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Passive IM, similar to active IM but occurs in passive devices, is present whenever RF signals at two or more frequencies are simultaneously present in a conductor of RF energy. Every passive RF device generates passive IM products when more than one frequency is present in the device. The signals are mixed by the non-linear properties of junctions between dissimilar materials. Typically, it is the odd-ordered products (e.g. IM3=2*F1-F2) that can be very problematic should they fall within an uplink, or receive band of the base station because they appear to the receiver's random noise floor.
In RF components (antennas, cables, filters, etc.), there are typically three causes:
This annotation is commonly used to specify the order of the IM product being discussed. The IM stands for "intermodulation." The numeric value that follows is the sum of the integer multipliers used for each of the two parent tones to realize the given IM product. This is best understood by reviewing the following table:
Most commonly, the lower order tones are of the largest magnitude. However, in frequency-selective systems, it is possible that an IM5 product might actually appear larger to the receiver than an IM3 product.
The required PIM performance for a given RF device is a strong function of where that device is located in the final system. For example, an antenna must have excellent PIM performance as the PIM generated in the antenna is both received and radiated by the base station. Further, the transmit antenna is subjected to nearly the full carrier power of the base station. On the other hand, the PIM performance of a receive "clean-up" filter need not be so stringent. This filter might be located on the other side of a diplexer thus preventing the full carrier power level from reaching its input connector.
Ultimately, it is up to the buyer to specify the maximum acceptable PIM level and carrier power levels. Commonly seen specifications for antennas are -100 to -110dBm IM3 levels with two, +43dBm (20 Watt) per carrier tones.
Yes. However, the relationship between the generated PIM power level and the parent carrier power levels is not always straightforward.
In simple, broadband devices terminated into a broadband termination, the IM3 response typically increases approximately 3 dB for every one dB in carrier power level (assuming equal carrier powers). However, there are many factors which tend to work against this nice, simple relationship. These include:
In general, as the transmitter power increases, the importance of PIM on the overall system performance becomes of increasing concern. As a TDMA system fills available frequency and time channel slots, or as a CDMA system increases forward power levels to increase capacity, PIM levels typically increase.
It depends. One postulate is that a single IM source which is located at a single point (not spatially distributed) and is matched in impedance to the incident transmission line (or source of stimulus RF energy) generates frequency-independent IM isotropically. This is the analog of the classical "Point Source" of RF in antenna theory.
Given that this point source of PIM exists (at least theoretically), real-world RF devices can be modeled as being comprised of multiple PIM sources. These sources generate IM which has a phase relationship with the parent RF carriers. Once the PIM is generated at each point source within the device, the PIM signals themselves can vectorally combine (either constructively or destructively) to produce a composite PIM response. The phase relationship between the PIM sources will depend upon their physical separation, the dielectric through which the RF must travel between the sources, and the frequency of the parent carriers.
Given that all real world devices have more than one source of PIM, it is quite probable that the device will have a frequency-dependent PIM response. However,
Even though electrically long (more than one-half wavelength) cables can have a frequency-dependent PIM response in the reverse direction, the periodicity of the ripple can quite often be related directly to the electrical length of the cable assembly. Yes, the cable PIM response is frequency dependent. However, if the cable assembly PIM is measured across a swept-frequency bandwidth that includes both peaks and nulls, the worst-case combination of PIM sources can be captured across the test band.
So long as the individual sources of PIM do not change in magnitude dramatically with frequency and so long as the loss of the cable assembly does not change appreciably with frequency, there is a good change that the PIM results measured at 1800 MHz will be representative of the performance expected at 900 MHz.
This topic does bring up an interesting note, however. As the carrier frequencies increase, the RF skin depth on the conductors of the device-under-test tends to decrease. For equal carrier powers, the current density at 1800 MHz will be higher than at 900 MHz. For this reason, testing a cable at 900 MHz may produce a PIM result which is better than the results which might be obtained if the same cable were tested at 1800 MHz.
The bottom line is as follows: To be absolutely certain of the PIM level for a cable assembly in a given band, you should test in that band. To characterize the approximate performance of a cable assembly (or the integrity of the mechanical connector-cable interfaces), testing in one band will most likely yield results which are representative of the overall cable performance.
