Passive intermodulation, PIM is a form of intermodulation distortion that can occur when no active components are present. It arises from the action of passive components or elements that have non-linear responses to any signals
Passive intermodulation, PIM can be generated by a variety of components and objects: everything from coaxial connectors to cables, even rusty bolts or any joint where dissimilar metals occur. Even some normally ‘linear’ components may generate PIM
Passive intermodulation, PIM can produce interference, and this can sometimes hide the wanted signal. Accordingly it is often necessary to try to remove the element that generates the passive intermodulation so that interference generated can be removed.
What is passive intermodulation, PIM?
Passive intermodulation occurs when two or more signals are present in a passive non-linear device or element. The signals will mix or multiply with each other to generate other signals that are related to the first ones.
The nature of passive intermodulation, PIM is that it occurs in elements that would otherwise be expected to operate in a linear fashion. Typically any mixing or multiplication in diode components in circuits is not termed PIM as the mixing is generally wanted and the diodes are expected to be in place. Instead passive intermodulation is normally as a result of the spurious generation of non-linearities - typically it may occur in connectors, switches, isolators of the like. Here oxidation or other effects may cause the generation of a non-linearity.
The passive intermodulation products caused by the non-linearity follow exactly the same principles of those of wanted modulation products in a mixer. It is found that the various harmonics of the input frequencies mix together to form products that can remain within the required operational band.
On high power transmitter towers it was found that bolts would rust after a while and the oxide would give a non-linear junction that would generate passive intermodulation. As a result, the effect is sometimes called the rusty bolt effect.
Passive intermodulation basics
Passive intermodulation requires two signals to be present.
If the two signals involved are F1 and F2, then the intermodulation products can occur at frequencies of ± M ⋅ f1 ± N ⋅f2.
When referring to intermodulation products, the order of the products is often mentioned. This is the number of times a product within the output is multiplied, i.e. one that is generated by 2 x F1 - 1 x F2 would be a third order product and 3 x F1 + 2 x F2 is a fifth order product and so forth.
|Passive Intermodulation Summary Table
|Product Order||Subtractive PIM Product Frequency||Additive PIM Product Frequency|
|3rd||2 F1 - 1 F2||2 F1 + 1 F2|
|5th||3 F1 - 2 F2||3 F1 + 2 F2|
|7th||4 F1 - 3 F2||4 F1 + 3 F2|
|9th||5 F1 - 4 F2||5 F1 + 4 F2|
The generation of passive intermodulation, PIM spurious signals may not be a problem if the signals do not interfere with other users of the system itself. However there are many times when these signals can interfere with other users. In the case of passive intermodulation products, the levels are normally relatively low so the interference is generally limited to local users or the system itself.
If the two initial frequencies have modulation applied to them, they will occupy a greater bandwidth than just a single unmodulated carrier. As the modulation product orders become higher, so the modulation spreads further out and can give rise to an overall background noise level which can degrade performance.
Typical PIM levels & relevance
Passive intermodulation is almost always a small effect. This arises from the fact that it is often caused by small spurious non-linearities. Typical signal levels may be 100dB down on the levels of the signals that give rise to the PIM products.
Active circuits such as amplifiers and the like generate much higher levels of intermodulation and other forms of distortion. However this can often be removed by filtering. In instances where passive intermodulation occurs this may not always be the case.
One major area where passive intermodulation is a major issue is on systems where both transmitter, Tx and receiver, Rx use the same antenna, and utilise a common feeder. In many systems the transmitter and receiver will run simultaneously using a duplex system where the transmitter and receiver use different frequencies.
The transmitter signal can be filtered and reduced to a level where it does not affect the receiver. Also the signal combiner will prevent direct signal from the transmitter reaching the receiver. However any passive intermodulation occurring after the signal paths are combined, e.g. in the feeder or antenna will cause a major issue. The passive intermodulation products can give rise to unwanted signals within the receive band - even the transmitter noise can cause an issue.
Even if the passive intermodulation products are 100 dB or even 120 dB down on the transmitted signal, these levels are still enough to cause the receiver performance to be impaired if the products fall within the receive band.
Examples of systems where transmitter and receiver use the same feeder and antenna include cellular base stations, satellite systems, and other radio or wireless systems where duplex transmission, i.e. simultaneous transmission in both directions is required, but on different frequencies.
Passive intermodulation causes
Passive intermodulation effects occur as the result of a non-linearity being experienced by two or more signals.
A whole variety of instances can give rise to a non-linearity that can cause PIM effects. Some of the more common ones include:
- Joints where dissimilar metals meet and oxidation, etc. occurs converting the joint into an area that exhibits some diode effects.
