Measurement of Passive Intermodulation Distortion in Communication Systems

Introduction In modern communication systems, when carrier signals of multiple frequencies pass through some passive components, intermodulation distortion will be generated. Passive components such as antennas , cables, filters, etc., due to their unreliable mechanical connections, the use of materials with hysteresis characteristics, dirty contact surfaces, etc., signals of different frequencies are nonlinearly mixed at the material connection, generating intermodulation products of different amplitudes, and these intermodulation distortion signals are manifested as interference signals in the communication frequency band, which reduces the signal-to-noise ratio of the system and seriously affects the capacity and quality of the communication system. In fact, in our usual design and measurement, we generally pay more attention to active intermodulation, such as intermodulation distortion generated by amplifiers, mixers, etc., and the measurement of active intermodulation is easy to achieve because the relative amplitude difference between intermodulation distortion and carrier is small. With the development of communication systems and the improvement of system quality, the measurement and analysis of passive intermodulation will receive increasing attention.

Measurement setup When measuring intermodulation distortion of a power combiner, a traditional measurement method can be used as shown below:

Figure 1

As shown in the figure, the high-power continuous wave signal output by Anritsu's 68347 signal source is input to the two ports of the power combiner. The frequency of each carrier is appropriately set within the bandwidth required for measurement. The power combiner has two functions: it is the device under test and it combines two signals into one signal. The intermodulation signal generated by the power combiner is transmitted to the duplexer port, and the intermodulation signal within the receiving bandwidth is measured with a spectrum analyzer.

Modern passive intermodulation analyzers can output pre-combined dual-frequency signals. The intermodulation analyzer has two RF ports. Port one can output two high-power dual-frequency signals, which enter port two of the analyzer after passing through the device under test. The reflected signal of port one also enters the receiver of the analyzer. The analyzer can work in transmission mode and reflection mode to measure the transmission intermodulation distortion and reflection intermodulation distortion of the device under test respectively.

In fact, for the DUT, the intermodulation distortions generated by different factors are all vector signals, and their relative phase relationship will determine the total amplitude of the intermodulation distortion of the DUT in a specific state. In transmission measurement, different intermodulation products are in phase when they arrive at port two, while in reflection measurement, the intermodulation distortion arriving at port one is the total response of port one and the phase shift response of the intermodulation source on port two. Therefore, the reflection intermodulation distortion is a function of frequency and the electrical length of the DUT.

Use an intermodulation meter to measure the intermodulation response of the power combiner above. The measurement connection is as shown below:

Figure 2

As shown in the figure, port 1 of the intermodulation meter is connected to the input port of the power combiner to be tested, so that the intermodulation distortion of ports A1, A2 and B of the power combiner can be measured. The transmission mode of the intermodulation meter measures the forward intermodulation distortion of port B, and the reflection mode measures the intermodulation distortion of port A1. As shown in the figure, if port A1 is used as the driving port, port A2 should be connected to a low intermodulation distortion load to ideally test the intermodulation distortion of the power combiner. By switching ports A1 and A2, the intermodulation of each input port of the power combiner can be measured.

Compare the above two methods: In Figure 1, the power combiner connection and port B carry two continuous wave powers, and the measured intermodulation distortion is the total intermodulation distortion of these two factors. If the incident signal at each port is a non-modulated signal, this method accurately measures the true intermodulation performance of the power combiner, but is limited by the inherent intermodulation distortion of the spectrum analyzer. If the power combiner has modulated signals at both the output and input ports, the measurement results provided in Figure 2 are more practical.

