Digital receivers for cellular communications systems require the highest performance levels from analog-to-digital converters (ADCs) and their supporting cast of RF components. The signal chain must be sensitive enough to capture low-level signals, while providing enough dynamic range to handle high-level interfering signals (blockers). Fortunately, the MAX1418 15-b, 65-MSamples/s ADC or MAX1211 12-b 65-MSamples/s ADC from Maxim Integrated Products (Sunnyvale, CA) in combination with the company's 2-GHz MAX9993 or 900-MHz MAX9982 integrated mixers provide exceptional dynamic range for two of the most critical stages in a receiver. In addition, the firm's MAX2027 and the MAX2055 intermediate-frequency (IF) digital variable-gain amplifiers (DVGAs) provide high third-order output intercept (OIP3) performance with the required gain adjustment range for many applications.

For the subsampling receiver architecture shown in Fig. 1, stringent noise and distortion requirements are placed on the ADC. In receiver applications, the lower level desired signal is digitized alone or in the presence of an unwanted signal(s) that can be significantly larger in amplitude. To properly design the receiver, the ADC effective noise figure must be determined under these two signal extremes. The converter's noise figure is determined by comparing its total noise power to the thermal noise floor. For small analog input signals, the thermal + quantization noise power dominate the ADC's noise floor, which is used to approximate the ADC's effective noise figure (NF).

In practice, once the ADC's effective NF is known under small-signal conditions, and the cascaded NF of the analog (RF and IF) circuitry is determined, the minimum power gain ahead of the ADC is selected to meet the required receiver NF. The amount of power gain places an upper limit on the maximum blocker, or highest interference level the receiver can tolerate before the ADC overloads. For BTS applications, the ADC often does not have sufficient dynamic range to meet both the NF requirements (receiver sensitivity) and maximum blocker requirements without implementing automatic gain control (AGC). The AGC can be included either in the RF stages, IF stages, or both.

Other converters in the MAX1418 family are optimized for baseband performance where the input frequency (f_INPUT) is less than one-half the clock frequency (f_CLOCK/2}. Operating in this frequency range and using these baseband-optimized parts provide the best possible converter dynamic range. These converters include the MAX1419, which is optimized for a sampling rate of 65 MSamples/s, and the MAX1427, which is optimized for a sampling rate of 80 MSamples/s, both with spurious-free-dynamic-range (SFDR) performance equal to −94.5 dBc at baseband.

As an example, the MAX1418 was used as the converter in a front-end signal chain (using specifications listed in Table 1). The MAX1418 can be used with a 14-b interface by not connecting the least-significant bit (LSB). If so used, there is a slight signal-to-noise-ratio (SNR) performance penalty and the SFDR performance remains essentially unaffected. Figure 2 shows the ADC noise contribution in the absence of a large-level blocker. Assume all the analog circuitry in front of the ADC has a cascaded noise figure of 3.5 dB. As a first approximation, suppose a designer's goal is for the ADC to degrade the overall receiver NF by no more than 0.2 dB to meet some target sensitivity in a code-division-multiple-access (CDMA) base-station receiver. This NF value should provide sufficient margin to the air-interface requirements, which is also dependent on the final detector's bit-energy-to-noise-power-spectral-density-ratio (E_b/N_o) requirement. If the MAX1418 thermal + quantization noise floor value from Table 1 is used, an equivalent NF of 26.9 dB can be calculated when the device is clocked at 61.44 MSamples/s (a 50× chip rate). The ADC noise in the 1.23-MHz CDMA channel bandwidth is 14 dB lower than the noise in the Nyquist bandwidth due to the processing gain achieved. An overall gain of 36 dB is needed to achieve the desired cascaded receiver noise figure value of 3.7 dB.

With 36-dB gain ahead of the ADC, a maximum single tone blocker level above −30dBm at the antenna terminal will exceed the ADC full-scale input. The cdma2000 cellular base-station standard specifies a maximum allowable blocker level of −30 dBm at the antenna terminal. For this example, a 6-dB gain reduction was used to increase the largest allowable blocker signal applied to the ADC providing margin to the standard's specification. Assuming allowable headroom of 2 dB, a 6-dB gain reduction results in a maximum blocker level of −26 dBm at the antenna and +4 dBm at the ADC input (Fig. 3). The cellular standards allow 3-dB degradation in overall (noise + distortion) relative to reference sensitivity when a single-tone blocker is present. The allocation of individual noise and distortion components is left to the designer.