dB = 37 + 20 log (915) + 20 log(.018939) =
37 + 59. 23 – 34. 45 = 61. 79 dB
Note: The log of fraction gives a negative
number that is the reason for the - 34. 45 number.
Received power = 3 - 61. 79 = - 58. 79 dBm
Finally, compare that to the receiver
sensitivity figure usually given in –dBm. The larger the
number, the better the sensitivity. A sensitivity of -100
dBm means the receiver can detect a signal that small.
A sensitivity of - 80 dBm is not as good as -100 dBm.
That means that in terms of power, - 80 dBm is greater
than -100 dBm. So, if the received power is greater than
the sensitivity figure, the signal will be received. Assume
a receiver sensitivity of - 70 dBm. Since the received
power is greater than the sensitivity figure, you will have
a good link.
Incidentally, this calculation assumes basic isotropic
antennas with a gain of one. If you use a dipole or ground
plane, the real power gain is 1.64.
THE WIRELESS MODULES
The wireless devices I bought were part of a whole
package called the Master Evaluation/Development
System, specifically the Linx Technologies MDEV-HP3-PPS-
USB. It comes with two development boards: one for
transmit and the other for receive. It also has two each of
the transmitter modules (TXM-900-HP3-xxx) and receiver
modules (RXM-900-HP3-xxx). The development boards
have a USB port on them but you can also get a version
for RS-232. Cables and programming software is included.
Figure 1 shows the whole package.
Figure 2 shows the transmit block diagram. It uses a
PLL frequency synthesizer programmed by an internal
microcontroller for any of the 100 channels in the 902-
928 MHz range. A 12 MHz crystal sets the precision
and frequency increment at 250 kHz. You can select
any of the existing frequencies by programming
via the PC, or any of eight frequencies that can
be set with a DIP switch on the transmitter
development board. The PLL and its VCO act
as a frequency multiplier to increase the output
frequency into the 902-928 MHz range. Then, a
power amplifier boosts the output signal to 0
dBm or in the - 3 to + 3 dBm range. An output
filter gets rid of any pesky harmonics.
The modulation is FSK and you can achieve
a data rate up to about 56 kbps. A 28 kHz low
pass filter on the data input filters the binary serial data to
help restrict the harmonics and the sidebands, and to keep
the frequency deviation down to less than 115 kHz.
Incidentally, you can also modulate the transmitter with
analog signals up to 28 kHz.
■ FIGURE 2. The Linx Technologies TXM-900-HP3-xxx
transmitter module block diagram. (Courtesy Linx Technologies.)
June 2010 15