2.2 Designing an LNA Scheme
The sensitivity of a receiver (S) is a function of its noise figure (NF), the minimum required signal-to-noise ratio
for detection of the modulation and the thermal noise of the system. S can be written as:
Equation 1: Receiver Sensitivity
where
is the thermal noise power of the receiver in dBm.
At room temperature (T = 290°K), in a 1 Hz bandwidth,
. For a 500 MHz channel bandwidth,
.
is the required output signal-to-noise ratio for detection of the modulation.
The QM33100 and DW3000 have a noise figure of 7 to 8 dB depending on the channel and the frame
parameters. The sensitivity of the DW3000 receiver is between -93.2 dBm and -100 dBm and for the QM33100,
between -92.8 dBm and -102.6 dBm.
Let us consider adding an LNA to the input of our transceiver. The overall sensitivity of the system can be
written as:
Equation 2: Overall system sensitivity
corresponds to the cascade of the LNA and transceiver. Referring to Equation 1 and Equation 2:
Equation 3: Change in system sensitivity
Thus, the overall increase in the sensitivity of the transceiver depends on the noise figure of the amplifying
stage. The lower the noise figure of the stages between the antenna and the transceiver, the greater the
increase in receiver sensitivity.
The LNA circuitry should be designed to give the lowest noise figure while not compressing the receiver’s
internal LNA. The strongest receiver signal presented to the QM33100 should not exceed -25.4 dBm for
channel 5 and -27.1 dBm for channel 9. For the DW3000 this signal should not exceed -25 dBm for channel
5 and -32 dBm for channel 9.
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