Digital Receiver Design: Basics of Software Radio

By Roger H. Hosking

December 18, 2006

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Digital receivers have revolutionized electronic systems for a variety of applications including communications, data acquisition, and signal processing. This article examines how digital receivers, the fundamental building block for software radio, can replace conventional analog receiver designs, offering significant benefits in performance, density and cost.

In order to fully appreciate the benefits of digital receivers, a conventional analog receiver system will be compared to its digital receiver counterpart, highlighting similarities and differences. The inner workings of the digital receiver will be explored with an in-depth description of the internal structure and the devices used. Finally, some actual receiver system implementations and available off-the-shelf board level digital receiver products for embedded systems will be described.

Analog Receiver

The conventional heterodyne radio receiver, as seen in Figure 1, has been in use for nearly a century. Let's review the structure of the analog receiver so comparison to the digital receiver becomes apparent. First the RF signal from the antenna is amplified, typically with a tuned RF stage, that amplifies a region of the frequency band of interest. This amplified RF signal is then fed into a mixer stage. The other input to the mixer comes from the local oscillator (LO) whose frequency is controlled by the tuning knob on the radio.

Figure 1: Typical analog receiver block diagram.

The mixer translates the desired input signal to the intermediate frequency (IF). (See Figure 2.) The IF stage is a bandpass amplifier that only lets one signal or radio station through. Common center frequencies for IF stages are 455 kHz and 10.7 MHz for commercial AM and FM broadcasts. The demodulator recovers the original modulating signal from the IF output using one of several different schemes.

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