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Does the passive device produce nonlinear intermodulation distortion? The answer is yes! Although there is no theoretical analysis of the system, it has been found in engineering that the passive device has intermodulation distortion under certain conditions, and it will be a communication system (especially It is a cellular system) that causes serious interference.
Passive Inter-Modulation (PIM) is caused by the nonlinear characteristics of various passive components in the transmitting system. In high-power, multi-channel systems, the nonlinearities of these passive components produce higher harmonics relative to the operating frequency. These harmonics mix with the operating frequency to produce a new set of frequencies, the end result being in the air. A set of useless spectrum is generated to affect normal communication.
All passive components generate intermodulation distortion. There are many reasons for passive intermodulation, such as unreliable mechanical contact, solder joints, and surface oxidation.
Five years ago, most RF engineers seldom mentioned the issue of passive intermodulation. However, with the continuous planning of new frequencies of mobile communication systems, the application of higher power transmitters and the increasing sensitivity of receivers, the system interference caused by passive intermodulation is becoming more and more serious, so it is increasingly being used by operators and system manufacturers. And device manufacturers are concerned.
For a long time, the intermodulation distortion measurement technology of passive components has been mastered by foreign companies and monopolized the measurement. Today, this situation has changed. The passive intermodulation measurement technology has been overcome by Chinese local RF engineers, and low-cost commercial passive intermodulation measurement systems have also been born.
2. Expression of passive intermodulation
Passive intermodulation has two expressions, absolute value and relative value. The absolute value expression refers to the absolute value of the 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 the passive device The magnitude of the carrier frequency power is related to), expressed in dBc.
A typical passive intermodulation indicator is a passive intermodulation distortion of -110dBm (absolute) generated by the DUT when two carrier powers of 43 dBm are simultaneously applied to the DUT of the device under test, with a relative value of -153 dBc.
3. Passive intermodulation measurement method
Passive intermodulation measurements are very difficult due to the very small passive intermodulation values. So far, there are no corresponding standards for measurement items and measurement methods for passive intermodulation, and the measurement methods recommended by IEC are usually adopted.
4. New challenges in passive intermodulation measurement
With the continuous development of communication technology, new system interference problems continue to emerge, bringing new challenges to surveyors.
(1) Reverse intermodulation measurement
In some power synthesis systems or shared systems with multiple carrier frequencies, when two high power signals are applied to the input and output of a two-port device simultaneously, a large intermodulation product will be produced at the output port. The situation is more complicated in a multi-system combined platform (POI) system. The carrier frequencies of different frequency bands enter the system at the same time. In addition to the intermodulation interference of this frequency band, intermodulation interference across frequency bands is also generated.
(2) Measurement range
Typical passive components, such as directional couplers, power splitters, duplexers, and components, have intermodulation products typically ranging from -120 to -100 dBm, which is -163 to -143 relative to 43 dBm measurement conditions. dBc; while some devices have larger intermodulation products, such as the intermodulation products of ferrite devices can reach -60dBc or even larger. For the former type of device, the measurement range of the measurement system is not required to be too large. At present, the upper limit of intermodulation measurement of similar products is -65 dBm, which is -108 dBc at 43 dBm. For the latter type of device, a general spectrum analyzer can be used for measurement. The spectrum analyzer is a general-purpose RF analysis instrument, also known as the "RF Multimeter". Given this reputation, the dynamic range of the spectrum analyzer must be large enough. Even for low-end spectrum analyzers, the measurement range can reach -150 to 30 dBm.
(3) Measurement accuracy
For the measurement accuracy of passive intermodulation measurement systems, although there is no corresponding international standard, the measurement accuracy of passive intermodulation is still rule-based. Factors related to measurement accuracy are power calibration and residual intermodulation of the system.
â—Power calibration
Power calibration has a lot to do with measurement accuracy. In theory, the carrier frequency is increased by 1 dB and the intermodulation product is increased by 3 dB. In the measurement method recommended by the IEC, it is recommended that the measured power loaded into the DUT is 43 dBm per carrier frequency, which has become the industry standard measurement power. As the power of the communication system continues to increase, the reference power standard is not static, and a higher reference power standard may occur.
To accurately calibrate the power at the measurement end, the spectrum analyzer is not the most suitable choice because the amplitude measurement accuracy of the spectrum analyzer is typically ±1 dB, and with the influence of the attenuator, the total power error may exceed ±1 dB. The best means of high-power measurement is the pass-through power meter, which uses a high-directional directional coupler to provide high-power online measurements.
â—Remaining intermodulation of the system
The residual intermodulation value of the measurement 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 difference between acceptable system residual intermodulation and DUT intermodulation recommended in the 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 measurements, this error is perfectly acceptable. For large intermodulation measurements (greater than -80dBc), the 10 dB margin seems to be smaller and 20 dB is more reasonable.
