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 (PIM) spectral products.
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 different frequencies.
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 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 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 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 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 great distances.
*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