In an effort to build smaller digital receivers, the aerospace and defense industry is embracing modern direct radio-frequency (RF) sampling analog-to-digital converters (ADCs). These ADCs eliminate RF mixing stages and are closer to the antenna, simplifying digital receiver designs while also saving cost and printed circuit board (PCB) area.

One critical (and often misunderstood) parameter is the ADC noise figure, which sets the amount of RF gain to detect very small signals. This article explains how to calculate the noise figure of an RF-sampling ADC, and illustrate how the ADC noise figure affects RF signal-chain designs.

The digital receiver operates in one of two distinct scenarios as illustrated in Figure 1. In the blocking condition, an interferer or jammer is present and the receiver has to operate with reduced RF gain in order not to saturate the ADC. In this setup, the ADC is driven close to full scale by the interferer; thus, the large-signal signal-to-noise ratio (SNR) of the ADC determines how weak a signal can be detected. There are additional degrading mechanisms such as phase noise and spurious free dynamic range.

In the second scenario, there is no interferer present. Detecting the weakest signal possible is solely dependent on the inherent noise floor of the receiver, a condition typically measured as receiver sensitivity. The noise figure measures the SNR degradation caused by components in the receiver signal chain.

The noise figure of the ADC is typically the weakest link of the receiver (approximately 25 to 30 dB), while low-noise amplifiers (LNAs) have noise figures as low as <1 dB. It is possible, however, to improve the ADC noise figure by adding gain to the analog RF front end (close to the antenna) using LNAs. The difference between a 1-dB receiver system noise figure and a 2-dB receiver system noise figure translates to approximately 20%. This difference means that a receiver with a 1-dB noise figure can detect signals with approximately 20% weaker amplitude. In a software-defined radio (SDR), that translates to radios with reduced output power – saving battery life – while in radar, that makes it possible to cover a longer distance.

Modern receiver designs in SDRs or digital radars use direct RF-sampling ADCs in order to reduce size, weight and power. This architecture simplifies receiver designs by eliminating the RF downconversion mixing stage. The better the ADC noise figure, the less gain required, which results in additional savings. Furthermore, using less additional RF gain means that when a jammer is present, there is less gain to reduce, with a higher dynamic range maintained in the receiver.