Intermodulation Distortion (IMD) Measurements Using the PNA-X

Application Notes

Introduction

This application note describes accuracy considerations when using the Keysight Technologies PNA-X microwave network analyzer for two-tone intermodulation distortion measurements. One of the unique attributes of the PNA-X is its inclusion of an internal second source and an internal combining network1, which together allow for an exceptionally convenient setup for two-tone intermodulation distortion measurements. Since these measurements are unique in a number of ways, it is important to understand how the attributes of the test system contribute to the measurement uncertainty, and how to best optimize the test setup to make accurate distortion measurements.

Table of Contents

 

  • Overview
  • Legend/Symbols
  • Receiver considerations
    • Low-level random noise
    • High-level random noise
    • Receiver generated intermodulation distortion
    • Combining receiver errors, calculating the error vector magnitude (EVM)
    • Measurement example
    • Setting the receiver attenuator
  • Source considerations
    • Port power available from combined sources
    • Impact of source harmonics
    • Source cross-modulation
  • Avoiding spurious signals in IMD measurements
    • 1. Center frequency protocol
    • 2. Tone frequency protocol
  • Appendix A: IMD Optimization tables for the PNA-X
  • Appendix B: The amplifier I want to test has an IP3 of +38 dBm. Can the PNA-X make this measurement?
  •  Appendix C: Quick checklist for two-tone CW IMD measurements using a PNA-X with dual internal sources

 

Overview 

Intermodulation distortion (IMD) is a measure of the nonlinearity of an amplifier. When two or more sinusoidal frequencies are applied to an amplifier, any nonlinear behavior of the amplifier will produce additional frequency components called intermodulation products. For an amplifier with input signals at f1 and f2, the output will contain signals at the following frequencies: nf1 + mf2, where n, m = 0, ±1, ±2, etc. The third-order products, 2f2 – f1 and 2f1 – f2, are a major concern because of their proximity to the fundamental frequencies; and the fact that their power levels increase by a factor of three, relative to an increase in the power level of the fundamental tones. Additionally, their proximity to the fundamental frequencies precludes their removal by filtering. The third-order intercept point (IP3) or the third-order intercept (TOI), often used interchangeably, are figures of merit for intermodulation distortion.

Receiver Considerations

The measurement receiver contributes to three important sources of error in an IMD measurement. These are low-level random noise, high-level random noise, and the intermodal generated by the network analyzer receivers. By individually characterizing these three error contributors, the total receiver error can be calculated. The errors due to low-level noise and high-level noise can be reduced by increasing the tone power or decreasing the IFBW. However, the problem is that once we increase the tone power, the error due to receiver distortion increases, and when we reduce the IFBW, the measurement speed is reduced. The key is finding the optimum receiver attenuator setting where the total error is at its lowest point for a given IFBW, not just one of the sources of error. In this section, we describe the method for optimizing a distortion measurement and finding the settings which provide the highest accuracy, in the shortest possible time.

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