leads are always on the opposite ends of the two coils
or at the two inside terminals. This rule holds good regardless
of the placement of the tuning condenser or condensers or the method
used in supplying power to the oscillator circuit. The tuning condenser
or condensers do not shift the phase of the voltage across the coils
sufficiently to stop oscillation.
Fig. 2. - Relationship between grid bias voltage, grid swing
and plate current for class A, class B and class C amplifiers.
In class A, the grid never swings positive; in class B the
grid swings positive only over the linear region of the
plate current characteristic; in class C the grid swings
beyond the plate current saturation point.
Method Of Feeding
Power To Oscillator Tubes: Figure 3B shows how the power
or electrode voltages are supplied to the tube so it can amplify
by what is known as the shunt or parallel feed method. The signal
voltage generated is in parallel with the path taken by the power
to the tube electrodes. In Fig. 3C the same circuit components are
shown but connected to give us the series feed method of supplying
power to the tube electrodes. Note that the coupling condensers
are now by-pass condensers and are connected to the cathode of the
tube. It is, of course, possible to use the series feed in the grid
circuit and the parallel or shunt feed method in the plate circuit
or shunt feed in the grid circuit and series feed in the plate circuit.
The method of feed selected by the engineer in the construction
of the device may be anyone of these combinations. The series plate
feed method being somewhat more efficient than the shunt feed method
as this method prevents Rp from shunting the plate circuit
of the oscillator.
Automatic C Bias Supply: Although the
circuit shown in Fig. 3A has a C battery to enable class A operation
of the tube at a given point on its Eg-Ip curve, it is possible
to obtain class Band C operation by omitting the battery. This latter
is possible because the upper end of the resistor Rg
will become negative when the grid is driven positive by the excitation
signal. The grid serves as the anode and the cathode of the tube
as the cathode of a rectifier tube. The positive end of the resistor
Rg being connected to the cathode. Rg may
be considered the load on the rectifier. The voltage across Rg
is dependent upon its current and resistance.
How Oscillations Are Maintained: Now that we know
how the proper polarity of the winding can be ascertained, how power
may be fed to the tube and how the grid bias can be obtained automatically,
let's determine how oscillations are developed and maintained.
Assume that the cathode of the tube in Fig. 3B is at its operating
temperature and that the B battery voltage is applied instantly.
Upon application condenser Cpc will start to charge up to the value
of the D.C. voltage dropped in the resistor Rp. Plate
current will start to flow through winding L2. This causes a magnetic
field to be present about coil L2. This field would appear to be
of a steady value because the flow of the D.C. plate current is
assumed to be constant. This is not, however, the case. The moment
the plate voltage is applied the magnetic field about coil L2 starts
to expand, which, according to the electro-magnetic law would link
the coil L1, due to its inductive relation and would consequently
induce an e.m.f. or a difference of potential across it. If then,
the coils L1 and L2 are wound as stated above where coil L1 would
produce a positive potential at the terminal a and a negative potential
at b, the grid which is connected to a would receive a positive
charge. This immediately partly neutralizes the space charge between
the cathode and plate and allows more plate current to flow and
at the same time causes the production of a negative grid bias.
This causes a greater field to exist around coil L2 and results
in a greater positive charge on the grid. The plate current then
increases and in turn applies a greater positive potential to the
grid. Of course this action continues until the plate current is
limited by the emission characteristics of the tube or by the automatic
C bias voltage which is developed by the rectified grid current
which is across the resistor Rg. The turns ratio and
amplification factor of the tube will also affect the peak value
of plate current.
When the peak plate current value has
been reached, the magnetic field collapses and as a result the grid
is driven negative. This causes a reduction in plate current which
tends to aid in making the grid more negative. The grid may be driven
so far negative that the plate current is completely cut off as
shown in Fig. 4c. No further changes will then occur in the negative
direction and again the magnetic field collapses. Then the complete
cycle of operation will be reversed and as before the operation
will start allover again. Thus it can be seen that the polarity
of the coils L1 and L2 must be correct to cause the proper changes
in plate current.
