Resolution of PIM vs Distance plot is poor compared with Return Loss vs Distance plot
It will often be the case that the resolution of the PIM vs Distance plot is not as good as the Return Loss vs Distance plot.
This is to be expected, due to differences in the sweep bandwidth and signal-to-noise ratio of each measurement.
Nevertheless, several strategies are available to maximize the resolution of the PIM trace:
• Move the F1 tone frequency to either the very top or very bottom of the instrument’s transmit band.
• Make the F2 sweep range as wide as possible, preferably as wide as the instrument’s transmit band.
• Change the PIM product from IM3 to IM5. (Note: This step should be taken only if the IM5 product is 10–20 dB
above the instrument’s noise floor.)
•
Make sure resolution enhancement is turned ON for the PIM plot, as described in Section 8.4.2
Instrument will not display resolution-enhanced results on PIM vs Distance plot (or Return
Loss vs Distance plot)
In some situations the resolution enhancement mode of the RTF system can fail to return a result, leaving the user with
only the standard resolution results. The three most common reasons for this are:
• Poor Signal-to-Noise Ratio (SNR):
The resolution enhancement algorithm typically requires an SNR of 20 dB or
better. Weak PIM sources (or well-matched devices, in the case of Return Loss measurement) often produce signals
that are close to the instrument’s noise floor, thereby resulting in poor SNR and a failure of the algorithm to converge.
• Unstable Faults:
The resolution enhancement algorithm requires all PIM faults and Return Loss faults in the system
under test to be stable for the duration of a measurement. If the magnitude of a fault changes significantly during the
course of a measurement sweep (due for example to mechanical instability, micro-arcing or RF heating effects), the
resolution enhancement algorithm may fail to converge.
• Dispersive Devices:
The resolution enhancement algorithm assumes that all cables and components in the system
under test are non-dispersive; that is, that they have a group delay response that is constant over the instrument’s
transmit and receive bands. Unfortunately, devices like filters and antennas are often highly dispersive, and therefore
don’t satisfy this requirement. In such cases the resolution enhancement algorithm can fail to return a result.
Note that while the resolution enhancement mode cannot always be guaranteed to converge, the standard resolution
mode will produce accurate results in all but the most extreme cases.
Peaks on PIM vs Distance plot do not line up properly (or at all) with peaks on Return Loss vs
Distance plot
The accuracy of PIM vs Distance measurements can be adversely affected by the presence of large return loss faults in
the device under test. Symptoms include:
• Peaks on PIM vs Distance plot do not line up properly with peaks on Return Loss vs Distance plot
• PIM vs Distance plot can contain spurious peaks that do not correspond to any components or interconnects in the
device under test
To avoid such problems it is strongly recommended that all major return loss faults in the device under test be repaired
before performing any PIM vs Distance measurements.
PIM vs Distance plot has a large peak at zero metres (or feet) which won’t go away even after
re-working the DUT several times
A persistent PIM fault at zero metres may be due to contamination on the 7-16 connectors of the RTF module or iQA/iPA.
Use the following procedure to remedy this problem:
•
Turn off RF power from iQA/iPA.
• Disconnect RTF module from iQA/iPA.
Operating Manual
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Passive Intermodulation and Return Loss Fault Ranging Device
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