Black Box MDR210A-485 manual Application Notes, Why does Sensitivity Matter?, Example

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 λ²

P( r ) =

F( s ) x 4 πr 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.

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