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.
National 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 entitled National Radio News
was published by them 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 are established.
More is known nowadays regarding actions at the atomic level regarding how oscillations
begin, but the fundamental principles are the same.
Oscillators - How They Work
By J. A. Dowie
J. A. Dowie, NRI Chief Instructor
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?
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.
Phase Relationship 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.
Figure 1A - Oscillator biasing.
Figure 1B - Operating point.
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.
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
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 an oscillator.
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.
In 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.
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.
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.
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.
Fig. 3 - Inductive feedback Hartley type configuation. (A) C
battery to enable class A operation, (B) B battery voltage is applied instantly,
(C) Automatic C bias supply.
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.
Excitation Regulation: 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.
Oscillator Output: 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 values.
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 circuit.
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
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.
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.
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.
The basic 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 plate current.
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 24, 2022
(updated from original post on 3/13/2014)