Nov. / Dec. 1941 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Images, harmonics of the intermediate frequency
(IF), harmonics of the local oscillator
(LO), multiple station IF mixing,
inductive and capacitive coupling, other types of noise can find
a way into circuits if sufficient shielding and judicious component
placement is not implemented. It is as true today as it was sixty
years ago when this article appeared in Radio-Craft magazine.
An interesting interference generator discussed is that of heterodyned
signals generated external to the receiver by means of random nonlinear
junctions reacting to multiple high power broadcasting stations
in a local area, as was fairly common when AM stations were the
norm. Rusty bolted joints in buildings, towers, even automobiles
can be the source of such phenomena. Even today it is not uncommon
for bolted and riveted junctions on antennas and RF connectors to
generate what are now termed passive intermodulation
W. J. Zaun
Many forms of radio interference occur, each arising from particular
causes or circumstances, and each producing typical effects upon
reception. Disturbances such as man-made and natural static are
rather complex in nature, and are not subject to simple analysis
nor cure. Other interference phenomena, however, which are associated
with signals having definite wave character, frequency disposition,
and intensity, and which bear a relation to receiver characteristics,
may be examined conclusively. The more commonly experienced troubles
of the latter type are treated herewith.
I. Image Response
This diagram shows graphically the action taking place
in a superheterodyne receiver.
On the basis of present practice in superheterodyne design, when
a receiver is tuned to a given signal, the local oscillator is at
a frequency the amount of the I.F. above the signal frequency. The
difference in the oscillator and station frequencies is the nominal
I.F. and signals of this frequency are amplified and transmitted
to the second detector of the receiver for demodulation. Should
a second incoming signal be present, whose frequency is above the
frequency of the local oscillator by the amount of the same I.F.,
it will likewise tend to combine with the oscillator and produce
a difference beat which will appear in the I.F. system, and finally
at the second detector stage. The interference is heard as a "whistle"
or as mixture of modulations of both signals. In this case, considering
the oscillator at a particular frequency, there is a signal below
it, by the amount of the I.F., and there is a signal above it by
the amount of the I.F. The undesired second signal when attenuated
or when not allowed to mix with the oscillator, causes no interference.
However, if it is possible for this signal to reach the first detector
stage, it will also beat with the local oscillator signal when tuning
to the desired station. This condition is referred to as "image
response." It is a function of the degree of selectivity ahead of
the input to the I.F. system.
Effects of interference from this cause may be reduced by suppressing
the strengths of the undesired local stations which are producing
the images. This can be done by reducing the receiving antenna efficiency,
or by using wavetraps tuned to the "image" station. It must be noted,
that harmonics of local broadcast stations, harmonics of the local
oscillator, and stations operating outside the limits of the standard
broadcast band, oftentimes are sources of "image" interference.
Particular attention must be given to the 1700 kc. Police Band,
the 2000 kc. Amateur Band, and the 2500 kc. Police Band, in cities
where image interference exists. Variation of the I.F. is another
means of correcting the condition.
II. Harmonic of I.F.
When a signal is being received whose frequency is twice that
of the nominal I.F.; or within a range of plus/minus 10 kc. of twice
the I.F., there will appear in the output of the mixer stage a second
order effect, or the difference between twice the signal frequency
and the oscillator frequency. Whenever the signal frequency is twice
the I.F., the normal I.F. will be produced in addition to the spurious
I.F. which is due to the beat between the second harmonic of the
signal and the heterodyne oscillator. Since the standard I.F. and
the extra I.F. vary at different rates as the receiver is tuned,
a whistle will be heard. Selectivity cannot discriminate against
this type of whistle as only a single signal is involved.
Since the number of cities having stations which operate on the
second harmonic of I.F. frequencies used at the present time, is
limited, this interference does not become of general concern, but
applies only to the particular locality where the station is situated.
Realignment of the I.F. stages of any receiver affected, is the
usual cure for trouble of this sort. It should be carefully noted
and checked as to whether the signal operating at the second harmonic
of the I.F. is being picked up on the underchassis wiring of the
receiver, in addition to the antenna. In this case the whistle produced
will be aggravated. In extreme cases, it is possible to eliminate
the whistle by providing a wavetrap, tuned to the second harmonic
of the signal, and placed in the circuit feeding the mixer stage.
III. Direct I.F. Response
When there is a signal present in the receiving locality, whose
frequency is the same as that used for the I.F. of the receiver,
or near thereto, direct pickup of the signal may take place and
interference will be reproduced. This interference is not affected
by tuning of the receiver, inasmuch as it has a frequency corresponding
closely to the fixed I.F. resonant circuits. It enters the receiver
through the antenna and first stage in most cases, but may be introduced
by direct induction to the I.F. system.