5. Elements to be considered for the implementation of passive intermodulation measurement systems
Passive intermodulation measurement actually reproduces 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 do this, the following major factors must be considered.
(1) The amplitude of the measuring terminal power
The principle of setting the power of the measuring terminal should be the upper limit of the maximum power that can be loaded to the DUT. It is mentioned in the IEC that the measured power loaded into the DUT is 2 x 43 dBm unless otherwise specified. Obviously, this is for early base stations, and until now, this power level is still suitable for most devices. As new digital cellular communication standards continue to emerge, larger and larger power levels 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 far exceeds the average power in the modulation state. Therefore, in addition to 43 dBm, measurement requirements as small as 26 dBm and as large as 51 dBm have occurred.
(2) Number of carrier frequencies
Most passive intermodulation measurements are made at two carrier frequencies, but there are also measurements at four carrier frequencies. With the increasing congestion of wireless channels, passive intermodulation measurements with multiple carrier frequencies may be included in the relevant measurement standards in the near future.
(3) Measuring the direction of power flow
Combining the two carrier frequencies and simultaneously injecting the DUT from one direction is already the inertial thinking of passive intermodulation measurements. However, in practical applications, devices in the system are subject to power from different directions. For this, early passive intermodulation measurement systems were not considered.
(4) Frequency configuration
In the early days, the measurers were concerned with the intermodulation that fell in the receiving band, and now more and more concerned about the intermodulation that falls into the transmitting band. Some standard passive intermodulation measurement systems can only measure intermodulation that falls within the receive band, and are incapable of intermodulation measurements that fall into the transmit band. In addition, due to the coexistence of multi-standard systems, inter-band interference will gradually appear. For passive intermodulation measurement systems, in addition to the receiving frequency band, intermodulation analysis and measurement of the transmission band and the cross-band are also important factors to consider.
(5) Measurement range
This problem has been described in detail above. The spectrum analyzer's own measurement range far exceeds that of dedicated measurement receivers. In addition, the spectrum analyzer is a general-purpose instrument that can fully improve the utilization of resources.
6, passive intermodulation measurement solutions
Through unremitting efforts, Shanghai Chuangyuan Information Technology Co., Ltd. successfully developed the first localized commercial passive intermodulation measurement system--PIM system. The PIM measurement system is developed with reference to the measurement methods recommended by the IEC and combined with various new measurement requirements. The entire design process fully follows 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 the general spectrum measurement technology. The second platform is the basic measurement system of GSM900 and DCS1800 respectively. On this basis, it can be upgraded to the CDMA800 and WCDMA bands respectively, thus covering The mobile communication band.
Thanks to this design concept, the upgrade and expansion of the PIM system has become very convenient and economical. To upgrade to the TETRA band and the E-GSM band, simply add the corresponding RF subsystem; even if you are upgrading to the 450MHz and 3.5GHz bands, the first-tier shared platform can still be utilized. With the emergence of new communication systems (such as POI systems), PIM systems can provide enough upgrade space to develop customized measurement solutions.
(2) Built-in signal source
The PIM system has a built-in signal source that is configured according to the frequency band required for measurement, in order to reduce the cost of the user.
(3) Flexible structure
The PIM system is divided into two types: highly integrated and 19-inch cabinet. The highly integrated structure has a small footprint and is suitable for single measurement functions. The 19-inch cabinet structure facilitates system upgrade and expansion. Each subsystem module is adopted. The 19-inch standard box allows you to add new functional modules as you wish.
(4) Adjustable high power source
In the case of forward intermodulation measurements, the measured power at the DUT can be greater than 44 dBm; in reverse intermodulation measurements, the power applied to the DUT can be as high as 49 dBm. The system power can be increased to 51.7 dBm (150w) if needed. With the standard signal source, the power of the measuring terminal can be adjusted arbitrarily. In order to ensure the measurement accuracy, the power of each measuring end is precisely calibrated by the 5012C pass-through power meter.
(5) General basic instruments
In addition to the built-in signal source, the PIM system is also compatible with general-purpose basic RF instruments, thus ensuring system versatility and scalability.
(6) Targeted measurement solutions
In addition to the basic measurement methods recommended by the IEC, the PIM system also provides a large number of highly targeted measurement solutions, including transmission band measurement, reverse intermodulation measurement, harmonic measurement, and intermodulation measurement of POI systems. And more powerful synthetic applications.
7, the conclusion
The analysis and measurement of intermodulation distortion of passive components is complicated. According to the "simulation principle" of passive intermodulation measurement, a set of passive intermodulation measurement systems should have combined functions, with good compatibility and convenience of upgrading. PIM passive mutual development developed by Shanghai Chuangyuan Information Technology Co., Ltd. The measurement system is well suited to meet these requirements. 
Passive intermodulation measurement and solution
1 Overview