Application Note: Intermodulation Distortion (IMD) Measurements Of Ferrites

By Cobham

Introduction
Intermodulation distortion (IMD) measurements are used to characterize the non-linearity of RF components, including ferrite circulators and isolators. Two CW tones (f1 and f2) are combined and fed into the Device Under Test (DUT). The resulting output is measured on a spectrum analyzer.

The third order, and occasionally the fifth order, products are the critical unwanted frequency IMD products that are measured.

3rd order products at: (2 x f₁) – f₂ and (2 x f₁) – f₁

5th order products at: (3 x f₁) – (2 x f₂) and (3 x f₂) – (2 x f₁)

Forward IMD Test Set Up
Figure 1 is a typical block diagram of a forward IMD test stand. M/A Com uses a Agilent 83712B signal generators and Amplifier Research 100W1000M1 amplifiers. The dual isolators provide >50dB of isolation and K&L's six cavity tuneable low pass filters remove system harmonics. The M/A-Com quad hybrid (QH32-0018-N) gives a further 18dB of isolation between the tones.

The Directional Couplers (CH25-0014-10N) must be term inated with a broadband cable load with superior VSWR (<1.1:1). A six cavity notch filter (WRCD800/960 0.2/40-6EEK) attenuates the fundamental frequencies (f1 and f2) to provide greater dynamic range.

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