Transformer Turns Ratio:
It should also be evident that the greater the turns ratio of L1
to L2, the higher the voltage across terminals ab. That is, the
voltage across winding L1 should be high and naturally the greater
the number of turns in coil L1, the higher the voltage developed.
This will mean more excitation voltage and also a greater plate
current as more power will be required to supply the extra excitation.
These facts also apply to the operation of the circuit shown in
Fig. 3A and Fig. 3B. The turns ratio factor is also present and
holds true when the inter-electrode capacity of the tube is used
in tuning the entire circuit and when the tuning condenser is connected
between terminals a and d in Figures 3A, Band C. When the tuning
condenser is connected across either coils L1 or L2 then the coil
without the condenser across it has the least number of turns. This
is due to the fact that the condenser tunes the circuit to resonance
and allows a higher voltage to exist across the coil and naturally
with a larger magnetic field.
For a given plate supply voltage it is possible to find the correct
excitation voltage by either selecting the proper number of turns
or regulating the coupling or both in an oscillator circuit. The
excitation voltage is also affected by the automatic bias voltage
placed on the oscillator tube and the load coupled to the output
circuit. For efficient operation of the oscillator circuit and for
a given power output we must select the correct amount of excitation
to give the class of operation consistent with the type of performance
we desire. This value will usually be for the least amount of plate
current that will give the most power output. There are other factors
such as frequency stability and wave form that must be taken into
consideration in the selection of the circuit values. It is the
work of the radio engineer to select the proper operating characteristics
of an oscillator circuit.
The output of the oscillator is affected by a change in the oscillator
plate voltage for a given turns ratio or coupling between the grid
and plate circuits. It is also a fact that an increase in the D.C.
plate voltage causes an increase in the D.C. plate current, the
generated R.F. tank voltage, the R.F. tank current, the R.F. grid
and plate current as well as the self-adjusting grid biasing voltage.
These factors are all related to the power supplied to the oscillator
for a fixed amount of coupling. It can also be stated that for a
given supply voltage it is impossible to change any of the other
currents or voltages in the oscillator circuit without changing
all other values. This means that an increase in the coupling of
the load to the oscillator circuit will affect all of the values
of currents and voltages, that is, their relationship to the other
General Discussion Of Oscillator Characteristics:
In discussing how oscillations are maintained we stated that the
plate current increased to a value established by the emission characteristics
of the tube. An oscillator tube functioning in this manner will
not operate very long as it will lose its emission and become defective.
It is for this reason desirable to provide a self-biasing resistor
having a value of resistance which causes the production of the
automatic C bias voltage that will give class B or C operation of
the oscillator tube. Lower efficiency of operation is obtained when
using either class B or A operation.
The self-biasing grid
voltage developed should limit the peak plate current rather than
the emission characteristics of a tube in a well designed oscillator
circuit. The ability for the self-bias voltage developed to limit
the plate current flow is often referred to as a "braking action"
that limits the grid A.C. voltages for a fixed amount of excitation
and prevents them from reaching unsafe values of operation. I will
add that if we get a clear picture of what takes place in an oscillator,
the effects of changing any factor in the circuit can be explained
very easily. Let us see what is the basic action in an oscillator
When the oscillator reaches its final oscillating
condition, we know that the grid is driven sufficiently positive
to produce grid current which in turn develops across the grid resistor
a definite negative C bias voltage. This voltage establishes a new
operating point on the Eg-Ip characteristic curve. The A.C. grid
voltage drives the grid positive and negative with respect to the
operating C bias value as shown in Fig. 1B, always sufficiently
positive so it creates this C bias voltage. The plate current flows
only during that portion of the grid cycle when the grid voltage
is less than the cut-off value. This is the point where the grid
voltage stops the flow of plate current. The higher the excitation
the smaller the operating angle for the plate current. For class
B operation the operating angle for the plate current will be less
than 180 degrees.
