RF Test Equipment Series: PIM Analyzers
Passive intermodulation distortion (PIM), is a prevalent issue with cellular networks and other wireless systems with relatively high broadcast power and dense channel plans. The results of PIM can be the desensitization of the receiver, dropped calls, and harmonic interference that can affect receivers for other frequency bands and services well outside of the carrier frequency of the PIM generating system. As the peak power of highly modulated digital communications continues to increase, PIM becomes a more vexing issue that requires on-site PIM measurements to diagnose and ensure that PIM isn’t the failure mode.
A PIM Analyzer is essential for measuring components, cables, and systems as part of a quality control check during manufacturing, as well as for on-site measurements of installed systems. PIM testing is most commonly done during component manufacture, initial installation of a wireless system, when upgrading a system with new bands or hardware, and if distortion is detecting that may be PIM.
The source of PIM can be as simple as a rusty bolt, or nearby air conditioning ducting. Essentially, every nearby conductive feature is a potential PIM source, but these conductive structures will only produce PIM if there is a non-linear behavior induced by some nuances of the structure, such as the rusty connection between a bolt and tower structure. This fact makes on-site PIM diagnosing and testing extremely challenging, and hence the reason for such stringent PIM requirements for modern cellular network components.
There are two main types of PIM testing, reflective/reverse PIM testing and forward PIM testing. A reflective PIM analyzer is a highly sensitive receiver and two, or more tone generator (one of which may be swept frequency generator) connected by a duplexer to a load device or antenna. In this case, two signals are generated, amplified, and combined before being sent to the load. If there is any PIM, the distorted reflections of the signals containing harmonics are then picked up by the receiver. This type of PIM testing is the most commonly used type of PIM tester, including the testing of base stations and radio transmitters.
Forward PIM testing involves the use of filter networks at the output of the DUT or an external antenna capturing propagated signals. The forward PIM testing method relies on separating the high-level output signals from the low-level PIM signals, which are further analyzed. In the case of forward PIM testing with an external antenna, the PIM analyzer may just be a spectrum analyzer with a highly selective filter on the input port.
Useful PIM Analyzers Links
• Pasternack VNA Calibration Kits
• Pasternack Phase Stable (VNA) Coaxial Cables
• Pasternack Cable Generic Test Cable Assemblies
• Pasternack Skew Matched Pair Coaxial Cable
• Pasternack Precision Coaxial Adapters
• Pasternack In-series and Between-series Adapters
• Pasternack RF Coaxial Probe
• Pasternack Coaxial Probe Leads
• Pasternack Antennas