What is intermodulation distortion?
The non-linear circuits in the transmitter output stage of the wireless microphone and the receiver input stage are very important for the design of RF components. If multiple frequencies enter the non-linear circuit at the transmitter output or receiver input stage, or the RF amplifier is over-driven for non-linear operation, the input signal will generate some additional frequencies. These additional frequencies are called intermodulation (IM) products. The interference they cause is called intermodulation distortion (IMD).
Under what circumstances will intermodulation occur?
Intermodulation can occur between transmitters too close, and the signal radiated from one transmitter produces intermodulation products in the output stage of a nearby transmitter. Then, the intermodulation products will be sent along with the tuned carrier frequency of the affected transmitter. If the receivers in the system are tuned to the same frequency as the intermodulation products, these receivers may experience intermodulation distortion. If the receiving antenna is too close to the transmitter, intermodulation will also occur, resulting in intermodulation products at the receiver input stage.
What are the factors that affect intermodulation?
The frequency at which the intermodulation product is generated is mathematically related to the original carrier frequency. The above table shows the frequencies of all 2 to 5 order intermodulation products that can be generated in a dual carrier system. The order of the intermodulation product refers to its relative power level. Regarding the intermodulation power level, there are two things to keep in mind. First, the power of the intermodulation product is proportional to the square of the carrier power and inversely proportional to the square of the physical distance between the transmitters. In other words, a transmitter operating at high power at a tight distance will produce stronger intermodulation products than a transmitter operating at a lower power at a distance. Secondly, the power of intermodulation products decreases as the order increases. In other words, the second-order intermodulation product contains more power than the third-order, and the third-order power is greater than the fourth-order, and so on.
Of the 20 intermodulation products listed in the table above, only four are close enough to the carrier frequency and may cause intermodulation distortion. Other intermodulation products are beyond the bandwidth of other wireless microphones or IEM systems that may be used, so they can be ignored. The power level of intermodulation products exceeding the fifth order is usually very low, even if they are within the bandwidth, they will not cause intermodulation distortion, so they can usually be ignored.
The figure above depicts the most common intermodulation situation. Two carrier frequencies can generate third-order intermodulation products at (2×f1)-f2 and (2×f2)-f1. In this example, for carrier frequencies of 590 MHz and 595 MHz, IM products can be generated at 585 MHz and 600 MHz. If another wireless microphone or IEM system is set to work at 585MHz or 600MHz, it may produce intermodulation distortion.
The figure above depicts the second most common intermodulation situation. Three transmitters used at the same time can produce six in-band third-order and three in-band fifth-order intermodulation products. In this case, the intermodulation products can occupy 9 frequencies around the original 3 carrier frequencies. To avoid potential intermodulation distortion problems, most manufacturers recommend a minimum margin of 250kHz between any third-order intermodulation product and any tuned carrier frequency.
Unfortunately, as more transmitters are included in the system, the intermodulation pattern increases exponentially. The following is a spectrum scan of the 9-channel VHF wireless microphone system. The carrier frequencies of the nine wireless microphones are shown in red. The extra peaks shown in gray are intermodulation products. This is a fully compatible frequency setting, so there is no risk of intermodulation distortion at any of the nine carrier frequencies. However, adding a 10th wireless microphone within this range will cause problems. Therefore, the generation of intermodulation products can severely limit the channel density and stability in wireless microphones and IEM systems.
▲With the increase of transmitters, the intermodulation situation increases exponentially
When controlling intermodulation in actual operation, the most important thing is to understand that the tuned carrier frequency may not be the only frequency generated in the system. We cannot artificially calculate the frequency of possible intermodulation products in a system composed of more than two or three transmitters. However, there are simple ways to coordinate these frequencies.
At present, many brands of wireless systems include a group channel matrix, which aims to provide a pre-configured compatible frequency set, while considering all potential in-band 3rd and 5th order intermodulation products. If you use this method to coordinate the system, it is important to remember the clean spectrum of the channel matrix.
The group channel matrix method is suitable for coordinating low channel counts in entry-level and mid-level systems. For professional systems with high channel counts, using software tools such as Wireless Workbench 6 is a more effective method.