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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.
Some of the other circuits are an
automatic gain control (AGC) that
helps keep the signal level to the
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June 2013