Radio Receiver Cross Modulation

- radio receiver cross modulation, what it is, the factors that affect it and how receivers can be designed to minimise it.

Cross modulation is an effect that affects receivers used for amplitude modulation or vestigial sideband, or other forms of modulation where there is an amplitude component.

Cross modulation traditionally occurred in receivers receiving an AM signal in the presence of other strong AM signals. It was found that the modulation from the strong signal was cross modulated and appeared on the weaker signal being received.

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

It is often necessary to define cross modulation in specific terms. 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   =   Mu (4  Pu) / (Rxip   +   2 Pu)

    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 not a specification that is as widely publicised as many others, however its effects are seen in a number of occasions:

  • 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:   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. As such it can give rise to cross modulation in receivers. In televisions sets it 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.
  • CDMA / WCDMA cellphones:   Cross modulation is also important in cellphones using the various forms of CDMA where there is an amplitude component on the signal. In particular it can be an issue as a result of the full duplex operation of these systems requires that the transmitter and the receiver operate simultaneously. It is most critical when a handset is operating at a distance from the base station within a given cell. Under these circumstances the handset transmitter will be operating towards its maximum output power to provide reliable communications with the distant base station. In addition to this the receiver will need to be operating at maximum sensitivity.

    The transmitter leakage power through the duplexer is usually significantly higher than the desired signal channel through the first few elements of the receiver. As a result, the transmitter leakage becomes the dominating effect for nonlinearities for the first amplifier in the receiver chain.

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 content.

By Ian Poole

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