· Definition of the term Intermodulation Distortion : A form of interference involving the generation of interfering beats between two or more carriers according to the frequency relationship f = nf1 ± mf2,
· where n and m are whole numbers (but not zero), with appropriate expansion for additional carriers
· For additional definitions click on http://www.google.com/search?q=define:Intermodulation+Distortion
· I've also Included an article I found in QST about Intermodulation which gives working definitions and methods for calculating the inputs and products and examples of fixing the problem
· I've also included an email (see below) Sent to Andy 12/03/2002 giving experiments that I had carried out and some examples of successful things I've done in the past to clear up intermod in repeaters I've had the responsibility of servicing
· Last but certainly not least I've included A rough update of some of the work carried out in the last few weeks
Email from k1ugm to Andy on 12/03/2002 ; Subject Intermod on 64
Hi Andy ,
I just want to pass along some tests and calculations that I made on and for the 64 repeater concerning our noise problem and the 147.240 repeater.
The following equation is for the intermod product that I feel is most likely giving 64 the noise issue that 64 has been experiencing over the last several weeks.
From the model or theory; The most likely intermod relationship is
The Possible Solutions
Brief Status 03/15/2004 k1ugm
QST May 1983 17
With examples for fixing the problem
Please note the red lettered parts of the article
A QST Article by David W. Potter, W1GZD
What is Intermodulation and how do you cure it.
Intel-modulation (IM) is defined as the undesired mixing of two or more frequencies in a nonlinear device, which produces additional sum-and-difference frequencies. Problems with IM seem prevalent in the vhf and uhf bands, because amateur repeaters tend to cluster around hills and mountain-tops in close proximity to commercial and government vhf and uhf radio services. The intermodulation problem has been understood for years, but a review of the subject can be helpful.
Types of Intermodulation
The simplest kind of intermodulation is the mixing of two frequencies.
The equation for second-order IM
fIM =fl ± f2
where fIM is the frequency of the IM product, and fx are the mixing frequencies. Intermodulation products are the sum and difference of the two mixing frequencies. These are similar to the familiar products deliberately generated by the mixing process used in super heterodyne receivers. Note that if f1 and f2 are frequencies within an amateur band fIM must be an out-of-band signal.
Third-order IM products can produce both in-band and out-of-band signals when f1, f2; and f3 are all in-band frequencies as given by the equation
flM = f1 ± f2 ± f3 (Eq. 2)
In-band signals are generated by the sum of any two frequencies minus the frequency of the third signal. The out-of- band signal is the sum of all three frequencies. A special case of Eq. 2 is
>>>>> fIM = f1 + f1 - f2 = 2f1 - f2 <<<<<<< (Eq. 2A)
Here, the second harmonic of an in-band signal can beat with a fundamental frequency to produce another in-band signal.
Fifth-order IM products are given by
flM = f1 ± f2 ± f3 ± f4 ± f5 (Eq. 3)
with special cases:
flM = 3f1 ± 2f2 (Eq. 3A)
fIM = 3f1 ± f2 ± f3 (Eq. 3B)
fIM = 2f1 ± f2 ± f3 ± f4 (Eq. 3C)
fIM = 2f1 ± 2f2 ± f3 (Eq. 3D)
Some fifth-order products are in-band and others are out-of-band. Notice that second- and third-harmonic signals may be involved. Odd orders of in-band mixing frequencies produce some in-band products, but when out-of-band mixing frequencies are involved, even-order products may fall in band!
The above equations are valid for steady carriers, and it is easier to understand the concept of intermodulation by using them. Most of the signals we deal with are not steady carriers, however, but modulated ones.' The bandwidth of an IM product may be wider than the' bandwidths of the individual signals. This is because the instantaneous frequency of the product is the algebraic sum of the instantaneous frequencies of the mixing signals. For fm, it would be equivalent to adding three voice signals together in a wide-band fm transmitter. Assume that the audio amplitude is limited on each signal to produce a deviation no greater than 5 kHz. When the three signals are added together, they could produce an IM product having much greater deviation than any of the individual signals.
Intermodulation may involve any number of frequencies, but let's concentrate on the more common third-order Types. I will show some examples to make These abstract concepts more meaningful
Field Examples of Intermodulation
Consider - two repeaters that are physically located close to each other. One repeater transmits on 146.70 MHz, with a 146.10-MHz input, and the second has a 145.31-MHz output and a 144.71-MHz input. Assume that the .31 repeater output causes the .70 repeater first receiver stage to be driven into nonlinear operation (overload). This means that mixing of all frequencies seen by the first stage will occur.
Case 1: The .70 repeater is off, but the 31 machine is operating and a local 145.50-MHz simplex signal is present. The 31 repeater input and output signals and the simplex signal mix to produce an IM product on 146.10MHz: 145.31 - 144.71 + 145.50 = 146.10 MHz. This signal can key up the .70 machine, which will then repeat both the .31 machine and the simplex conversation. We will disregard the other IM products that are generated.
