Direct RF Sampling (DRFS) or Direct RF Conversion (DRFC) is the sampling of RF signals and conversion to digital signals without the need for an intermediate frequency (IF) stage to translate the frequency of the RF signals to a lower frequency more accessible to older digital sampling electronics. Using a DRFS analog-to-digital converter (ADC) allows for a massive shift in the RF architecture from a heterodyne architecture to a direct RF architecture where there is not a need for a mixer stage in the receive signal chain.
This use of DRFS allows for the removal of a mixer, IF filter, local oscillator (LO) and possibly other RF filters or RF amplifiers. Instead, the RF ADC can be directly connected to the antenna receive port (circulator/isolator, switch, or filter depending on the RF front-end (RFFE) design), which both simplified the architecture and enabled advanced digital signal processing (DSP) options that were previously unavailable without direct sampling. Though it is possible with adequate processing and DSP technology to implement much of the receive signal filtering once the signal has been digitized, anti-aliasing or reconstruction filters are likely to still be needed. Moreover, to achieve desirable signal-to-noise ratio (SNR) for a DRFS receiver, additional low-noise amplifier (LNA) gain may be needed compared to a heterodyne receiver, as the mixer often provides gain within the signal chain prior to input to the ADC.
DRFS has become possible for frequencies well into the gigahertz, and in some cases, tens of gigahertz. This is a very recent advancement, as just over a decade ago, the fastest ADCs were sampling at a rate of hundreds of megasamples, now there are ADCs with acceptable resolution for many RF applications with sampling rates in the tens of gigasamples. The dynamic range of DRFS ADCs has also improved to a point where it is now possible to use DRFS in many common communications and sensing applications, such as Wi-Fi, 4G/5G cellular telecommunications, troposcatter communications, and even radar applications.
The physical hardware for a direct RF architecture receiver can be much more compact, lower power usage, and allow for much greater software configurability. This allows for greater regimes of hardware reuse and more capable software-defined radio (SDR) systems that can be made to more readily adapt to the electromagnetic (EM) environment than heterodyne receiver technologies. As the DRFS technology matures, it is also likely that these technologies will continue to enhance in dynamic range and reduce in cost. DRFS systems can already capture very wide-bandwidth signals and directly digitize the RF signals, which enables easier band planning and may ease the radio design burden in many applications.
Learn more about Fairview Microwave’s RF/microwave hardware that can be used in DRFS receiver signal chains: