Figure 6. The Aerocomm ConnexLinks 900 MHz wireless modems
for connectivity up to 20 miles with clear line of sight antennas.
The LNA input gives the receiver a -112 dBm
sensitivity. This receiver is a direct conversion type,
meaning the local oscillator PLL is set to the receive
frequency producing a zero IF. To detect the FSK, dual
mixers fed 90 degrees apart produce in-phase (I) and
NUTS & VOLTS
Circle #30 on the Reader Service Card.
quadrature (Q) outputs that are filtered in active filters
and a highly selective, switched capacitor, low pass filter.
The cut-off frequency can be set to 100 kHz, 150 kHz,
230 kHz, or 340 kHz to match the chosen data rate. The
filter output goes to the demodulator that recovers the
serial binary signal.
Micrel also has a wide range of wireless chips that can
be reviewed on their website at www.micrel.com
A really interesting new wireless product by Cypress
Semiconductor is called WirelessUSB. It is a radio chip
that incorporates a transceiver and is designed to work in
the 2.4 GHz band, where one of the versions of ZigBee
works. You could, in a way, call this chip ZigBee-lite, since
it is not as fast and does not include all of the automatic
networking capability, which is overkill for many
There are two versions of the WirelessUSB — short
range and long range. Both have a basic data rate of 62. 5
Kbps. The short range device is good up to about
10 meters. It was optimized for things like wireless game
controllers, computer mice, keyboards, and toys. The
longer range device operates at up to a 50 meter range. It
is ideal for many industrial applications, such as reading
Figure 4 shows a typical application, where multiple
sensors send their data back to a central station for
monitoring and control.
What makes this chip special is that it uses direct
sequence spread spectrum (DSSS) with Gaussian
FSK modulation. Spread spectrum is a far better
communication method in noisy, industrial
environments because it is more robust and tolerant of
interference and multipath signal conditions, which are
typical in such applications.
For more information, go to the website for the data
sheets, app notes, etc. ( www.cypress.com).
A really sophisticated wireless product is DPAC
Technologies’ Airborne radio modem. This is a complete
IEEE 802.11b standard transceiver.
Also known as Wi-Fi, this is the technology used in
laptops to link to the hot spots in airports, hotels, and
other public places for Internet access. It is also widely
used in home and office wireless LANs. This radio
modem operates in the 2.4 GHz band with DSSS. The
maximum data rate is 11 Mbps, but it can drop back to
5. 5, 2, or 1 Mbps for longer range. Figure 5 shows the
The applications processor supports lots of I/O
types, including an eight channel analog multiplexer
and A-to-D converter. The processor has a TCP/IP
stack, making Internet access fast and easy. Also
included is a web server on-chip that lets you monitor or
control this device remotely via the Internet with any
While this module is overkill for some industrial
wireless applications, it is ideal for many uses, like