Direct conversion receivers (DCRs) are prevalent in mobile communication systems due to their simplicity, low cost and for the reason that much of the signal processing can be achieved in the digital domain. A drawback to the elimination of an intermediate frequency stage is an increased susceptibility to second-order intermodulation effects. For example, intermodulation products generated from strong interfering signals can be downconverted to baseband with relatively weaker signals of interest, thereby desensitizing the DCR to the desired signal. Such interfering signals, referred to herein as blocking signals, are ubiquitous; they may originate from communication signals in adjacent channels and/or from transmission sources that, even when far removed in frequency from the channel of interest, transmit at a power level sufficient to cause significant interference problems in a DCR, including bit error rate (BER) and/or signal-to-noise ratio (SNR) degradation.
In view of the susceptibility to distortion by second-order effects, design specification of DCRs typically include high input-related second order intercept point (IIP2) criteria. IIP2 is the theoretical input signal level at which second-order intermodulation products are equal in power to that of a desired signal. Thus, if the IIP2 can be made higher, the power of an interfering signal must reach a correspondingly higher level to have an equivalent detrimental effect on the DCR. It is thus clearly desirable to establish IIP2 at as high a level as possible.
Second-order intermodulation distortion is an effect of non-linear behavior of components in a DCR and downconverting mixers are often mostly responsible for limits on the level to which IIP2 can be established. Compensation techniques can be applied to increase the IIP2, but changes in temperature and/or frequency can reduce the IIP2 to effectively undo applied compensation measures. Indeed, an IIP2 established at 100 dBm can be reduced to 50 dBm in response to a change in frequency or temperature.
Given the potential improvements in DCR performance that can be expected upon overcoming the foregoing limitations in the art, substantial resources are increasingly devoted toward developing interference-tolerant designs in direct conversion receivers.