Passive Inter-modulation, also known as PIM, is an unwanted effect where spectral distortion is generated as a result of non-linear effects in a Multi Transmitter (co-site) set up like you find in mobile telephone networks, i.e. base stations. The handling of traffic through a base station is in general handled by more than one transmit frequency.
With more than one transmitter active on the base station (co-site operation), there is always a serious chance that PIM generation plays a role and that the additional and unwanted spectral components are falling into the base station's receive channels. This noise in the reception channel reduces the base stations' capability to handle the traffic as planned because the BER number is increasing.
What is the cause of PIM? In the close vicinity of the base station, metallic junctions of different materials with different voltaic voltages can cause non-linear and diode like effects in the close vicinity of the transmit antennas as the field strength close to these metallic junctions is quite high.
Annalise how non-linear metallic effects can cause inter-modulation signals falling into your RX channels with the WinRFCalc PIM analyzer. What are the calculation methods used in this tool:
- Define the TX and RX frequencies of the station This is done by defining the TX frequencies and calculate the RX frequencies by taking the TX-RX offset into account.
- Create a multi tone time domain signal with all the defined channels.
- When the Non-Linear transfer function is selected, then during the calculation of the time domain multi tone signal a non-linear operation is applied to the time domain signal. This non-linear operation is the source of the inter-modulation products.
- The next step is to run a Fast Fourier Transformation (FFT) where the time domain signal is converted from the time to frequency domain.
- In the spectral display, the green curve is the RF spectrum calculated by the FFT.
- The program does not analyze at present the effects of different power levels and is just frequency driven. You can check with the chosen frequency layout if there are chances of PIM interference in your bases stations receive channels
The PIM analyzer is undergoing a major redesign for the 2024 release.