Digital Receiver Design: Basics of Software Radio Part 1

Digital receivers have revolutionized electronic systems for a variety of applications including communications, data acquisition, and signal processing. This series shows 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.

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.

For example, AM uses an envelope detector and FM uses a frequency discriminator. In a typical home radio, the demodulated output is fed to an audio amplifier which drives a speaker.

2. The mixer translates the desired input signal to the intermediate frequency.

The mixer performs an analog multiplication of the two inputs and generates a difference frequency signal. The frequency of the LO is set so that the difference between the LO frequency and desired input signal (the radio station you want to receive) equals the IF.

For example, if you wanted to receive an FM station at 100.7 MHz and the IF is 10.7 MHz, you would tune the local oscillator to: 100.7 - 10.7 = 90 MHz

This is called "downconversion" or "translation" because a signal at a high frequency is shifted down to a lower frequency by the mixer. The IF stage acts as a narrowband filter which only passes a "slice" of the translated RF input. The bandwidth of the IF stage is equal to the bandwidth of the signal (or the "radio station") that you are trying to receive. For commercial FM, the bandwidth is about 100 kHz and for AM it is about 5 kHz. This is consistent with channel spacings of 200 kHz and 10 kHz, respectively.

Digital Receiver
Take a look at the digital receiver block diagram shown in Figure 3. Note the strong similarity to the analog receiver diagram; all of the basic principles of analog receivers still apply. Right after the RF amplifier and an optional RF translator stage, we use an A/D (analog-to-digital) converter to digitize the RF input into digital samples for the subsequent mixing, filtering and demodulation that are performed using digital signal processing elements.

3. Typical digital receiver block diagram.