Passive Intermodulation Measurement and Solutions

1. Overview

Will passive devices produce nonlinear intermodulation distortion? The answer is yes! Although there is no systematic theoretical analysis, it has been found in engineering that under certain conditions, passive devices have intermodulation distortion and will cause serious interference to communication systems (especially cellular systems).

Passive inter- modulation (PIM) is caused by the nonlinear characteristics of various passive components in the transmission system. In high-power, multi-channel systems, the nonlinearity of these passive components will produce higher harmonics relative to the operating frequency. These harmonics will mix with the operating frequency to produce a group of new frequencies. The final result is a group of useless spectrum in the air, thus affecting normal communication.

All passive components will produce intermodulation distortion. There are many reasons for passive intermodulation, such as unreliable mechanical contact, cold solder joints and surface oxidation.

Five years ago, most RF engineers rarely mentioned the problem of passive device intermodulation. However, with the continuous planning of new frequencies for mobile communication systems, the application of higher-power transmitters and the continuous improvement of receiver sensitivity, the system interference caused by passive intermodulation has become increasingly serious, and has therefore attracted more and more attention from operators, system manufacturers and device manufacturers.

For a long time, the intermodulation distortion measurement technology of passive devices has been mastered by foreign companies, and they have monopolized the measurement product market. Today, this situation has changed. The technical difficulties of passive intermodulation measurement have been overcome by Chinese RF engineers, and low-cost commercial passive intermodulation measurement systems have also been born.

2. Expression of passive intermodulation

Passive intermodulation can be expressed in two ways: absolute value and relative value. The absolute value expression refers to the absolute value of passive intermodulation in dBm; the relative value expression refers to the ratio of the passive intermodulation value to one of the carrier frequencies (this is because the intermodulation distortion of passive devices is related to the size of the carrier frequency power), expressed in dBc.

The typical passive intermodulation indicator is that when two 43 dBm carrier powers are simultaneously applied to the device under test (DUT), the DUT produces a passive intermodulation distortion of -110 dBm (absolute value), and its relative value is -153 dBc.

3. Passive intermodulation measurement method

Since the passive intermodulation value is very small, it is very difficult to measure passive intermodulation. So far, there is no corresponding international standard for the measurement items and measurement methods of passive intermodulation, and the measurement method recommended by IEC is usually used.

4. New challenges facing passive intermodulation measurement

With the continuous development of communication technology, new system interference problems continue to emerge, bringing new challenges to measurement workers.

(1) Reverse intermodulation measurement

In some power synthesis systems or multi-carrier frequency sharing systems, when two high-power signals act on the input and output of a two-port device at the same time, a large intermodulation product will be generated at the output port. The situation is even more complicated in the multi-system combined platform (POI) system. Carrier frequencies of various frequency bands enter the system at the same time, and in addition to the intermodulation interference of the frequency band, cross-band intermodulation interference will also be generated.

(2) Measurement range

Typical passive components, such as directional couplers, power dividers, duplexers, connectors, and cable assemblies, usually have intermodulation products between -120 and -100 dBm, which is -163 to -143 dBc under 43 dBm measurement conditions. Some components have even larger intermodulation products, such as ferrite components, which can reach -60 dBc or even greater. For the former type of components, the measurement range of the measurement system is not required to be too large. The upper limit of intermodulation measurement for similar products is currently -65 dBm, which is -108 dBc under 43 dBm conditions. For the latter type of components, a general-purpose spectrum analyzer can be used for measurement. A spectrum analyzer is a general-purpose RF analysis instrument, also known as an "RF multimeter." Given this reputation, the dynamic range of a spectrum analyzer must be large enough. Even a low-end spectrum analyzer can measure within a range of -150 to 30 dBm.

(3) Measurement accuracy

Although there is no corresponding international standard for the measurement accuracy of passive intermodulation measurement systems, there are still rules to follow for the measurement accuracy of passive intermodulation. Factors related to measurement accuracy include power calibration and residual intermodulation of the system.

Power calibration

Power calibration has a great impact on measurement accuracy. Theoretically, if the carrier frequency increases by 1 dB, the intermodulation product will increase by 3 dB. In the measurement method recommended by IEC, the recommended measurement power loaded to the DUT is 43 dBm per carrier frequency, which has become the industry standard measurement power. As the power of communication systems continues to increase, the reference power standard is not static, and higher reference power standards may appear.

To accurately calibrate the power at the measurement end, a spectrum analyzer is not the most suitable choice, because the amplitude measurement accuracy of a spectrum analyzer is usually ±1dB, and with the influence of the attenuator, the total power error may exceed ±1 dB. The best means of high-power measurement is a through-type power meter, which uses a highly directive directional coupler and can provide high-power online measurement.

●Residual intermodulation of the system

The residual intermodulation value of the measuring system itself is one of the most important indicators of the system. The difference between the system residual intermodulation and the DUT intermodulation determines the accuracy of the measurement results. The acceptable difference between the system residual intermodulation and the DUT intermodulation recommended in IEC is 10 dB. This means that the measurement error of the system is +2.4/-3.3 dB. In the case of small intermodulation measurement, this error is completely acceptable. For large intermodulation measurement (greater than -80 dBc), the margin of 10 dB seems to be a little small, and 20 dB is more reasonable.

