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Cross modulation vs intermodulation - Difference between cross modulation and intermodulation

This page cross modulation vs intermodulation describes difference between cross modulation and intermodulation.It explains basics of cross modulation and intermodulation and mentions useful difference between terms.

Both of these terms are vary useful in RF as they result into receiver not functioning as desired due to overload and generation of unwanted harmonics and other inter-modulation products.

What is cross modulation?

The Cross modulation is the intermodulation distortion caused by multiple carriers within the same bandwidth. This can happen when strong nearby unwanted AM signal gets added with the wanted AM signal. The effect will be observed on the weaker input signal. This results into receiver gets overloaded.

Examples:
Cross modulation is observed in cable TV and OFDM based systems. In cable TV, same amplifier is used for multiple channels for amplification. OFDM uses multiple carriers within the bandwidth of the single amplifier. OFDM(Orthogonal frequency division multiplexing) modulation technique is used in WiMAX or WLAN based technologies.

Measurement:
Turn on all tones or carriers except the one and measure the power at the frequency that was not turned on.

Fig-2 depicts frequency spectrum of the various signals. As shown the jammer signal is present just outside the edge of channel filters. Hence large part of cross modulation signal falls within passband of filter. If LNA IP3 point is not higher, the cross modulation power within filter pass band exceeds total thermal noise power.

Cross modulation power equations is as follows.
Total unwanted signal at the LNA input is sum of first jammer and transmitter leakage.
x(t) = Aj*cos(Wj*t) + r(t)*cos[WTt + θ(t)]
Average cross modulation power is expressed as
Pcrossmod = 6 + 2*( Ptxin - PIIP3 ) + Pj
All the units are in dBm.

For the smaller jammer signal, cross modulation can be observed as a form of a time varying gain compression of LNA. This can results due to transmitter leakage signal which uses non-constant envelope modulation. The cross modulation noise power is the total power in the AM spectrum which is present around the single tone jammer. Small cross modulation power leads to larger IP3 for the LNA.

Cross modulation testing

Cross modulation testing is done using two tone tests. Some frequency spacing is kept between desired and undesired signal. Desired signal is set to -53 dBm power. Undesired signal is amplitude modulated with 30% level and kept at 20KHz away. Now the power of undesired AM modulated signal is increased until unwanted AM output is 20 dB below the desired signal power.

Cross modulation specification of >=100dB is considered to produce good system performance. Receiver having higher TOI point also results into better cross modulation system performance.

intermodulation

Intermodulation distortion is relevant to devices which handle multiple frequencies. There are second order and higher order intermodulation distortions. Refer second order vs third order intercept point which describes intermodulation products, TOI point and IP2 point.