■ FIGURE 3. The Linx Technologies RXM-900-HP3-xxx
receiver module block diagram. (Courtesy Linx Technologies.)
the crystal oscillator. The mixer output is a 10. 7 MHz
second intermediate frequency (IF) signal that is filtered —
limited to remove any amplitude variations — and sent to
the quadrature demodulator where the signals are
recovered. A slicer/shaper cleans up the digital signal.
A neat feature of this module is the received signal
strength indication (RSSI) circuit. It is a calibrated output
that indicates the level of the received signal. It generates
a DC signal in the one to three volts range, corresponding
to input power levels in the - 40 to -110 dBm range. Using
this feature lets you actually measure the signal level being
DEMONSTRATING THE MODULES
To get the boards ready for use, you plug in the
transmit and receive modules, screw on the antennas, and
install a nine volt battery in each. The battery drives a five
volt regulator that supplies power to the modules.
Once you set the frequency on both the transmit and
receive boards, are can demo them. Turn on the power
switches. Then, press the yellow button on the transmit
board that will sound the buzzer on the receive board
with a beeping tone. The red transmit button causes a
relay on the receive board to close. You can hear it click.
■ FIGURE 4. The Antenna Factor ANT-916-YG5-N Yagi
antenna. It has a 50 ohm cable and N-connector. I had
to buy two extra cables with different connectors to
match up with the SMA connector on the transmit and
The relay contacts have connectors
on the receive board so you can
hook them up to control some
other device like a light or motor.
The relay contacts can handle as
much as five amps up to 30 volts
DC or 120 volts AC.
As a next step, you can take
the boards outside and test the
range. I took the boards out in the
street with my wife and tried to
estimate the maximum range.
Keep the line of site path clear to get the best results. You
should easily get several hundred feet. I got 300 feet
before a hill cut my line of sight path. (I live in central
Texas where it is called the hill country.)
For further experimentation, you can put obstacles
between the transmitter and receiver. Walls, trees, cars,
whatever. The units still communicate, but the obstacles
add considerable extra attenuation to the path thereby
shortening the range.
One other demo you can do is check out the RSSI
voltage. Get a digital or analog multimeter and connect it
up to the RSSI pins on the receiver board. Turn on the
units and measure the RSSI voltage over a decent range.
The output voltage is roughly linear from about 1.2 volts
DC at -110 dBm received power to 2. 7 volts at - 40 dBm.
You could calculate the received power using the formula
above, then attempt to verify it with the RSSI reading for a
given power and distance. Incidentally, the transmit power
is nominally 0 dBm or 1 m W. The specs indicate a
possible range from - 3 dBm to + 3 dBm so any errors will
probably be in transmit power or range measurements.
One final thing I did was to buy an accessory five-element Yagi antenna (Figure 4). The antenna is made by
Antenna Factor, and is available through Linx. It has a gain
of 9 dBi (isotropic gain). It gets that gain by focusing the
power in one direction so the effect is that of a transmitter
power boost or improved receiver sensitivity. I used the
antenna on the receiver. The effect was to greatly increase
the range of transmission by a factor of three or more. Not
bad. You can note that change on the RSSI reading.
That is all the time I had for the demos, but I do plan
to do more. I particularly want to plot the Yagi
antenna pattern by using the RSSI readings as I
rotate the antenna horizontally. Should be
Wireless is fun to play with, but I do admit it is
also frustrating at times. Things don’t always work
as they should, as they seem, or as you want.
Mostly, this is due to obstacles in the path like trees
or walls, multipath signals developed from nearby
reflecting bodies (cars, water tower, etc.), or you
personally being too close to either the receive or
transmit antennas. So, you need to experiment for
best results. Once everything is set up correctly, the
communications link is remarkably reliable, almost
as good as a wire. NV