|
I think this is a good comparison, however I have a possible explanation. One critical thing is to understand that there are great inconsistencies between the noise figures of different cards. This is because most of the WLAN PCMCIA cards are made from G 10 epoxy circuit board, which, from board to board, has inconsistent performance, regardless of how well the WLAn receiver chip sets are 'laid out'. The reason WLAN manufacturers use G10 is cost, although they would get much better performance from expensive 'microwave board' like FRR 10.
The sensitivity of a WLAN receiver is usually rated by noise figure (NF) which is specified in dB. The effective noise temperature of a typical WLAN receiver using the Harris PRISM chip set is in the order of 4 dB. When the chip set is implemented on the cards, there may be up to 2.5 dB (or higher) increase in 'system' noise figure. This means that there is a difference in the sensitivity between individual cards as those with higher noise figure are less sensitive.
A 2.4 GHz receiver with a 1 MHz bandwidth and a noise figure of 4dB has a sensitivity of approximately minus 142 dBW. A noise figure of 6.5 dB gives a sensitivity of about minus 138 db.
Clearly, noise figure affects sensitivity, as does the relative atmospheric noise figure in the region where the measurments are taking place.
It is possible that the Air Magnet cards are 'hand-picked'.
It would be interesting to see the sensitivity tests reproduced when you run a 3.5 dB noise figure pre-amplifier ahead of each system (that's how I stumble). With a pre-amp, you will get more cionsistent results.
|