Role of Analog-to-Digital (ADC) Converters in Spectrum Analyzers

Analog-to-digital converters (ADCs) are the backbone of virtually all modern test and measurement equipment. These devices are responsible for converting the real-world RF signals to digital signals that can then be further processed/stored and displayed to the user. There have been significant advances in ADC technology in recent years, with much faster ADCs with higher bit counts that can operate to several gigahertz without frequency translation.

These new extremely fast ADCs have enabled spectrum analyzer capability that allows for functions like direct conversion, digital downconversion, direct IQ data recording, among others. However, to reach beyond a few gigahertz or to capture very short duration signals, often more than a single ADC is needed. For instance, spectrum analyzers with higher frequency capability may employ multiple ADCs, one directly converting the lower frequency range with another, possibly several more, with frequency translation hardware (downconverters) translating higher frequencies to an intermediate frequency (IF) the ADCs can handle.

A spectrum analyzer may even use multiple ADCs for the same frequency band, depending on the intended performance. In this case, a faster but lower precision ADC can be used to rapidly capture a frequency sweep while a higher precision ADC could provide signal monitoring and analysis functions at a slower rate. Moreover, multiple ADCs may be used in conjunction with staggered sampling windows to enhance the probability-of-intercept (POI).

A challenge with this approach is that the digital backend of the spectrum analyzer will have a limited bandwidth for real-time spectrum analysis captures, so the observation spectrum will be less than what is captured by the ADC. This is because that a spectrum analyzer ADC is typically sampling at a constant rate, but the resampler and digital down converter, which are often implemented with field-programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs), convert the digital signal from the ADC to a digital baseband (IQ data) with bandwidth, frequency span, and sampling rate as required by the Nyquist criterion.

The choice of ADC for a given spectrum analyzer is often a significant portion of the cost, design complexity, and a performance bottleneck. This is mainly due to how digital electronics have scaled in performance much faster than analog electronics, and the input section of an ADC is intrinsically analog in nature. This is why it is critical in a spectrum analyzer design to ensure that the RF Front-end (RFEE) of the spectrum analyzer minimizes noise, phase noise, and is set at the optimal signal strength for dynamic range and precision for the ADC. This includes low-noise amplifiers (LNAs), limiters, power splitters, attenuators, filters, and interconnect.