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Intermodulation Distortion

Intermodulation occur when the non-linearity of a device or system wth multiple input frequencies causes undesired otputs at other frequencies. In communication system (include in satellite communication), this means that signals in one channel can cause interference with adjacent channels. As the spectrum become busier and the channels become more tightly spaced, minimizing intermodulation distortion becomes more important.

To measure this Intermodulation, we can combine two equal power signals with a set frequency spacing at the input of the device-inder-test DUT). The output spectrum will look like figure below ;

The two largest signals are the amplified carrier signals, and the smaller signals, moving away from the carries in both directions, are the 3rd order, 5th order, and 7th order intermodulation products, respectively. The frequency spacing between all the signals is equal.


The interest parameters here as follows :

Carrier (C) : This is the power of the carrier signal in dBm units. It is similar to P(out) parameter, except that C is measured with a spectrum analyzer, and P(out) is measured with the power meter with only one RF input signal.

3rd Order Intermodulation Product (IM3) : This is the power of the spurious 3rd order intermodulation signal in dBm units, measured with the spectrum analyzer.

Carrier to 3rd Order Intermodulation Ratio : This is the ratio of the carrier power to the spurious 3rd order intermodulation power in dB.

3rd Order Intercept Point (IP3) : This is a figure of merit in dBm for the device and normally varies with tuning.

If we talk about Intermodulation, we should know about the term OBO (Output Back Of).  OBO is essensial in reduction oiwer appliead to SSPA (Solid State Power Amplifier) or TWTA (Travelling Wave Tube Amplifier)  to minimize the effect of any Intermodulation product created by modulated carriers. The OBO is proportional to the number of carriers in operation and increases by 3 dB as the number of carriers doubles, but that rule cannot be defined absolut as amplifiers vary.
The amout of OBO needed to avoid Intermodulation is typically 1 to 4 dB, but it depends on the SSPA design. Usually, for SSPA, OBO is about 3 dB. Due to the physic of TWTA which 1dB compression point [P1] is at least 3dB below saturation this means operating the amplifier with around 6 to 7dB OBO to avoid Intermodulation.

Example :
When operated single carrier you may operate the SSPA at its rated power e.g 400W or +46 dBm
When operated multi carriers, you will operate the SSPA backed off several dB from its rated power e.g 400W minus 3 dB OBO = 200 W or 43 dBm.. Note that 200W is the total aggregate power of all the carries (e.g 10 carries at 20W).

The acceptable Intermodulation value in amplifier is around 30 dBc. You can look the value in specification brochure of the amplifier (SSPA or TWTA) or you can test it personally with your test equipment (Power Meter and Spectrum Analyzer).




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