Passive IM is typically specified in absolute power (units of dBm) or power relative to only one of the test tones (units of dBc). For example, a -110dBm IM signal caused by two +43dBm tones is also specified as a -153dBc IM level. In the case of unequal carrier power levels, Summitek Instruments has established the convention that units of dBc are relative to the largest of the incident carriers.
It is important to note that a carrier power level must always be specified with the given PIM performance level. This applies equally to PIM performance specified in units of dBm and dBc.
There is a clear distinction between random noise floor (kTBF) and the "PIM Noise Floor." The latter is more accurately restated as "Residual IM Level". These two parameters are discussed below:
The noise floor of the PIM test system is typically defined as the mean value of the measured signal when the receiver is terminated into 50 Ohms and the RF is turned off. If there is a coupling mechanism for noise from the high power amplifiers to appear in the receiver, this source of noise must also be included in the noise floor test. Noise is random and is typically due to a combination of phase noise in the local oscillator, kTBF noise from the receiver's pre-amplifier(s), and noise from the transmitter. The noise floor of a PIM test receiver (or spectrum analyzer/LNA combination) typically varies from approximately -120dBm to -140dBm depending upon the selected averaging level (or resolution bandwidth). You cannot make a meaningful IM measurement at a level below the noise floor of the receiver.
The residual IM level of the analyzer is caused by internally generated IM within the analyzer's cabling, internal connectors, filters, and duplexers. This level is typically larger than the noise floor of the receiver for the third-order IM product (IM3). When an IM measurement is performed on a DUT whose true IM level is near that of the analyzer, significant measurement errors can occur. This is because the residual IM of the analyzer vectorially combines with the true IM of the device-under-test thus producing a measurement with a high uncertainty level. The residual IM level of a test system can be reduced through the use of high-quality diplexers, filters, and carefully constructed, Low-PIM interconnecting cables.
Note: Averaging (or reducing the resolution bandwidth of a spectrum analyzer) cannot reduce the residual IM level. Averaging is only useful in reducing the level of the receiver noise floor. For the most efficient measurement time, and to maximize the test system's responsiveness to transient PIM, use only enough averaging (or use the widest possible resolution bandwidth) to maintain the receiver noise floor at least 10 dB below the expect minimum PIM level.
Possibly: Two types of PIM generation are typically found. The first type is of a "burst" nature and is commonly associated with the periodic breakdown of poor mechanical junctions exposed to high RF power levels. With this type of PIM, the IM will appear as a short (less than 1 second) burst of broadband, noise-like energy. On some devices and systems, these bursts have been measured at random intervals from 2 or 3 seconds to several hours.
The second type of PIM generation is a more steady state, and coherent in nature. RF heating within RF conductors and around RF interfaces can causes minute changes in the contact integrity. The result is a PIM level which changes with time. A classic example of this can be found by measuring the PIM from a cable assembly which is poorly constructed or has been subjected to mechanical stress. The PIM performance of the cable assembly may appear quite good at first, only to degrade as the assembly heats up. Interesting enough, the opposite has been found to happen. The cable assembly is poor at first, but as the RF heating causes the mechanical interfaces to expand (and compress), the PIM performance improves with time.
Consider an operational and fielded base station. Wind, Rain, and Sun-induced thermal cycling are all at work to continuously stress the mechanical interfaces within the antenna, the cable assemblies, and the connections to the shelter. As the sun rises and heats the RF connections. The PIM levels can rise (or fall) if the cables, connectors, and antennas are not functioning properly. The result can be increased levels of IM only at certain times of the day.
Because the intermodulation signals created at various PIM sources within an assembly are vectorial in nature, their relative phase relationships will determine the overall magnitude of (scalar) PIM measured at a particular location in a Device Under Test. Using the model developed in the application note Measuring the Passive Intermodulation Performance of RF Cable Assemblies, we find that all the IM responses arrive in-phase at Port 2 of a through IM measurement, independent of the IM frequency, while the reflected IM response present at Port 1 is a combination of the Port 1 response plus a phase-shifted response from the IM sources at Port 2. Because there is a vector combination of IM sources with differing phases, it is expected that the reflected IM response is a function of both frequency and the electrical length of the assembly.
In the real world, however, the forward IM response may not measure as being frequency independent and may not be the worst-case IM response. This is due to differences between the real world and the very simple model used in the above reference. Using more complex models which account for complex impedances and losses at not only the IM frequencies, but also at the harmonics of the carrier frequencies is a step forward towards more accurately predicting the results of a passive IM measurement.
