Sensitivity measurements
Test results for the noise figure NF of the HackRF, obtained at a
frequency of 435MHz:
NF = 22dB LNA=OFF
NF = 10.8dB LNA=ON
NF = 1.1dB LNA=ON, additional external LNA "LNA4ALL"
RTL-SDR, tuner 820T or 820T2
NF = 3.7dB
The HackRF really is extremely "deaf" when the internal preamplifier is
turned off. Turning it on and installing
a good external amplifier brings NF down to a very good value.
Applying so much gain at the preamplifiers in front of the mixer will
degrade the intermodulation properties.
Adding an external LNA will block the transmitter path to the antenna.
The noise figure is the fundamental parameter to specify the sensitivity
of a receiver. All the other, more common,
sensitivity specifications can be derived from it. For example NF=3dB
will give an input sensitivity of
S=-174dBm+NF+48dB=-123dBm for typical NBFM signal. The factor "48dB"
somewhat depends on the test parameters.
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