Case 2: The simplex station is off the air. The .31 machine is repeating, and the input to the .70 repeater drops, but the output is still up. An IM signal is produced: 146.70 - 145.31 + 144.71 = 146.10 MHz. The 0.70 machine will now repeat the signal from the .31 repeater until someone overrides the IM signal — provided that the 0.70 repeater doesn't time out beforehand. If the system gain is adequate, the repeater could feed back on itself with a resulting characteristic audio howl.
Case 3: The 0.31 repeater shuts down. The input to the 0.70 machine drops, but the output is up and stays up. You may hear another signal on the output, or the repeater may break into oscillation. Why? You find a strong signal at 147.30 MHz that is' overloading the repeater receiver, causing it 'to be nonlinear. The strong signal mixes with the second harmonic of the repeater: 2(146.70) - 147.30 = 293.40 - 147.30 = 146.10 MHz. This signal falls on the .70 repeater input.
These cases use the popular 2-meter band for illustration. Keep in mind that intermodulation can occur on any band, and can entail an endless combination of frequencies.
Receiver as the Culprit
Usually, the first stage of a receiver is the one most likely to overload, causing susceptibility to intermodulation. In some cases, however, later stages may be at fault. Receiver front-end nonlinearity occurs at relatively low input voltage levels, so the signal power involved is small. Therefore, the IM products generated there are also low in amplitude. These signals are fed to the receiving Antenna and are radiated. If you track down these weak IM signals, they will lead you back to the receiver site!
There is a big temptation to use a preamplifier to increase the sensitivity of a receiver or the range of a repeater. The use of these devices in repeater service is strongly discouraged.
Preamplifiers may not have the dynamic range that early stages of communications receivers have, and they usually lack the front-end selectivity found on well-designed receivers. Therefore, they are red-hot candidates for intermodulation. Preamplifiers may generate signals on the input frequency of the repeater, and users have to override these signals in order to be heard. The use of a preamplifier can sometimes degrade system performance!
Reducing Receiver IM Susceptibility
It is important to realize that it takes only one signal, located anywhere in the spectrum, to drive a circuit into nonlinearity, which could produce IM products when one or more other signals are present. In order to minimize intermodulation, your receiver circuitry should provide great attenuation to all frequencies except the band of interest. The receiver dynamic range should also be as large as possible. For this idealized case, only a huge signal in the pass band could possibly cause overload and nonlinear operation. Any resulting IM signals falling outside of the receiver pass band would be severely attenuated.
Transmitter as the Culprit
Serious intermodulation problems can be generated by transmitters when other strong signals present on the antenna (and coupled to the final amplifier) mix with the fundamental and its harmonics. Here the voltage and power levels are much greater than those associated with receiver circuits, so relatively strong IM signals may be coupled to me transmitting antenna and be radiated. The transmitter final amplifier is essentially an rf switch, and unless it is operating Class A (which is uncommon) it will be a nonlinear stage. Class C operation is more likely to cause intermodulation than Class AB1 because it is more nonlinear, wide-band, solid-state amplifiers with low-Q circuits tend to be more susceptible to intermodulation than are narrow-band, high-Q configurations.
Reducing Transmitter IM Susceptibility Fm transmitter final amplifiers are quite nonlinear, and intermodulation can occur if other signals mix in this stage. The Q of most final stages is not high, even for tuned final amplifiers, so the resulting bandwidth is wide. Increasing circuit Q helps the IM problem, but it may be undesirable for other reasons. Eliminating intermodulation in transmitters is, therefore, more difficult than in receivers. The use of a circulator is effective because It presents a very low impedance to signals Going from the transmitter to the antenna, But the high attenuation to signals going from The antenna to the transmitter. Since the circulator must carry full transmitter power, it is an expensive cure for inter- modulation. When duplexers are used at repeater sites, a degree of attenuation is introduced for out-of-band signals, but this alone may not be sufficient if other transmitters are nearby.
Conclusions In-band intermodulation products will degrade or destroy your station performance, in addition to interfering with other amateur communications. Out-of- band IM products may play havoc with non-Amateur Radio services. Further- more, transmitting spurious signals, such as IM products, is illegal. Conscientious operators are knowledgeable about inter- modulation and ensure that their stations are free from it.
Article written By David W. Potter* w2gzd QST May 1983 17 OCR-ed from a copy of the article was done by Jim Morris K1UGM
David W. Potter, W1GZD, is a spokesman for the Long Island CAME Association (Group Against Malicious Emissions). If it sounds like a hunting club, you're right! The group is engaged in tracking down interference/or local repeater clubs. David's inspiration to write this article came about as a result of his involvement with the group. Persons interested in working with the GAME Association are invited to contact him.