In Fig. 4A and B we find the plate and
grid voltage curves respectively. Note that the A.C. grid voltage
decreases to a maximum while the A.C. tank circuit or plate voltage
increases to a maximum. This is the correct phase relationship between
the input and output voltages of a tube used in an oscillator. The
plate current as shown in Fig. 4C and as ip pulse represents the
driving power to sustain oscillations for it is this change in current
that is fed back into the tank circuit to set this resonant circuit
into natural oscillation. The area of this pulse when we view it
as a graph represents available oscillating power, the greater the
area of the plate current pulse the more the power available. Technically
speaking any increase in peak current, any increase in the operating
angle, and any trend to make the size of this pulse steeper and
more flattened on top indicates more operating power for all of
these factors increase the area of the plate current pulse ip.
The amount of power consumed by the oscillator has a number
of important functions to perform. It must overcome the losses in
the tank circuit (resonant circuit), overcome the power 'lost in
the grid resistor, the power lost in the grid-cathode of the tube,
overcome the power dissipated in the plate-cathode circuit of the
tube supply power to the load and any other incidental circuit losses
as well as to develop enough excitation to drive the grid circuit
of the tube in order to produce the correct amount of plate current.
If we assume that there is a tendency for the A.C. grid voltage
to increase then immediately the grid current increases and consequently
the bias voltage becomes greater. This in turn reduces the operating
angle of the plate current pulse, even though the current peak may
tend to rise. Less power will then be available for oscillation
and the braking action takes place preventing more power from reaching
the circuit and consequently preventing the grid excitation to increase.
If the grid A.C. voltage drops, the C biasing voltage is automatically
reduced as the grid current is reduced and in turn the plate current
flows over a greater portion of the cycle, resulting in the application
of more power to overcome losses and again drive the grid up to
a point where all losses are supplied and the grid excitation is
sufficient to sustain the oscillating condition.
Fig. 4. - These plate and grid currents and voltages represent
operating conditions in the oscillator circuit of Fig. 3.
Remember that graphs like these are always read from left
to right. When comparing two voltages, that one which reaches
a positive peak closest to the vertical reference line is
said to lead the other; thus, νT in A leads eG in B.
braking action is the inability of the oscillator circuit to draw
enough power to take care of all current demands, and as a result
the circuit sets itself to a definite operating condition and balance.
For example, if we increase the grid resistor value from a low value
to a slightly higher value the initial action results in an increased
C bias voltage. As this takes place the operating point on the Eg-Ip
curve of the tube is further negative. The grid, however, must draw
current to supply automatic C bias voltage and to do this the A.C.
grid excitation increases. Because of this grid circuit action the
peak plate current goes up slightly but at the same time the operating
angle decreases. The power drawn by the circuit depends on both
the operating angle and the peak plate current. If we increase one
and decrease the other by a small amount the circuit will draw more
power increasing both the grid excitation and C bias voltage. Again
the peak plate current increases but it is perfectly possible for
the operating angle to decrease so much that the amount of power
drawn starts to decrease at the point where maximum power is drawn
from the supply balance occurs and the circuit conditions are stabilized.
Of course we can make the grid resistor so high in its ohmic value
that this condition of maximum power is far below any condition
which would exist for normal circuit values, and a large grid resistor
may actually produce less power in the oscillator circuit than normal
low grid resistor values. Increasing the grid excitation using a
given grid resistor may also decrease the power developed because
the operating angle is decreased more than the peak increase in
If we increase the D.C. plate voltage, the
Eg-Ip characteristic instead of being held at the operating point
as shown in Fig. 1B will move toward the operating point b. Therefore
for an increase in plate voltage the grid current will be greater
as the grid voltage is increased. and consequently a higher negative
C bias will be produced as it will be required in order to cut off
plate current, Since the grid A.C. voltage must always drive the
grid positive to produce the automatic C bias voltage and as this
C bias voltage is greater than the cut-off bias, the result is a
much stronger current pulse; the plate current increases, as explained
above. Under this condition more oscillating power is available
and greater power will be received from the oscillator.
With the usual testing instruments available to servicemen, only
the D.C. plate voltage, the D.C. plate current and the self-biasing
grid voltage can be measured with a voltmeter having a high resistance
per volt rating. When there is any increase in the automatic D.C.
grid voltage for a given value of grid resistance we have an indication
of more A.C. tank voltage. This fact should be remembered and taken
into consideration when servicing oscillators.
Posted March 13, 2014