Because the phase of the individual sources of IM within a device-under-test is related to the phase of the parent carriers, changing the carrier frequencies will change the phase relationship of the PIM signals. Depending upon where the composite PIM response is measured, the resulting composite PIM level may change as the carrier frequencies are changed.
Not if you are testing with two tones. When two carrier tones are used, the relative rate of phaser rotation between the carriers is determined by the frequency separation. The carriers will periodically combine in and then out of phase at a fixed rate for the given frequency separation. Phase locking the carriers together will force the carriers to cross at a known instant in time, relative to the phase of one of the carriers. However, this won't impact the magnitude of the generated PIM levels.
If three or more carriers are utilized for testing, the phase of the third carrier now becomes important. By phase locking the three carriers together, and adjusting their relative phases, a specific phase point on the third carrier can be made to align with a known phase crossing point of the first two carriers. This could be used, for example, to establish a worst-case current density at a set of fixed frequencies at a specific point within the device-under-test.
Whether you are using 2, 3, or 100 carriers to perform PIM testing, it is good practice to connect the clocks together to minimize the impact of RF frequency drift on the measurement. This is especially important if you are using a very narrow receiver to perform the PIM testing.
When measuring PIM with a 2-tone test, there is only one IM response of each order which is of interest. This is in contrast to a 16-tone PIM measurement. In this case, there is a "picket-fence" of, say, IM3 responses to choose from displayed on the spectrum analyzer. Depending upon the characteristics of the device- under-test, the picket-fence may be flat, or have a more complex shape. Typically, the largest of the observed responses of which is reported as the device's PIM level.
The 16-tone test has the advantage of allowing the device's PIM frequency response to be observed in a single measurement. This same frequency response display is obtained with the Kaelus Passive IM Analyzer (or other computer-controlled PIM systems) by sweeping the carriers across a pre-defined band, much the same as a conventional network analysis measurement.
Comparing the 16-tone and 2-tone measurement results is a difficult task. Some users have reported the swept 2-tone test is much more difficult to pass, while others say the 16-tone test is more rigorous. As results from comparison testing become available, Kaelus will continue to update this FAQ.
Ultimately, it is the performance of the integrated base station that is important. Although most wireless transmit and receive frequency bands are carefully selected to avoid landing the largest IM products within the receive band, self-generated higher order products (IM5, 7, 9) do land within some communication bands. More frequently, IM products from a nearby (or co-located) competitor's site can become troublesome sources of interference.
To the receiver, PIM products appear as interference. Once the PIM power level rises above the random (kTBF) noise floor of the receiver, the system C/I becomes adversely impacted. Because PIM products typically increase significantly as the average transmit power level increases, the impact of PIM on a base station may only become significant when the base station becomes fully loaded. Just when the most capacity is needed, passive IM level can rise and interfere with normal base station operation.
The International Electrotechnical Commission (IEC) has formed a Technical Committee (TC46/WG6). The assigned task of this committee is as follows:
"To prepare test methods and to investigate relevant limits, for Passive Intermodulation in the RF and microwave frequency range for passive components (i.e. connectors, cables, cable assemblies, waveguide assemblies and components...). To closely liaise with TC 102 for matters relevant to antennas and with SC 48B for connectors with respect to PIM. To liaise with other relevant committees, subcommittees, working groups, organizations and individuals, in order to ensure the widest appropriate awareness of, and the greatest relevant participation in and contribution to the work being carried out."
This group has been meeting for several years, and the first release of a standards document is imminent. Contact the IEC http://www.iec.ch for additional information or to obtain a copy of this document when it becomes available.
Only a return loss measurement on the Termination uses equipment (network analyzer) that is traceable by the ANSI standard. We do not do a return loss measurement unless the Termination has failed. No test equipment or serial number, that is traceable, is on the calibration certificate. It is the level of residual PIM of my equipment. There is no provision on the calibration certification for traceability to the Summitek equipment.
The engineering department has reviewed the calibration status of the SI-20A and SI-30A Terminations. They have concluded that a calibration certificate and calibration requirement for the Terminations does not apply. We no longer issue calibration certificates for Low IM Terminations as of January 1, 2008 and you should discard any certificates that you have. Only a verification certificate/plot is now issued for new Terminations.