The following mainly discusses the methods for improving the measurement accuracy when using a passive intermodulation analyzer:

(i) When measuring the forward passive intermodulation distortion of a two-port device, a direct connection method can be used: the input port of the device under test is connected to port one of the analyzer, and the output port is connected to port two of the analyzer. The measurement error of this method varies with the frequency and the length of the cable connecting port two and the device under test. In addition, since ports one and two of the intermodulation meter only achieve impedance matching within the measured transmission and reception bandwidth, a large standing wave will be generated within the harmonic frequency range of the analyzer output carrier signal. In this way, even if the device under test has good impedance matching characteristics within the fundamental and harmonic frequency range of the high-power carrier, the establishment of this measurement method still produces different intermodulation levels. The method shown in the figure below can achieve ideal measurement results.

Figure 3

As shown in Figure 3, first of all, the directional coupler used must have sufficiently low inherent intermodulation characteristics, and its coupling degree is between 10 and 30 dB. Too large a coupling value makes the measured intermodulation signal submerged in the noise floor band of the second port of the analyzer, and too small a coupling degree will increase the measurement error. The directional coupler is connected so that both the dual-frequency carrier and the generated intermodulation can be transmitted to the coupling port, and the transmission arm of the coupler is connected to a low intermodulation distortion terminal load. The reverse coupling port of the coupler is matched to a standard fifty-ohm terminal load. Before measurement, first directly connect the directional coupler (good cable and adapter) to the two ports of the analyzer to check for residual intermodulation. This measurement setup provides broadband impedance matching, effectively reducing the standing wave in the harmonic frequency range of the carrier, and stable test conditions to obtain more meaningful measurement results.

2. Passive intermodulation distortion measurement with high intermodulation level:

Generally, the passive intermodulation distortion analyzer system has a linear working area, such as -75~-125dBm. If the IM level of the receiver is greater than -75dBm, the measurement error of the receiver will increase. For measuring the forward intermodulation level, the test method shown in Figure 3 can be used. This connection method of the directional coupler allows the dual carrier and the generated intermodulation signal to flow to the coupling port, and the port of the transmission arm of the coupler can be connected to a low intermodulation distortion load.

The same method can be applied to the measurement of reverse intermodulation, and the following diagram is established:

Figure 4

The figure shows the reverse intermodulation measurement of the DUT. Note that the directional coupler in Figure 4 is reversed compared to Figure 3. The forward and reverse coupling ports of the directional coupler are connected to standard 50-ohm loads, the transmission arm is connected to the DUT, and a low intermodulation distortion load is connected to the output end of the DUT, so that the intermodulation signal transmitted to port one is finally measured at port two.

In both of the above establishments, the coupling port of the directional coupler is connected to a fixed attenuator. The value of the attenuator is determined by the expected intermodulation level. The role of the attenuator is to further reduce the intermodulation level to a level lower than that when the directional coupler is used alone. In both methods, the measurement system should be established to avoid the generation of effective residual intermodulation levels. During measurement, the attenuation value of the attenuator can be changed from small to large so that the measured intermodulation level is attenuated to reach the linear working area of ​​the intermodulation meter receiver. The measurement result should take into account the attenuation value of the attenuator and the coupling value of the directional coupler.

Summary: At present, the theory and methods of passive intermodulation distortion measurement are still in the preliminary stage, and some measurement methods are not mature enough. With the development of RF technology, the measurement of this parameter will be more and more valued, the measurement equipment will be more perfect, and the measurement accuracy will be greatly improved.

Attach English title, abstract and keywords:

Measurement Of Passive Intermodulation Distortion In Communication System

Abstract This paper introduce the measurement method of passive intermodulation distortion, and mainly discuss the test principle of modern passive intermodulation distortion analyzer, the setup of measurement system and means of improve the measurement accuracy.

Keywords Passive Device Intermodulation Distortion Power combiner

Directional Coupler

References:

1．http:www.tec.ch To closely liaise with TC102 for matters relevant to antennas and SC48B for connetors with respect to PIM.

2.Passive Intermodulation Distortion Analyzer, User's Guide , Summitekinstruments

3.application note ＃1900-1，application note ＃1900-2，application note ＃1900-3，

Summitekinstruments