High-Speed Data Converters Make Direct-Sampling Receivers Practical (.PDF Download)

Communications receiver architecture has evolved slowly over the years. The most common and successful configuration is the superheterodyne architecture, which translates the incoming signal to a lower (or higher) intermediate frequency (IF), where bandwidth and gain can be controlled. Since then, newer digital modulation modes have fostered some new architectures. The most recent configuration is the direct-sampling architecture that has been enabled by super-fast analog-to-digital converters (ADCs).

Receiver Architectures in Review

The earliest receiver architecture was of the tuned radio frequency (TRF) type with multiple amplifiers, each tuned to the desired frequency. Then the regenerative receiver came along and added feedback to an RF stage that improved gain and selectivity.

Both of those types went away quickly once Edwin Armstrong invented the flexible superheterodyne (Fig. 1a). A low-noise amplifier (LNA) provides input gain. The incoming signal is mixed with a local-oscillator (LO) signal in a mixer to downconvert the incoming signal to a lower IF where a fixed bandwidth is established with a bandpass filter (BPF) along with high-gain IF amplifiers just prior demodulation. The superhet is still widely used. Dual-conversion and triple-conversion versions solve the image and selectivity problems that occur.