The degree of interference is related to the amount of I.F. attenuation
provided in the pre-selector circuits ahead of the I.F. system.
It is usually evidenced in the form of a "birdie," or in the form
of a tone, depending on whether the interfering signal is using
CW or ICW during its transmission. The stations which are apt to
give interference in the 450-470 kc. intermediate frequency range,
are used for code communication and are generally coastal shore
to ship stations.
Wavetraps in the antenna circuit tuned to the exact frequency
of the interfering signal, are effective in reducing this type of
interference. In some cases it is necessary to shift the I. F.,
either up or down, to get away from the interfering station. Use
of an RCA Magic Wave Antenna provides an attenuation of approximately
6/1 in the I.F. range.
IV. Harmonics of Oscillator
How "beats" are produced by the combination of slightly
The presence of short wave code or short wave broadcast signals
within the standard broadcast band is generally due to their combination
with the upper harmonics of the receiver's oscillator; the difference
of the station frequency and harmonic frequency being equal to the
I.F. Spurious reception of this type is most prevalent on receivers
employing loop antennas.
Electrically, an antenna loop has the character of a long line
having several resonances in addition to its fundamental tuning.
The secondary resonance effects may fall into and provide substantial
gain in causing an appreciable level of short wave signal to appear
at the first stage.
When this signal is of such a frequency as to combine with a
harmonic of the oscillator, and produce I.F., reproduction takes
place the same as with the fundamental signals.
Proper treatment of this type of interference should be along the
line of (1) Orienting loop for minimum pickup of interfering SW
station, (2) Re-aligning loop carefully, (3) Substituting conventional
antenna coil for loop, (4) Decreasing oscillator excitation by shunting
tickler section with a resistor, or taking turns off this same winding.
V. Combination of I.F.
Two stations in the same locality having frequency assignments
differing by the amount of the receiver I.F., may combine in the
early stages of the receiver, forming a difference beat frequency,
equal to the particular I.F. This combination usually occurs when
the first tube in the receiver is the mixer stage. It is not uncommon,
however, for an undesired I.F. signal to form in an R.F. stage or
possibly later in the I.F. stages, when the signals are of sufficient
intensity or the later circuits are not completely protected against
signal pickup. The presence of an extra I.F. signal, such as brought
about by this mixture of local stations, causes a "whistle" or "birdie"
to be reproduced when receiving carriers (not related in frequency
to interfering signals) over extensive sections of the tuning range.
The "birdie" is created by the audible beat resulting from mixture
at the second detector of the normal I.F. and the superfluous I.F.
signals. The latter is a constant frequency, while the former varies
with tuning. Therefore, a variable pitch audio note is produced.
Zero beat is obtained at the point of exact tuning.
Discrimination against this type of interference is gained by
providing ample selectivity ahead of the receiver stage which is
susceptible to the mixing phenomena.
For service, discrimination can be provided at the frequency
of one of the interfering signals, preferably the strongest, by
use of wavetrap or attenuator circuit, tuned to that particular
frequency so as to suppress its strength at the input of the susceptible
stage. In applying further practical remedies for this interference,
it often is essential to reduce antenna efficiency by decreasing
its length or adding a small capacitor (50-200 mmf.) between it
and the receiver input antenna terminal. If this treatment is not
effective, back-door points of signal entry, such as under-chassis-wiring,
grid leads and power circuits should be investigated. Shielding
of the susceptible circuits, and filtering of the power line at
the receiver with standard units, may be required. In some cases,
each possible point of entry may be contributing a component of
interfering signal and each must, therefore, be corrected separately
before satisfactory performance can be obtained. Realignment of
the circuits to a higher or lower I.F. will be beneficial. Change
of alignment by 10 kc. can usually be accomplished without serious
effect on the receiver.
The fact that harmonics of the local stations may subtract with
each other or with fundamentals of the same stations, to produce
a beat of the nominal I.F., must be considered as possible causes
for this type of interference.
Phenomenon of "beats," produced bythe combination of sound waves
of slightly different frequencies. The two light curves represent
sets of simple sound waves having a vibration frequency ratio of
8 to 5. The heavy curve, represents the resultant sound wave obtained
by adding the amplitudes of the individual curves together at various
points, due regard being taken of the relative directions at these
instants. Four points (R) of reinforcement (beats) and three of
interference (I) are produced.
