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 in 1941 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 (PIM) spectral products.
This diagram shows graphically the action taking place in a superheterodyne
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
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 different
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
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
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 produced.
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 been combined.
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
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
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
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 interference.
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.
When 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 audio signal.
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 chan-nel 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 great distances.
*Service Department, RCA Manufacturing Co., Inc. Illustrations courtesy "Radio
Physics Course" - An Elementary Text Book on Electricity and Radio, by Alfred A.
Posted December 15, 2021(original 12/1/2014)