April
2006, Issue 189
Low-Cost
2.4-GHz Spectrum Analyzer
WHICH
COMPONENTS?
To
build my spectrum analyzer, you need a receiver that’s
sensitive to frequencies higher than your range of interest
(see Photo 2). If you’re like me, you’re interested
in the 2.4-GHz ISM band, which extends from 2.40 to
2.485 GHz. The chip must be digitally tunable. You also
need a way to know the strength of the signal it’s finding
at each frequency (usually referred to as received signal
strength indicator, or RSSI). Chipcon’s CC2500 chip
meets all of these requirements.
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(Click
here to enlarge)
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Photo
2—The spectrum analyzer circuit board includes a
Chipcon CC2500 transceiver chip. An Atmel ATmega48
processor and a CP2102 USB interface are also on
board. |
The
CC2500 is a low-power 2.4-GHz transceiver with a sophisticated
packetized data transfer and forward error correction.
Using the CC2500 is serious overkill for this application,
because you only need a receiver (not a transceiver).
You don’t even need to demodulate and receive data.
But, the benefit is that it has a sensitive receiver
with digital tuning and digital RSSI.
In
case you’re wondering if such a powerful chip will fit
your budget, how does $2 per chip sound? You can thank
all the people demanding cheap PCs for the semiconductor
technology that makes this chip possible.
In
addition to the CC2500, you need an embedded processor
to control it. I chose Atmel’s ATmega48, which has 4
KB of flash memory and 0.5 KB of RAM (see Figure 1).
Its SPI interface communicates with the CC2500. Its
UART allows data to be sent to and from the ATmega48.
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(Click
here to enlarge)
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Figure
1—The ATmega48 microcontroller (U1) controls the
RF receiver’s (U2) functions. U1 is connected via
a UART connection to USB bridge chip U3. J1 enables
the microcontroller to be programmed in-circuit. |
Now
that you’ve identified the hardware that will acquire
the spectrum, you need a user interface to control the
hardware and to display the results of your spectrum
analysis. Rather than trying to reinvent an embedded
system keyboard and display, I decided to use a commercial
PC. You could use a hand-held device such as a Pocket
PC or a larger machine running a desktop operating system.
I used a laptop running Windows.
You
also need a way to connect the PC to your microcontroller.
Fortunately, this is easier than it was a few years
ago. Silicon Laboratories (formerly Cygnal) makes a
USB-to-UART bridge chip called the CP2102. Like the
radio receiver part of this design, the CP2102 has a
high level of integration, requiring only a couple bypass
capacitors. Just connect the CP2102 to the ATmega48’s
UART and send serial data back and forth to the PC.
In addition, the CP2102 has an on-chip voltage regulator
that you can use to power the rest of your circuit,
so no batteries or power supplies are needed. Everything
is powered from the PC’s USB port.