VI. Heterodyne Oscillator Radiation
Another illustration showing how a "beat" frequency is
The tendency of the oscillator in a super-heterodyne to radiate
over a limited area occasionally produces interference in another
receiver, evidenced as a "whistle" appearing, disappearing and changing
frequency at random. This interference becomes prevalent and of
consequence in localities where two popular stations are separated
by the amount of customarily employed intermediate frequencies (I.F.).
For example, in a community having a station "A" at 600 kc. and
another "B" at 1060 kc., receivers using a 460 kc. when tuned to
"A" will have an oscillator frequency at "B" which will cause interference
on nearby receivers which are tuned to the 1060 kc. station.
Service procedure on cases of this nature should include one
or more of the following measures: (1) Install filter in power circuit
of receiver affected. (2) Install noise reducing antenna such as
RCA Magic Wave type on receiver causing radiation. (3) Realign the
radiating receiver to new I.F. (4) Position leads of radiating receiver
to reduce oscillator/antenna coupling. (5) Reduce excitation of
radiating oscillator. (6) See that good ground is attached to radiating
receiver. (7) Reduce size of antenna used with interfering receiver.
(8) Completely shield oscillator stage and filter its supply leads.
The production of a "beat frequency" (D) in an electrical circuit,
by the combination of two currents or voltages (A) & (B) of
differing frequencies. The "beat frequency" is equal to the difference
between the frequencies of the two voltages or currents which have
VII. Cross-Modulation Within Receiver
Two signals are said to be cross-modulated when the program of
an undesired station is superimposed upon the program of the station
to which the receiver is tuned.
As implied, the secondary modulation is directly associated with
the carrier being received, and is not evidenced except when tuned
to a carrier. In some cases, more than two or more stations may
be causing cross-modulation on another. Occasionally, cross-modulation
effects will produce extra responses at random points on the dial,
usually showing up as a mixture of two signals and their respective
modulations. Cross-modulation may occur on TRF as well as Superheterodyne
receivers. Its basic cause is usually related to demodulation of
an abnormally strong signal in the early stages of a receiver, and
the tendency to remodulate on other carriers existent in the same
circuits. Non-linearity of the circuit element or tube is, of course,
essential to this process. The degree of susceptibility of the first
stage to extraneous modulation is a matter of tube and circuit design.
Tubes employed in the first stages of modern receivers are of the
variable-MU types which have an extended cut-off characteristic,
enabling the application of a higher bias for reduction of signal
strength, without increasing the susceptibility of the stage to
detection or cross-modulation. Selectivity ahead of the first receiver
stage goes a long way to avoid the presence of undesired signals
on the grid of the first tube. In some receivers extra link circuits
are included for this purpose. The amount of cross-modulation varies
with the grid bias of the tube and since this bias is a function
of the developed automatic volume control voltage, the cross-modulation
is affected by the strength of the input signals.
Where it is necessary to make a service investigation of cross-modulation,
the identity of the station causing interference should be established.
Where a reasonable amount of selectivity is provided in the head-end
of the receiver, an ordinary wavetrap having good attenuation, and
tuned to the frequency of the interfering signal, will be effective.
It is possible, however, for the abnormal signal to enter the receiver
on circuits other than the antenna input. These circuits may be
the power line supply, direct pickup on the underside of the chassis,
direct pickup on the grid leads of the receiver, direct pickup on
the tubes of the receiver (if not shielded), and in some cases direct
pickup either on the chassis or on the ground circuit where this
is mutual to an R.F. circuit. A change of the voltage or operating
characteristic of the stage affected is not usually beneficial,
inasmuch as design determines the optimum point for the minimum
of interference. The principal idea to be kept in mind when working
on a receiver in an attempt to eliminate cross-modulation, is to
protect the susceptible circuits and to reduce the level of the
interfering signal voltage.
The importance of filtering the power circuit, having a short
low R.F. impedance ground and elimination of ground circuits that
are mutual to R.F. circuits, must not be minimized nor overlooked.
In many cases, wavetraps will not be sufficient where used singly,
but two or more may have to be employed. A parallel-tuned wavetrap
in series with the antenna and a series-tuned wavetrap in shunt
with the receiver input is the best combination for obtaining the
utmost attenuation against an interfering signal.
VIII. Cross-Modulation External to Receiver
This type of interference has become prominent in recent years,
due to the trend of increase in power ratings of transmitting stations.
