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Receiver Sensitivity

Greater sensitivity = longer range
Range is an important requirement for most any RF application. Long range achieved through greater receiver sensitivity is an envied position among RF product manufacturers. Extended range achieved in this fashion provides an excellent cost benefit to the customer. Digi achieves its long range with proprietary modulation and demodulation techniques coupled with one with of the best receiver sensitivity specifications in the market.

Receiver sensitivity is the lowest power level at which the receiver can detect an RF signal and demodulate data. Sensitivity is purely a receiver specification and is independent of the transmitter. As the signal propagates away from the transmitter, the power density of the signal decreases, making it more difficult for a receiver to detect the signal as the distance increases. Improving the sensitivity on the receiver (making it more negative) will allow the radio to detect weaker signals, and can dramatically increase the transmission range. Sensitivity is vitally important in the decision making process since even slight differences in sensitivity can account for large variations in the range. To better understand this relationship, the following example is provided.

Compare the Digi 9XStream radio (with -110 dBm sensitivity) to a commercial radio receiver with a sensitivity of -90 dBm. The Friis transmission formula can be used to calculate received power (or signal strength) at any receiver location under line-of-sight conditions. This formula is given by:

P R = [P T G T G R ? 2 ] / [F S (4pr) 2 ]

P R = received power
P T = transmitted power
G T = gain of transmit antenna
G R = gain of receive antenna
F S = losses due to multipath, obstacles, etc.
? = wavelength
p = pi
r = distance between transmitter and receiver

The following values were used to compare the range limitations of these radios:

P T =100 mW
G T and G R =2 dB, or 1.585
F S = 21 dB, or 125.89 (experimentally determined)
? = 0.333 meters

The table below demonstrates the power received at the receiver over the specified range between the TX and RX antennas, assuming line-of-sight conditions.

Detectable by
Digi 9XStream
Wireless Module
Detectable by
Wireless Module
100 -68.526 dBm Yes Yes
500 -82.506 dBm Yes Yes
1000 -88.526 dBm Yes Yes
3000 -98.068 dBm Yes No
5000 -102.506 dBm Yes No
8000 -106.588 dBm Yes No
10000 -108.526 dBm Yes No
(7 miles)
-109.561 dBm Yes No
13000 -110.805 dBm No No

Since the range doubles every -6 dB in line-of-sight conditions (-12 dB in urban environments), the 18 dB sensitivity difference in modules corresponds to 23= 8 times the range using the Digi wireless module!

In similar fashion, Digi wireless modules offer 32 times the range of -80 dBm modules and over 100 times the range of -70 dBm modules!