by Louis E. Frenzel
The Latest in Networking and Wireless Technologies
A Radio That Thinks — An Introductory Look at
Those of you who follow electronics either as a career or
hobby know that over
the past years, electronics has
evolved from an all analog world to a
mostly digital world. Most electronic
products today incorporate digital
techniques. And in fact, it is hard to
name a modern electronic product
that does NOT contain a single chip
computer called an embedded
controller that runs the show.
Yet, analog has not disappeared
completely. It won’t ever go away
because all physical phenomena in
nature are analog. Voice and music
are analog, video is analog, and
most signals from sensors and transducers are analog. Yet despite all this
natural “analogness,” so much of it
has already become digital. Music
CDs and MP3 players are digital. The
new high definition TVs are digital.
Cameras are digital. And so it goes.
Now, the final frontier of electronics, radio — or wireless as we call
it again today — is now going digital.
Cell phones are now mostly digital.
Satellite and High Definition AM/FM
radio is all digital. This rampant conversion is happening right now, but
thanks to some breakthroughs and
continuously improving integrated
circuits, the digitization of radio is on
the fast track.
One term for it is software-defined radio (SDR). Not only is the
radio going digital, but it is becoming
software, programs that run on a
digital computer, or processor. My
column this month is to introduce
you to SDR and the new companion
technology cognitive radio.
DSP to the Core
Figure 1. Digital Signal Processor block diagram.
NUTS & VOLTS
The core technology in SDR and
cognitive radio is digital signal processing (DSP). DSP is the computer
equivalent of analog signal processing. All radios process signals from
antenna to speaker and from microphone back to the antenna. Those
processes have in the past been
analog processes are
compression, modulation and demodulation, encoding and
decoding, and so on.
Today, much of this is
In a nut shell, a
digital version of the
analog signal is fed to
a fast microprocessor
along with a special
program written to perform one of
the analog functions mentioned
above. The new digital output of that
process is then converted back to
analog. And the results are similar to
what you would expect from an
analog circuit. And no one can tell
Figure 1 shows a simplified block
diagram of a digital signal processor.
The analog signal is applied to a fast
analog-to-digital converter (ADC).
This device digitizes the analog signal
into a stream of binary words that
represent the analog voltage variations. Remember, to capture the full
content of the analog signal, the sampling and digitization rate has to be at
least twice the highest frequency in
the analog signal.
For example, to convert analog
audio with frequencies to 20 kHz into
digital, the sampling rate has to be
no less than two times that or 40 kilo
samples per second (KSPS). You
may already know that CD audio is
sampled at 44.1 KSPS. That is a
sample every 22.675 microseconds.
A 4.2 MHz video signal needs a
converter that will sample at 8. 4
mega samples per second (MSPS)
or faster. A 10 MSPS rate would work
well or one sample every 100
To do radio, we need ADCs that
can sample at many hundreds of
MHz. Current ADC technology easily
converts signal at frequencies up to
about 250 MHz. And the technology
is continually improving. A New York
company called Hypres uses a cryo-cooled (< 4 degrees Kelvin!!) ADC
that can sample at well over 1 GHz.
The stream of binary words from