IP3, or third order intercept point is a way of stating how non-linear a system is. For example, often seen on datasheets, it is an easy metric to compare linearity between amplifiers. Linearity is important in a communication system as it effects the BER vs EbNo curve, the spectral regrowth (or adjacent channel power) and EVM (error vector magnitude) – if a circuit block is not designed to spec or the spec is not correctly defined, then the product may fail verification to a standard or the link performance (eg range or bit-rate) may not meet the target specification.
The graph above shows the output power in dB on the Y-axis and the input power in dB on the X-axis. With a small input signal, the amplifier is said to be operating in small signal conditions and the output is simply a copy of the input amplified (see blue curve). As the input is increased, there comes a point when the output increases less than the input – for example, a 2dB increase in the input may produce only a 1dB increase in output – the amplifier is said to be compressing.
When two signals at different frequencies (f1 and f2) are applied to the input, then two signals of increased amplitude are observed at the output. (Note: it is much easier to view this with a spectrum analyser (X-axis = frequency, Y-axis = power, dB) than with an oscilloscope (X-axis = time, Y-axis = amplitude).) However, the non-linear effect of the amplifier also creates some additional unwanted frequencies at the output of the amplifier that were not provided at the input. The third order products of interest are usually 2f1 – f2 and 2f2 – f1 as these are close to the wanted output f1 and f2 and cannot easily be removed by filtering.
On the graph, the purple line shows the output power level of the 3rd order spurs on the Y-axis and the input power of the two tones on the X-axis. The red line shows the linear region of the power gain plot extrapolated upwards (a region that is not found in practice). The point where the red line and purple line intercept is called the third order intercept point. It can be read on the Y-axis as output intercept point 3 (OIP3) or on the X-axis as input intercept point 3 (IIP3)
For wideband systems, such as HF radios, the second order distortion capabilities of the RF circuit are also important. For example, the HF radio could be receiving a strong signal at 5MHz and a strong signal at 7MHz. The front end of the radio would create an unwanted spurious product at 2MHz (f2-f1) and 12MHz (f1+f2). A desired wanted signal at 2MHz or 12MHz could be blocked by the unwanted spurious falling at the same frequency. OIP2 and IIP2 can be found at the intercept of the red line and the dark green line.
Next time you read a datasheet for an amplifier, have a look for IP3 and IP2!