### 1. Measuring IMD

If *F*1 and *F*2 are the frequencies of the two tones, then the third-order distortion products occur on both sides of these tones at 2*F*2 – *F*1 and 2*F*1 – *F*2. Assuming that the power levels of the two tones are equal, *IMD*3 is the difference between the power of the fundamental signals and the third-order products, as defined in the following equation:

where *o* refers to the output of the UUT, *Po*3 is the power level of one of the output third-order products, and *Po* is the power level of one of the fundamental tones.

Once the *IMD*3 is measured, calculate the UUT output third-order intercept point (*OIP*3) using the following equation:

The input third-order intercept point (*IIP*3) is defined as:

where *G* is the gain of the device. The *IIP*3 number quantifies the third-order linearity of a device. Use the *IIP*3 specification of the RF Vector Signal Analyzer (VSA) as a guide to optimize its settings when measuring the *IMD*3 of an external device.

The two tones injected into the UUT must be free from any third-order products. These two tones are combined, or summed, at or before the UUT input. If they are not well isolated, they intermodulate with each other and cause distortion. A signal combiner with good input-to-input isolation is recommended to minimize distortion of the input tones.

### 2. Measurement Setup

A typical *IMD*_{3} measurement setup is shown in the figure below. Lowpass filters are employed at the PXI-565x RF Signal Generator outputs to suppress harmonics.

**Typical IMD_{3} Measurement Setup**

### 3. Understanding Two-Tone Third-Order Intermodulation Distortion Limits of the RF VSA

The RF VSA generates its own distortion spurs, which are capable of swamping the *P*o3 of the UUT and giving rise to an erroneous measurement. Too much power at the signal input of the RF VSA may drive the system into a nonlinear region of operation and produce very large distortion products. Choosing an appropriate attenuation setting for the RF VSA minimizes its *IMD*3 contribution to the measurement. The *IMD*3 improves by 2 dB for every 1 dB of input power decrement.

To measure the *IMD*3 of a UUT, input power to the RF VSA mixer must satisfy the following condition:

where *IIP*3rfsa is the input third-order intercept point of the RF VSA (about 10 dBm). For example, to accurately measure an *IMD*3 of 80 dBc the input power to the mixer must be less than –31.5 dBm.

If the powers of two-tone signals generated by the RF signal generators are larger than this optimal level, they must be attenuated, either with the attenuators internal to the RF VSA or with external attenuators. However, as attenuation raises the noise floor of the RF VSA, there is a limit to how much attenuation can be used before noise overwhelms the distortion spurs. Its spurious-free dynamic range (SFDR) specification indicates the largest *IMD*3 value the RF VSA can accurately measure, assuming 0 dB attenuation and input signals whose powers satisfy the above equation.

### 4. Choosing an Optimal Setting for the RF VSA

Complete the following steps to set optimal attenuation levels for an IMD3 measurement when the level of the third-order distortion spur (*P*o3) is unknown:

1. Set the attenuation so that the input power at the mixer is about –30 dBm. When using the RF VSA Demo Panel, *mixer level* = *reference level* – *attenuation*.

2. Tune to the third-order distortion product frequency of interest, either 2*F*2 – *F*1 or 2*F*1 – *F*2. Then decrease the resolution bandwidth until a distortion spur appears.

3. Increase the attenuation level.

4. If the harmonic spur decreases, repeat step 3.

5. Repeat step 4 until the harmonic level does not decrease any further. Attenuation does not lower the distortion products of the signal; it only lowers the distortion products generated internally to the RF VSA. Decrease the resolution bandwidth to lower the noise floor.

This setting is the optimal attenuation setting.

### 5. Resources

Common RF and Microwave Measurements

This is the main page of a series of tutorials focused on common RF measurements involving signal generators and analyzers

### 6. Related Products

NI PXI-5660 2.7 GHz RF Vector Signal Analyzer The National Instruments PXI-5660 is a modular 2.7 GHz RF vector signal analyzer with 20 MHz of real-time bandwidth optimized for automated test. NI PXI-5671 2.7 GHz RF Vector Signal Generator The National Instruments PXI-5671 module is a 3-slot RF vector signal generator that delivers signal generation from 250 kHz to 2.7 GHz, 20 MHz of real-time bandwidth and up to 512 MB of memory. NI PXI-5652 6.6 GHz RF and Microwave Signal Generator The National Instruments PXI-5652 6.6 GHz RF and microwave signal generator is continuous-wave with modulation capability. It is excellent for setting up stimulus response applications with RF signal analyzers. NI RF Switches The National Instruments RF switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz to bandwidths as high as 26.5 GHz. NI LabVIEW The National Instruments PXI-5660 is a modular 2.7 GHz RF vector signal analyzer optimized for automated test. It provides high-throughput RF measurements in a compact, 3U PXI package. NI Modulation Toolkit The National Instruments Modulation Toolkit extends the built-in analysis capability of LabVIEW with functions and tools for signal generation, analysis, visualization, and processing of standard and custom digital and analog modulation formats. NI Spectral Measurements Toolkit The National Instruments Spectral Measurements Toolkit provides a set of flexible spectral measurements in LabVIEW and LabWindows/CVI, including power spectrum, peak power and frequency, in-band power, adjacent-channel power, and occupied bandwidth, as well as 3D spectrogram capabilities. NI Advanced Signal Processing Toolkit The National Instruments LabVIEW Signal Processing Toolkit is a suite of software tools, example programs, and utilities for time-frequency analysis, time-series analysis, and wavelets. It also includes a full version of the NI LabVIEW Digital Filter Design Toolkit. which is also available separately. NI Digital Filter Design Toolkit The National Instruments Digital Filter Design Toolkit extends LabVIEW with functions (LabVIEW VIs that install into the palette) and interactive tools for design, analysis, and implementation of digital filters. |