cts and works with a predis- tortion block to compensate for intermodulation and adjacent channel leakage power. The DPD bandwidth expansion requires the DPD observation receiver bandwidth to be five times the data bandwidth. As an example, in 100 MHz carrier aggregation scenarios, this means that the DPD bandwidth must be at least 500 MHz. Also, the observation receiver cannot add impairments to the observed signal because they cannot be discerned from the main transmit- ter (TX) path impairments. Consequently, the DPD observation path must have excellent lin- earity which adds cost and complexity. Conversely, if the main TX path has negligible impairments, then the DPD path impairments can be corrected. Since the RF DAC does not introduce gain or phase errors, there are negligible TX path impairments. Therefore, a low- cost and lower performance DPD receiver such as a ZIF receiver can be employed. There are three reasons that the ZIF architecture has lower cost compared to high-IF or direct RF sam- pling: (1) quadrature demodulation enables a lower conversion rate, baseband sampling, dual- channel high-speed ADC because it only needs to quantize one-half of the DPD expansion bandwidth; (2) the ADC samples baseband signals, which means that the ADC does not need 714 Chapter 5 pico- or femtosecond aperture jitter; 13 and (3) the baseband I/Q anti-alias filters are lower cost and easier to design compared to IF or RF filters used in high-IF or direct RF architec- tures. In summary, the RF DAC transmitter relaxes DPD receiver signal-path performance requirements, thus further reducing system cost and design complexity [16]. 5.5.3 Direct RF Sampling Receiver Design In a direct RF-sampling architecture the data converter digitizes a large segment of spec- trum directly at RF frequencies and transfers the information to a digital signal processor. This is a paradigm shift that takes what has traditionally been handled by analog compo- nents (i.e., mixers, LOs and their associated filters and amplifiers) into the digit