- Coaxial connectors - connectors are often prone to oxidisation. Especially silver plated connectors where the silver oxide is generated as a result of weathering or even just age. Silver oxide forms an excellent diode which will give rise to passive intermodulation. Connectors need to be high quality components, assembled correctly and water proofed well if they are to be used externally. But even then they will deteriorate with time. Connections made with poor connectors, or of the connectors have been assembled poorly will generate higher levels of passive intermodulation faster.
- Feeder lines - feeders such as coaxial feeders generate levels of passive intermodulation. The braid in the outer conductor provides many metallic interfaces in which passive intermodulation, PIM can be generated. Even if foil is used, there are interfaces between the layered construction of the foil wound round the dielectric. For applications where passive intermodulation is important coax with a good PIM performance can be bought. Some manufacturers may guarantee levels to below a given level, e.g. -140dBc, although at a much higher cost.
- Loose connections and irregular contact areas, on a macroscopic or even a microscopic scale, can cause an inconsistent flow of current and generate inhomogeneous electromagnetic fields.
- Dirty connections - investigate any areas where dirt may collect and cause connections to deteriorate.
- General anodic effects - this typically happens where water or even just moisture ingress occurs.
- The use of ferromagnetic metals, like iron, nickel and steel can give rise to passive intermodulation. These metals show magnetic hysteresis effects when energy is applied. This means that the resulting signal level is dependent, even to a small degree on input level. Hence this is a non-linear effect.
- Spark discharges can give rise to passive intermodulation. The spark will cause craters to be formed and some oxidation that will generate a diode effect. Sparks can result from poor connections or even the hot connection or disconnection of a connector.
It can be imagined that one of the most common components where passive intermodulation is generated is within coaxial connectors. If they are not tightened sufficiently, or if the connections have become oxidised, then they are prime candidates for being the source of any passive intermodulation products that are being experienced.
One of the difficulties when trying to locate passive intermodulation is that the nonlinearity that gives rise to them does not manifest itself at low input signal levels. This may result from a number of reasons including:
- PIM levels are normally low and at low power levels the passive intermodulation products may fall below the thermal noise level
- Not enough stress may be placed in any material involved. If the passive intermodulation arises from a hysteresis effect, then the activating signal may not be large enough to drive the material round its hysteresis loop.
Passive intermodulation prevention
Most passive intermodulation problems are associated with transmission lines and in particular the connectors associated with them.. To reduce the likelihood of PIM, there are several points to note and take action upon. Prevention is always better than cure.
- Inspection: When the connectors are apart, inspect them for physical damage. The centre connector element should not be loose, and should have no visible dents or scratches. Any damage or contamination may allow micro arcing or the diode effect to occur, causing some level of PIM.
- Don’t use worn connectors: Wear on connectors is always an issue. Repeated connect / disconnect cycles can give rise to wear. Manufacturers specify the number of mating cycles they can guarantee. Particularly those that need to be tightened to a given torque will have a lower number of permitted cycles. Some devices cannot handle more than a few complete torque cycles and care must be exercised so the device is not technically “worn out” before it is even installed.
For optimum permanent installation, very few connector designs appear to be able to handle any more than a couple of cycles before the connector base loosens from the chassis. This generally causes the antenna to fail any passive intermodulation tests and it will also most likely show a questionable line sweep.
- Cleaning: It is necessary to ensure that connectors are as clean as they can be as dirt will give rise to PIM in the short term and more so in the longer term:
- Remove loose particles: This can be achieved by using low-pressure compressed air. Low pressure air spray cans can be sued for this if a low pressure line is not available. A toothpick can be used to remove any small particles that the air does not dislodge. Don’t use a sharp metallic pick as this may damage the connector contact surface.
- Clean with Isopropyl alcohol: Use Isopropyl alcohol on cotton swabs to clean the rest of the surface. Use only enough to do the job because it may melt any plastic parts, if too much is applied.
- Blow again with low pressure air: Use the low-pressure air again to remove any remaining small particles and dry the surface.
- Care in mating connectors: Take care not to twist connectors when reassembling or mating them. If they are twisted, small scratches will form on the centre pin, which can destroy its precision. The small scratches can generate both excessive VSWR and PIM.
- Torque: Proper torque on the connector is required and will help minimize PIM. Low torque will allow gaps and PIM from the centre connector. High torque will damage the centre connector, again causing PIM. For 7/16 DIN connectors, 20 foot-pounds is an accepted value, and for Type N connectors, 12 inch-pounds is common. If the manufacture specifies slightly lower values, use them. Make sure to tighten connectors with the proper torque wrench.
These measures will help reduce the likelihood of the possibility of passive intermodulation being developed. These steps will also help ensure that no additional power is reflected as a result of poor connections - an added advantage.
The levels of passive intermodulation products are often very low, although not always. This can make them very difficult to track down and identify. Typically it is best to ensure good practice is adhered to during manufacture and then to be aware of the areas that can cause passive intermodulation. If connectors and other areas of electronics are exposed to the weather, then this can be an area to investigate if passive intermodulation becomes an issue.
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