66 System Level Architecture and Design Considerationswhere N is the number of subcarriers. The above equation has been verified through extensive simulations and comparison to measured results by the authors. However, note that if the output power is close to the 1-dB compression point, this formula will become less accurate. Note also that narrowband systems rely on filtering to minimize the number of tones that can intermodulate, making TB tones much less of an issue. However, OFDM uses multiple subcarriers that are always present; hence, it is much more like a video system in which a whole range of tones can in-termodulate, even though EVM or BER, but not CTB is typically used as a measure of performance.If the transmitter is backed off and transmitting at a low power level then the noise in the transmitter circuits can become an important contribution to the EVM of the system. If the total noise power density coming from the transmitter is No then the EVM contribution from noise is given by: onoisetN BWEVMP×= (3.42)where BW is the bandwidth of the channel. 3.2.11 ADC and DAC SpecificationsWhile this book will focus on RFICs in detail, it is important to say at least a few words about the analog-to-digital converters (ADC) and digital-to-analog convert-ers (DAC), as these blocks are the interface between the radio and the DSP. Gener-ally from a system perspective it is important to specify both the number of bits and the jitter or phase noise requirement of the reference clock, which drives the converters. Both these properties will affect the BER by influencing the SNR. Suppose that a quantizer converts a continuous analog signal to a discrete digi-tal signal with a characteristic shown in Figure 3.20, where x is the analog input Figure 3.20 Transfer characteristic of a multibit quantizer.