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to a block of digital signal processing (DSP) circuitry that performs the filtering for selectivity and the demodulation. The recovered signal in digital form is then sent to a digital-to-analog converter (DAC) where the original analog voice or music is recovered. The analog DAC output is then amplified as usual before going to the speaker.

As for tuning, this is usually accomplished with a frequency synthesizer. This is a mixed-signal (analog and digital) circuit called a phase-locked loop (PLL) that generates the LO signal for the mixers in frequency step increments, rather than a continuously variable frequency.

The SDR architecture greatly reduces the number of components needed to implement the radio. In addition, it also significantly improves the receiver selectivity over any analog design.

; FIGURE 3.

Block diagram of the Si4825 single chip radio.

A SINGLE CHIP RADIO

There are multiple vendors making what we call single chip radios. A good example are the radios from Silicon Laboratories. They have been making AM/FM and TV tuners using a mixed-signal approach for several years. One of the more recent is the Si48xx series that puts an entire AM/FM and SW radio in one chip; check out Figure 2. It is designed for making auto radios, tabletops, portables, and other form factors. It uses an SDR approach. There are three versions: Si4825/27/36; each has a slightly different tuning option. The Si4836 has stereo outputs while the other two are monaural output.

; FIGURE 2.

Silicon Labs Si48xx series single chip radios cover worldwide AM, FM, and shortwave bands to make any type of consumer radio product.

chip radio is shown in Figure 3. The antenna connects to the input with some inductors for impedance matching to the input amplifier. The input RF amplifier is referred to as a low noise amplifier (LNA) because it amplifies without adding any significant amount of noise to the signal. The amplified signal is sent to two mixers. The local oscillator is a synthesizer that generates two LO sine waves; one shifted 90 degrees from the other. The mixers get the phase shifted signals at the same frequency and then produce two low frequency IF signals shifted 90 degrees from one another. These are called the I and Q signals, meaning in-phase and quadrature (90 degree) phase. The IF is low, in the 100 kHz range. The analog signals are then digitized by the ADCs producing two quadrature bit streams to the DSP. The DSP circuits implement mathematical algorithms to do the IF filtering and demodulation. The DSP output then is a digital form of the original analog modulation that is then sent to the DAC for conversion back to analog for amplification. Another IC power amplifier drives the speaker.