11
Application Notes
Why does Sensitivity Matter?
Receiver sensitivity is the lowest power level at which the receiver can detect a wave and
demodulate data. Sensitivity is purely a receiver specification and is independent of the
transmitter. As the wave propagates away from the transmitter, it attenuates as the
distance increases. Lowering the sensitivity on the receiver (making it more negative)
will allow the radio to detect weaker signals, and thus increase the transmission range.
Sensitivity is vitally important since even slight differences in receiver sensitivity can
account for large discrepancies in the range. To better understand this relationship, the
following example is provided.
Example:
Compare the MDR210A-485 module (with –110dBm 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(
t
) x G(
t
) x G(
r
) x
λ
²
P(
r
) =
F(
s
) x 4
π
r
²
P(r) = received power (mW)
P(t) = transmitted power (mW)
G(t) = gain of transmit antenna (linear)
G(r) = gain of receive antenna (linear)
F(s) = fading margin (linear)
λ
= wavelength (meters)
r = distance between Transmitter and Receiver (meters)
The following values were used to compare the range limitations of these modules:
P(t) = 100mW
G(t) and G(r) = 2dB, or 1.585 linear
λ
= 0.333 meters
F(s) = 21dB, or 125.89 (experimentally determined.
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.