If you are not sure that an analyzer is measuring IM correctly, the PIM Source is a quick way to affirm that the readings are accurate, at least within a few dB. This would be a good tool for the production floor as you could verify the analyzer’s operation at the beginning of each day. A receiver failure might go unnoticed for a while during testing.
The unit has unstable readings or IM spikes with the RF off has been due to stray RF. The unit is picking up a nearby cell tower or cell phone and the signals can get very high. This can get so bad that it’s impossible to do a reliable test because you can’t separate the PIM from the stray stuff. Stray RF can occur with an open port at the analyzer or with the analyzer connected to cable runs/antenna at the base station. Sometimes you can point the antenna around and get an idea of where it is coming from.
The cable in the low IM termination is not shorted at the end and sometimes the load itself will act as an antenna. You may pickup stray RF with the load on the unit. The PIM source has a resistive load and will make a good termination. Put the PIM source on the analyzer and keep the RF turned off. If the IM spikes/instability stop, then the problem is stray RF. You can also use the spectrum mode to listen and see the stray RF.
The output port is dirty or damaged. Clean the output port and retry the process. If this does not solve the problem, please send unit in for repair.
Try testing the unit using a PIM source. If it is still reading perfect, send the unit in for repair.
Test reports can be saved to a USB memory stick in PDF format. Insert the USB memory stick. Press the Save as PDF button to save the report in *.pdf format on the memory stick.
Instructions also available in the iQA Operating Manual found in section 1.6.8 "Saving Reports"
The PIM Level Measurement and test point label can be added to the report by pressing “Record Point” during the period while the RF is on. Or, you may wait until the RF times out and then press the Record Point button. In both cases, the PIM level that was last seen by the instrument before the RF turned off will be the level that appears in the report.
The Peak PIM level will reflect the highest IM level that occurred while the RF was turned on. This may be the same as the PIM Level Measurement or higher, depending on how stable the IM source was during the measurement period.
Touching the letters “dBm” or “dBc” directly to the right of the large PIM value display in the center of the main screen will toggle the display between the two types of readout.
When the camera icon is pressed, a screen shot is saved in .png format to the root D: drive (there is no option to save it in any other location). There are two ways to retrieve the picture:
No. The test tones are controlled by a feedback loop to keep PIM measurements consistent across environmental factors.
Yes, All three hubs can be used at the same time. Using a USB keyboard and mouse will make processes easier to control.
The internal Solid State Drive has been dislodged and the unit will need to be returned for repair.
For Touch Screen calibration, do not use fingertip; a small plastic pointer is required for accuracy.
You have a bad antenna connection. The unit needs to be sent in for repair of the bad connection. It cannot be repaired on site.
There is an issue with the motherboard that cannot be fixed on site. Please send unit in for repair.
If your unit is not properly plugged into a grounded source it will shock you. Make sure your unit is plugged into proper a grounded power source to avoid personal injury.
Check the time and date on the unit to ensure it is current and correct. Technical support can correct the date remotely if mismatched dates occur.
The error occurs when you connect the USB port before you connect the monitor port. Disconnect both ports. Connect the monitor port first, and then connect the USB port. This will ensure you have the proper RTF connections in place.
Solution: Some models of the computer in the iQA will try to boot from the USB device before booting from the hard drive. Remove the USB device when powering up the unit.
To interchange between DBM and DBC simply click on the DBM or DBC icon and it will toggle between the two.
Make sure the COM port you’ve selected is the appropriate port for your COM device (i.e. a USB-to-Serial adapter cable might be on COM7, or COM 27). Also check your device properties to ensure COM settings are set to the following:
Also, if you are using a serial adapter (i.e. USB-to-Serial), make sure the adapter is compatible with your operating system (many adapters are not compatible with Windows 7 operating system). Check the vendor website or documentation to confirm compatibility.
If the battery light is on, the unit is working. However, if the battery light does not turn on and the system does not light up, you must send your unit in for repair.
It is possible that all of the test point labels have been deleted, which in the current version of software causes the application to shut down on starting. A work around until the next release is to delete the usersiHAconfig file. There should be one or more folders starting with iHA_interface.exe located on XP in the following location: C:\documentsandsettings\"username"\appdata\local\kaelus\.
Delete all of these folders to rectify the problem. We are working on a new release to address the issue.
