Inter Modulation Distortion is one of the important parameters of linearity in every RF and microwave component. Intermodulation happens whenever multiple signals at two or more frequencies interact in any passive or active devices. IMD is one of the significant factors which limit the system capacity in modern wireless communication systems.
Harmonics
Harmonics are generated as multiples of fundamental frequencies in a system, like second harmonics (2f1, 2f2) and third harmonics (3f1, 3f2), and so on. Harmonics signals combine with fundamental signals and produce third-order IMD products like 2f1-f2 and f2-2f1. The combination of fundamental signals is also analyzed as second-order IMD products. These combinations are f1-f2, f2-f1, and f1+f2.
Second Order Intermodulation
Second-order intermodulation is caused by the interaction of two fundamental signals in a high-power system. These combinations of f1+f2 and f2-f1 are categorized as second-order IMD products.
Two-tone Intermodulation (Third Order – IMD3)
The most common IMD test case is two-tone third-order intermodulation distortion (IMD3). It is the measure of the third-order distortion products produced by a nonlinear device when two tones closely spaced in frequency are fed into its input. This distortion product is usually so close to the carrier that it is almost impossible to filter out and can cause interference in other channels of communication equipment. The most critical measurements are 2f1-f2 and f2-2f1, which fall adjacent to the fundamental signals.
Two-Tone IMD Measurement Setup Of A Passive Duplexer
The measurement setup shown in the below figure is the standard IMD test setup. A high-power transmit signal is injected into the transmitter port, and another signal with a lower power level (to emulate the blocker signal) is injected into the antenna port of a duplexer. The signals are generated by two signal generators which have to be highly isolated from each other. This is to prevent intermodulation products generated internally inside the signal generators, which can contribute to the total intermodulation results.
The intermodulation products which fall at receiver frequencies will be measured by a spectrum analyzer at the receiver port of the duplexer. In some cases, a bandpass filter before the spectrum analyzer is necessary when the power of transmit and blocker signals is high enough to generate intermodulation products in the spectrum analyzer itself. LPF, HPF, or BPF are used for better isolation between Tx, and Blocker signals in the test system.
Harmonics Measurement
Harmonics are the multiples of the fundamental transmit signals or any high-power signals in a communication system. In a harmonics measurement setup, multiples of high-power input signals are measured at the output of the DUT. A low pass filter or band pass filter is used before DUT to stop high-frequency reflection from DUT and filter out any high-frequency noise from the power amplifier. A high pass filter is used after DUT. It allows only high-frequency harmonic signals to reach the spectrum analyzer and gives maximum attenuation at the input signal range. Suitable attenuators are used at input and output for better impedance matching and measurement accuracy. The high-power input signal must be prevented from reaching the spectrum analyzer. The figure below shows the typical harmonics test setup.
Note: The test setup needs to be configured based on the power requirement, device specification, test setup limitation, environmental conditions, etc.
Find more about How Intermodulation Distortion Occur
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