How to Make Third Order Intercept (TOI) Measurements

Learn how to characterize weak nonlinear systems and devices.
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Transcript:
When two or more signals are modulated, they produce a form of distortion called intermodulation products. These intermodulation products are distortion that results from nonlinearities in a system. Intermodulation products can prove to be quite problematic because of their close proximity to fundamental frequencies. Today, we’ll discuss how you can characterize your device’s third order intercept value so that you can measure for weak device components and model your system’s nonlinearity.

Hi, I’m Ally, and welcome to Keysight’s Rapid Measurement Series. In this video, we’ll be exploring the third order intercept or TOI.

The problem is that intermodulation products tend to be in the same band as our fundamental frequencies, so we can’t just filter them out.

By intermodulation products we mean that if you have a device that is transmitting a modulated signal, it will create additional signals at frequencies that are not just harmonics. It also creates the sum and difference of the original frequencies.

Another issue we need to be wary of is the fact that intermodulation products’ power levels increase by a factor of three, relative to an increase in the power level of the fundamental tones.

TOI is useful for characterizing weak nonlinear systems and devices such as amplifiers.

The TOI measures your device’s linearity.

To find the TOI, you have to plot out two things- the output power of your fundamental and the output power of your intermodulation products over varying input powers.

It should look something like this.

The third order intercept point, is the extrapolated intersection point of the fundamental power curve and the intermodulation power curve. We can’t measure it directly because our amplifiers aren’t designed for that level of input power, but you can find it mathematically by extrapolating the two curves.

At this point, the power of the fundamental is the same as the power of the intermodulation products.

A high TOI value means your system has good linearity. A low TOI value means your intermodulation products could interfere with your fundamental signal.

The specific TOI requirements depend on the technologies you are working with.

TOI can be measured manually by sweeping through input powers and measuring the fundamental and intermodulation powers. Or some signal analyzers can do it automatically- kind of like a bode plot.
Using this signal analyzer, we are measuring a two-tone modulated signal. We can automatically compute the third order intercept of these signals and place markers on the trace to indicate the measured signals and their third order products.

Here we’ve calculated the third order intercept of a two-tone signal. The TOI measurement begins by taking a sweep of the incoming signal, then it measures the two fundamental tones, and finally measures the third order intermodulation products.

With this measurement table we can also see the absolute power of each fundamental tone, the absolute and relative power for both the upper and lower intermodulation products, and the power of the third order intercept.

This information helps us accurately measure intermodulation and determine how linear our system is. Having a signal analyzer with good dynamic range also gives us the ability to measure the TOI point we are looking for. The better the dynamic range, the improved ability our signal analyzer has to measure harmonically related signals and intermodulation products needed to be able to characterize the system under test.