What is Intermodulation Distortion?

Intermodulation Distortion (IMD) occurs when two or more signals are used in a non-linear system. The spectrum at the output of the non-linear device will not only consist of the original signals but will also contain the sum and difference of the input signals along with their harmonics. Hence, if a non-linear system has two signals at its input, say at frequencies f1 and f2 then the non-linearity will give rise to other output signals at various frequencies, i.e., f1 + f 2, f2 – f1, 2f1 and 2f2 which are also known as 2^nd order intermodulation products. The products 2f1 and 2f2 are known as “Harmonics” which are nothing but replicas of a signal appearing at integer multiples of the fundamental signal. These sideband frequencies are considered to be undesirable.

In the next step, these 2^nd order intermodulation products will mix with the original signals that will give rise to 3^rd order intermodulation products. It should be noted that each individual signal, including original signals (f1 and f2) and 2^nd order intermodulation products (f1 + f 2, f2 – f1, 2f1 and 2f2), will be added and subtracted with/from each other to give rise to more signals. Most of these intermodulation signals are not close to the original signal so can easily be filtered out or do not cause issues. However there are two intermodulation products i.e., 2f1-f2 and 2f2-f1 which can be troublesome and can cause interference as they are close to the original signal. These products are known to cause intermodulation distortion.

The typical way of dealing with these troublesome IMD products is through filtering, but this becomes difficult when the products are very close to the desired (fundamental) frequencies. These 3rd order intermodulation products are of great concern as they are difficult to filter out, unlike 2nd order distortion products, which appears to be at a greater distance w.r.t the original signals (f1 and f2). The additional frequency content created by the 3rd order distortion is often referred to as ‘Spectral Regrowth’. In a transmitter, spectral regrowth resulting from poor linearity can interfere with other wireless channels. In a receiver, by contrast, it can cause out-of-band signals to distort the signal of interest. IMD and harmonics create leakage into adjacent channels, noise or distortion, which degrades the overall systems performance.

Some Interesting Things to Note:

• The amplitude of harmonics usually decreases as the harmonics order increases. Higher-order harmonics have very low amplitudes and thus can usually be ignored. And most of the higher-frequency products often fall outside of amplifier bandwidth, filter passbands, etc.
• Third-order intermodulation products have an amplitude proportional to the cube of the input signal whereas second-order components have an amplitude proportional to the square of the input signal.
• The best way to avoid IMD products is to operate the device in the linear region before it starts generating distortions.
• The order of harmonics and intermodulation is the sum of their (unsigned) coefficients:
  • 2f1 is second-order (2)
  • f1 + f2 is also second order (1+1)
  • 3f1 is third order (3)
  • 2f2 – f1 and 2f2 + f1 are both third-order (2 + 1)