Inter Modulation Distortion is one of the important parameter of linearity in every RF and microwave components. Intermodulation happens whenever multiple signals at two or more frequencies interact in any passive or active devices. IMD is one of the significant factor which limiting the system capacity in modern wireless communication system.
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 combines with fundamental signals and produces third order IMD products like 2f1-f2, f2-2f1. The combination of fundamental signals 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 interaction of two fundamental signals in a high power system. These combinations of f1+f2, f2-f1 are categorized as second order IMD products.
Two tone Intermodulation (Third Order – IMD3)
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 to the transmitter port and another signal with lower power level (to emulate blocker signal) is injected to the antenna port of a duplexer. The signals are generated by two signal generators which have to be highly isolated to 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 are 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 power amplifier. A high pass filter is used after DUT. It allows only high frequency harmonics signals to reach spectrum analyzer and gives maximum attenuation at input signal range. Suitable attenuators are used at input and output for better impedance matching and measurement accuracy. High power input signal must be prevented from reaching at the spectrum analyzer. The figure below shows the typical harmonics test setup.
Note: The test setup need to be configured based on the power requirement, device specification, test setup limitation and environmental conditions etc.
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