Fanalog is the higher than that of the converter. Therefore, to safely use analog IF frequency, tjrms is the aperture uncertainty, E is this converter, noise generated by the front end must be the average DNL of the converter, Vnoiserms is the thermal about -131 dBm/Hz. Modifying Equation (2.3.2) be- noise of the converter, and N is the number of bits. cause the NF of the AFE is not yet known, gives Equation Many data sheets will include information on the (2.3.9), and the gain and noise requirements. With this effective number of bits that a converter possesses. equation, the combined front-end gain and noise figure Software defined radio CHAPTER 2.3 must be 43 dB to ensure that the ADC does not domi- With the total integrated channel noise and the re- nate performance. quired SNR for the modulation standard, the reference Gmin + NFAFE = NFADC 10 sensitivity can be determined. Keep in mind that the required SNR may be positive or negative depending on (2.3.9) the amount of digital gain provided by detection algo- rithm. As an example, GSM requires about 5 dB SNR while IS-95 requires an SNR of about-16 dB. If the AFE generates a noise density of -131 dBm/Hz Noise figure for an ADC (the ADC is 10 dB below this and not a contributing If it is not possible to design the system such that the factor) and the channel bandwidth is 200 kHz, then the converter noise is significantly below the remainder of the total channel noise is -78 dBm/200 kHz. If the required system, then the noise must be included in the calcula- signal level is 5 dB above this, the smallest signal that can tion. This can be accomplished by using Equation (2.3.9) be detected as presented to the ADC will be -73 dBm. to determine the noise from the ADC, or the NF of the If the conversion gain of our signal chain is known, then converter can be calculated and included in the cascaded the sensitivity at the antenna can be calculated. In order NF of the signal chain analysis with the other linear de- to achieve the noise of -131 dBm/Hz, a gain plus NF of