3.2 System Level Considerations 61There are a few things that can be done about DC offset. If the radio uses a modulation type where there is not much information at DC (such as an OFDM signal where the first subcarrier does not contain any information), then a blocking capacitor can be placed right before the ADC. This will act as a highpass filter and will prevent the DC offset from entering the ADC. Since DC offsets are often not time variant it may also be possible to calibrate much of it out of the signal path. This can be done by sensing the DC offset in the baseband and adjusting the output from the mixer to compensate for it. 3.2.8 Second-Order Nonlinearity IssuesSecond-order nonlinearity is also very important in transceiver specifications. In this case, the main cause of nonlinearity is the IQ mixer that downconverts the signal to baseband. Consider again the case where there are two in-band interfering signals. If these signals are close to each other in frequency, then the difference between these two frequencies may fall into the baseband bandwidth of the receiver as shown in Figure 3.17. If this happens then the signal-to-distor-tion ratio must still be large enough to make sure that the signal can be detected with sufficient BER. It is usually only the final downconversion stage that is the problem, as prior to the mixer itself, a simple ac coupling capacitor will easily filter out any low frequency IM2 products produced by the earlier stages in the radio. Example 3.8: IIP2 Calculation A direct downconversion receiver is required to detect a signal at 2 GHz with a power level of -80 dBm at the input of the downconversion mixer. An SNR of 15 dB is required and the signal has a bandwidth of 20 MHz. Two interferers are pres-ent in band, each with a power level of -20 dBm. They are located at 2,100 MHz and 2,110 MHz. Determine the required IP2 for this system.Solution: The second order nonlineaity of the mixer will produce a tone at a frequency of 2,110 - 2,100 = 10 MHz, which will fall into the band of the desired channel. Since the signal strength is -80 dBm and the SNR required is 15 dB, the power Figure 3.16 LO self-mixing can cause DC offsets.