| Radiocommunications Agency
What this technique is used for
Spurious noise frequencies are created when the amplitude of a signal at one frequency is affected by the amplitude of a signal at another frequency when both are applied to a non-linear circuit at the same time. Combination frequencies are produced which are equal to the sum and difference of the two frequencies, and to a lesser extent their harmonics. These frequencies, not present in the sources, may coincide with those of a victim receiver and create unexpected interference.
How this technique is used
In radio receiver and transmitter circuit design: bandpass and notch filtering is employed to ensure that only the wanted frequencies are applied to non-linear devices.
In other types of circuit design: filtering and shielding is used to prevent unwanted frequencies from being applied to non-linear devices.
As a general rule, non-linearity should be kept to a minimum in all circuit design, but there are bounds to this approach. Any extraneous sources of non-linearity, such as inadequate bonds should be eliminated.
Key issues in employing this technique
Non-linearities and intermodulation
A non-linearity is a circuit feature which has a gain, attenuation or impedance that varies according to the magnitude of the voltage or current it is exposed to.
It is very difficult to get electrical and electronic circuits linear enough for many applications. Indeed, in radio receivers it is necessary to use non-linearity deliberately in order to create intermediate frequencies at which narrowband filtering can be applied.
When signals at two different frequencies (f1 and f2) are applied to a non-linear circuit such as, in the simplest case, a semiconductor diode, they create “intermodulation products” at their sum and difference frequencies (f1 - f2 and f1 + f2). Any harmonic distortion present on the original signals also causes its own intermodulation. Multiple signals create multiple intermodulation products.
Because of the nonlinearities, the amplitude of intermodulation products depend on the signals that gave rise to them in a square-law or cube-law fashion, and are always lower in amplitude than these signals. In many cases the resulting interference is only of concern to very sensitive radio receivers operating on weak signals. But this is still significant for the design of radio receivers themselves, and it can also be important for radio installations where many transmitters are co-located with receivers operating on different frequencies.
All semiconductors and thermionic valves are non-linear
All active devices (e.g. semiconductors, valves) are non-linear. When they are used in ICs and in circuits they are biased into their ‘linear region’ – but this is not usually very linear so a number of techniques (such as feedback) are used to improve linearity.
Intermodulation caused by two or more frequencies being present at the same time in a semiconductor is sometimes called ‘active intermodulation”. Once the two (or more) RF signals have got into an active device (e.g. one through the input pins and another through the power supply pins) intermodulation products will be created regardless of any linearising techniques (such as feedback) that are being used.
Since feedback and other linearising techniques don’t prevent intermodulation, it is necessary to resort to careful cabling routing, cable shielding, PCB layout, filtering and shielding to ensure that active devices are only exposed to the frequencies that are supposed to be present.
Electrical joints and connections are non-linear
“Passive” intermodulation is caused by two or more frequencies being present at the same time in a non-linearity in an electrical joint (connection) or similar situation. This is more commonly an installation problem.