When carrier power levels fall outside of your analyzer’s specification, the software produces an UNLEVELED or CHECK TX error to reflect this error condition. Check to make sure all ports on the back of the analyzer are properly connected (on D series analyzers check the Detector cable between the PA and RF modules). If problems continue, the unit may need to be returned for repair. Please contact support for additional assistance.
i. D and E series BPIM analyzers require National Instruments DAQmx drivers to be installed to your machine. The latest version of these drivers may be downloaded directly from National Instruments (http://www.ni.com).
ii. A and B series BPIM analyzers require National Instruments Traditional NI-DAQ drivers to be installed to your machine. The latest version of these drivers (v7.4.4) may be downloaded directly from National Instruments (http://www.ni.com).
Some newer versions of the Traditional NI-DAQ drivers also include a version of NI-DAQmx which causes the software to look for a D series PIM analyzer by mistake. This problem can be corrected by making the following modification to the Windows registry.
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A bug discovered in the 11.3.1 software release caused the D-series analyzers to show a higher receiver measurement (both on noise floor and in PIM measurements) if the VFP was started after having previously connected to an E-series analyzer. The workaround for this bug is to (1) EXIT VFP, (2) EXIT BPIM WEB ENGINE, (3) START BPIM WEB ENGINE, (4) START VFP with D-series analyzer only.
The invalid configuration file message is related to the VFP attempting to load a configuration which will not work for the connected analyzer. The message relates to the “instrument state” of the VFP and tries to automatically configure the power, frequency, reference level and various other settings in the VFP. This is not a serious error and the correct configuration (or “Default Configuration”) can be manually selected after you click OK. The VFP configuration file should be automatically corrected upon exit, and will not throw the error again the next time the program is opened.
Check NI-MAX (National Instruments’ Measurement and Automation Explorer) to confirm all DAQmx devices are present. If devices are displayed with a warning icon (yellow triangle) this may indicate that there is a potential problem communicating with the USB boards. This warning may be related to a Windows Hotfix which is available through your Microsoft Windows Update. The following instructions demonstrate how to apply the hotfix to your PC. The warning errors will go away following a reboot.
Important Notice: Microsoft Hotfix for USB DAQ Devices and Windows XP
If you are using NI-DAQmx with a USB DAQ device on Windows XP, you should install the Microsoft hotfix reported in Microsoft Knowledge Base 969238.
The USB/interface cards in the FE and PA modules are sometimes affected by a static discharge shock. The discharge can cause the USB card to lose connection and give the error you see. On the early USB cards, we found that NI did not properly ground the connector shield. To re-connect the USB card and clear the error, close the VFP and unplug the USB cable at the FE module. Wait a couple of seconds and plug the USB cable back in, listening for the recognition ding. The VFP should open normally. Preventative measures such as proper grounding and the use of ESD wrist straps may alleviate these errors. If the error persists the unit may require an engineering fix which can be installed at the nearest service facility.
Sometimes the PIM Web Engine (or Hardware Engine) is mistakenly blocked by the windows firewall (even when it’s been disabled). There’s a fairly easy way to unblock the process through the Windows firewall settings panel. Follow these instructions to reset the PIM Web Engine (or Hardware Engine) in the Windows firewall settings, and it should work again.
This should reset the program access restrictions in Windows firewall and allow the program to be visible again.
Calibration files are stored in the BPIM software program directory (“C:\Program Files\Kaelus\BPIM Suite”, or “C:\Program Files\Summitek\PIM” for legacy A/B series PIM analyzers).
Follow these instructions to make sure you have all of your switch positions mapped according to your unit serial numbers.
Legacy A|B series BPIM analyzers can be controlled using the Advanced Programming Interface protocol which is provided through the latest BPIM software release. Due to the requirement for Traditional-DAQ device drivers for the legacy BPIM analyzers, the system configuration consists of a hybrid installation of both legacy PIM software (i.e. BPIM Distribution Software v7.9), and the latest BPIM software release (both are available for download from our website). The following procedure demonstrates the process for configuring your Legacy PIM system for control using the BPIM API.
Once you’ve confirmed that the VFP software v7.9 continues to operate correctly then you may leverage the BPIM API protocol language to control your legacy BPIM analyzer programmatically and collect measurement data in your customized application.
View the iVA iOS user manual for information including setup, software downloads, navigation menu and much more!
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