5. Factors to consider in implementing the passive intermodulation measurement system

Passive intermodulation measurement is actually to reproduce the passive intermodulation generated by the device under actual working conditions in the laboratory. Therefore, how to realistically simulate the actual working environment is the key to the passive intermodulation measurement system. To achieve this, the following major factors must be considered. [page]

(1) The amplitude of the power at the measuring end

The principle of setting the power size at the measurement end should be the upper limit of the maximum power that can be loaded to the DUT end. In IEC, it is mentioned that unless otherwise specified, the measurement power loaded to the DUT is 2×43 dBm. Obviously, this is for early base stations. Until now, this power level is still applicable to the measurement of most devices. With the continuous emergence of new digital cellular communication standards, power levels with larger amplitudes and larger ranges have emerged. For example, CDMA and WCDMA, because these modulated signals have a high peak-to-average power ratio, in order to meet the requirements of the system, the 1 dB compression point power of the amplifier must be much higher than the average power under the modulation state. Therefore, in addition to 43 dBm, there are also measurement requirements as small as 26 dBm and as large as 51 dBm.

(2) Number of carrier frequencies

Most PIM measurements are performed under two carrier frequencies, but there are also measurements under four carrier frequencies. As wireless channels become increasingly crowded, multi-carrier PIM measurements may be included in relevant measurement standards in the near future.

(3) Measuring the direction of power flow

The conventional thinking of passive intermodulation measurement is to synthesize two carrier frequencies and inject them into the DUT from one direction at the same time. However, in actual applications, the devices in the system must withstand power from different directions. Early passive intermodulation measurement systems did not take this into consideration.

(4) Frequency configuration

In the early days, the measurement was concerned about the intermodulation falling into the receiving frequency band, but now more and more people are concerned about the intermodulation falling into the transmitting frequency band. Some standard passive intermodulation measurement systems can only measure the intermodulation falling into the receiving frequency band, and are powerless to measure the intermodulation falling into the transmitting frequency band. In addition, due to the coexistence of multi-standard systems, cross-band intermodulation interference will gradually appear. For passive intermodulation measurement systems, in addition to the receiving frequency band, the intermodulation analysis and measurement of the transmitting frequency band and cross-bands are also important factors that need to be considered.

(5) Measurement range

This problem has been described in detail above. The measurement range of the spectrum analyzer itself far exceeds that of a dedicated measurement receiver. In addition, the spectrum analyzer is a general-purpose instrument that can fully improve resource utilization.

6. Passive intermodulation measurement solution

After unremitting efforts, Shanghai Chuangyuan Information Technology Co., Ltd. successfully developed the first localized commercial passive intermodulation measurement system - PIM system. The PIM measurement system was developed with reference to the measurement method recommended by IEC and combined with various new measurement requirements. The entire design process fully complies with the " simulation principle" of passive intermodulation measurement.

(1) Shared measurement platform

The PIM system adopts the "shared platform" design concept. The basic platform of the system adopts universal spectrum measurement technology. The second-level platform is the basic measurement system of GSM900 and DCS 1800, which can be upgraded to CDMA800 and WCDMA frequency bands respectively, thus covering the mobile communication frequency band.

Thanks to this design concept, the upgrade and expansion of the PIM system has become very convenient and economical. If you want to upgrade to the TETRA band and E-GSM band, you only need to add the corresponding RF subsystem; even if you want to upgrade to the 450 MHz and 3.5 GHz bands, the first-layer shared platform can still be used. With the continuous emergence of new communication systems (such as POI systems), the PIM system can provide enough upgrade space to develop customized measurement solutions.

(2) Built-in signal source

The PIM system has a built-in signal source, which is configured according to the frequency band required for measurement in order to reduce the user's investment cost.

(3) Flexible structure

The PIM system is divided into two structures: highly integrated and 19-inch cabinet. The highly integrated structure occupies a small area and is suitable for applications with a single measurement function. The 19-inch cabinet structure is more convenient for system upgrades and expansions. Each subsystem module uses a 19-inch standard plug-in box, and new functional modules can be added at will.

(4) Adjustable high power source

In forward intermodulation measurement, the measured power at the DUT end can be greater than 44 dBm; in reverse intermodulation measurement, the power at the DUT end can be as high as 49 dBm. If necessary, the system power can be increased to 51.7 dBm (150 W). With the standard signal source, the power at the measurement end can be adjusted arbitrarily. In order to ensure the measurement accuracy, the power of each measurement end is accurately calibrated by the 5012C through-type power meter.

(5) General basic instruments

In addition to the built-in signal source, the PIM system is also compatible with common basic RF instruments, thus ensuring the versatility and scalability of the system.

(6) Targeted measurement solutions

In addition to the basic measurement methods recommended by IEC, the PIM system also provides a large number of highly targeted measurement solutions, including intermodulation measurement in the transmit frequency band, reverse intermodulation measurement, harmonic measurement, intermodulation measurement of POI systems, and higher power synthesis applications.

7. Conclusion

The analysis and measurement of passive intermodulation distortion is relatively complex. According to the "simulation principle" of passive intermodulation measurement, a passive intermodulation measurement system should have combined functions, good compatibility and convenient upgrade. The PIM passive intermodulation measurement system developed by Shanghai Chuangyuan Information Technology Co., Ltd. meets these requirements well.