Cross modulation is a parameter used for radio receivers where strong signals with an amplitude modulated component are used. Amplitude and quadrature amplitude modulation are two examples.
It is found that the strong signal may cause sections of the receiver to become non-linear and in this way the varying amplitude transfers over to other signals.
Effectively, cross modulation is the transfer of modulation from one signal, typically a much stronger one, to another signal, typically a weaker one, die to non-linearities in the receiver chain.
Cross modulation basics
Cross modulation normally arises out of imperfect mixer performance in the radio, although it can easily occur in one of the RF amplifiers. As it is a third order effect, a receiver with a good third order intercept point should also exhibit good cross modulation performance.
To specify the cross modulation performance the effect of a strong AM carrier on a smaller wanted signal is noted. Generally the level of a strong carrier with 30% modulation needed to produce an output 20 dB below that produced by the wanted signal. The wanted signal level also has to be specified and 1mV or -47dBm (i.e. a signal 47 dB below 1 mW) is often taken as standard, together with an offset frequency of 20 kHz.
Cross modulation specifications
In instances where cross modulation is an issue it needs to be specified. To ensure that it is possible to compare the performance of different radios and also to enable benchmarks to be set, it is necessary to have a standard method of specifying it.
Cross modulation is the amount of amplitude modulation which is transferred from an off-channel and unwanted signal to the required one being received.
To define the parameters and conditions correctly, the cross modulation specification must include the percentage of modulation of the interfering signal, its signal power, and frequency offset from the tuned frequency.
It is possible to calculate the level of cross modulation from a knowledge of the third order intercept performance. The percentage of modulation arising from cross modulation effects on the wanted signal is equal to the percentage of modulation of the unwanted signal multiplied by four times its power and divided by the sum of the third-order intercept power and twice the undesired power, or:
CM = percentage of modulation on the wanted signal due to cross modulation
Mu = percentage of modulation on the unwanted signal
Pu = power of the unwanted signal
Rxip = receiver third-order input intercept point power
Cross modulation affects
Cross modulation is really only applicable to instances where amplitude modulation is used. In the early days of radio, it was noticed on amplitude modulated signals like broadcast transmissions, but today there are other signals that have amplitude modulation components.
- AM broadcast receivers: This is traditionally the area where cross modulation effects had been noticed. When listing to weaker AM broadcast signals in the presence of very strong off channel signals, the modulation of the stronger signal or signals was transposed onto the weaker wanted signal. When broadcast receivers were located close to a broadcast transmitter, it could become an annoying problem.
- Analogue television receivers: Although analogue television has now virtually been overtaken by digital television, it is still used occasionally and it was found that cross modulation occurred in some areas.
The most widely used form of modulation used with analogue television systems is vestigial sideband, VSB. It is effectively a form of AM where one of the sidebands has a reduced upper bandwidth. In televisions sets cross modulation manifests itself by creating a ghost image under the wanted on-channel station being received. It can be annoying if television stations are not all geographically co-located, and a more distant signal is being received in presence of a much more local and stronger off-channel signal.
While radio receiver cross modulation may not have the visibility in terms of receiver specifications that it previously had, it is still very important, especially in scenarios in which the modulations schemes used have an amplitude component.