To understand the direct RF architecture, you need to know how it differs from other RF architectures.
In a heterodyne architecture, after the receiver receives the signal at RF frequencies, it downconverts the signal to a lower intermediate frequency (IF) where it is digitized, filtered, and then demodulated. Figure 1 shows a heterodyne receiver block diagram. As you can see, the instrument has an RF front end that consists of a bandpass filter, low-noise amplifier, mixer, and local oscillator (LO).
Figure 1. This heterodyne receiver block diagram shows an instrument with an RF front end that consists of a bandpass filter, low-noise amplifier, mixer, and local oscillator.
A direct RF sampling receiver architecture, however, consists of just a low-noise amplifier, the appropriate filters, and the ADC. The receiver in Figure 2 does not use mixers and LOs; the ADC digitizes the RF signal directly and sends it to a processor. In this architecture, you can implement many of the analog components of the receiver in digital signal processing (DSP). For example, instead of a mixer, you can use direct digital conversion (DDC) to isolate your target signals. Also, in most cases, you can replace much of the analog filtering with digital filtering except for the anti-aliasing or reconstruction filters.
Because analog frequency conversion is not required, the overall hardware design of a direct RF sampling receiver is much simpler, thus allowing for smaller form factors and lower design cost.
Figure 2. A direct RF sampling receiver architecture can consist of just a low-noise amplifier, the appropriate filters, and the ADC.