When two radio waves of sufficient strength encounter any elevated
system of electrical conductors in which system there is existent
anything that causes partial rectification or detection, numerous
new spurious radio frequencies are created which radiate from the
system to nearby receiving antennas. When one of these interfering
frequencies happens to fall on a desired station frequency, interference
results which no receiver can avoid. The interference has no relation
to receiver design and will be present on all types including the
automotive, battery, A.C. or D.C. It is generally localized in a
particular community. The electrical system, whether it be power
distribution, telephone system, or other aerial network of conductors
and particularly any network or system which is resonant to the
local station frequency, can produce this interference if it has
a rectifying tendency. Rectification may occur from poor joints
or contacts, special non-linear devices intermittent or poor contacts
to earth or to other objects, and rectification due to chemical
action at a joint or splice. The neutral or grounding system for
power circuits is a frequent cause for generation of this type of
Wherever this trouble develops, it should be definitely identified
by checking with various types of receivers, preferably the battery
loop antenna type, so as to isolate the source and to determine
the limit of the area affected.
Most receivers suppress the 10,000 cycle beat from adjacent carriers
either by limitation of fidelity, high degree of selectivity, or
by design of the loud speaker unit to prevent its reproduction of
such a note. Filter circuits having sharp cut-off below 10,000 cycles
are sometimes provided in high-fidelity receivers for elimination
of the beat. A very effective means of accomplishing the same end
is through use of a tertiary circuit, consisting of a parallel-tuned
coil associated with the loudspeaker matching transformer. This
coil is tuned to a frequency slightly below 10,000 cycles and gives
a sharply defined attenuation and cut-off of high frequencies.
High-fidelity receivers usually contain a control for reduction
of selectivity which makes possible two degrees of fidelity. Where
interference from an adjacent channel beat note exists this control
may be reduced to effect its elimination. Tone controls also, are
normally included on modern receivers and are arranged to reduce
the audio response at the higher audio frequencies, including the
possible 10,000 cycle interference.
Under ordinary conditions, the ten kilocycle beat is not frequently
encountered, since the receivers subject to this interference are
usually in the higher-price brackets and elaborate filter protection
is justified in the original design. When encountered, however,
there are two methods of treatment; the one being suppression of
the adjacent channel causing interference with a sharply-tuned wavetrap;
and the second, reduction of the high frequency response in the
audio system of the receiver. Precise alignment of the receiver
may also be beneficial.
two signals occupy adjacent channels, separated as to carrier frequency
by 10 kc., the side-band frequency of one station is very close
to the side-band frequency of the adjacent station. If either station
is modulating more than 5 kc. of audio range, the two side bands
will overlap. In such a situation, if the side band of one signal
enters the second detector stage along with the side band and carrier
of the other signal, a peculiar combination of frequencies will
result. The most troublesome frequency formed by this combination
is that which is produced by the difference between 10 kc. and the
modulation frequency involved. For example, if the case is taken
where a 3000 cycle note is modulating the adjacent undesired channel,
it will produce an interfering side band which will be superimposed
upon the desired signal as a 7000 cycle note. That is to say, the
side bands of the adjacent channel station form a difference beat
against the carrier of the desired station, or the one to which
the receiver is tuned. This beat will be in the audible range and
will have the character of "inverted speech." This means that modulation
on the interfering station of low frequency will create an audible
signal of 10 kc. minus this frequency, or a resultant high frequency.
High frequency modulation, conversely, produces a low frequency
Since this interference is an inversion of the adjacent channel
modulation, it appears as an unintelligible mixture, commonly termed
"monkey chatter." Receiver selectivity discriminates against this
type of interference. It is also limited by proper restriction of
the high frequency audio response. The selectivity ahead of the
second detector is, of course, the principal factor in preventing
response to the adjacent channel modulation.
"Higher fidelity" receivers are generally the only types affected
by "monkey chatter" and their circuits are designed to afford the
necessary protection against same. A sharp cut-off filter circuit
included in the audio system is common practice in the design of
high fidelity instruments. Provision of control for the high frequency
end of the audio band and the broadness of I.F. tuning is also common
in high fidelity design. Over-modulation of the adjacent channel
station accentuates the interference due to "monkey chatter" because
of the higher frequency side bands which are generated by such over-modulation.
Over-modulation, however, is an unusual condition and should not
be investigated as the most prominent cause for this type of interference.
In general, "monkey chatter" interference will be more prevalent
at more points on the tuning scale in localities where the number
of popular stations is limited, and where such stations are at relatively
*Service Department, RCA Manufacturing Co.,
Inc. Illustrations courtesy "Radio Physics Course" - An Elementary
Text Book on Electricity and Radio, by Alfred A. Ghirardi. E.E.
Posted December 1, 2014