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Amplitude modulated envelop on 1 Mbps signal (WLAN)

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bizoo

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I am observing baseband signals coming out of 802.11b card at 1, 2 ,5.5 and 11Mbps.

At 1Mbps, entire signal goes through scrambler,DBPSK modulation, 11 bit barker multiplier and raised cosine filter. At other data rates, only the preamble goes through same sequence and the remaining signal has other modulation schemes.

The problem is that when i receive this 1Mbps signal at baseband, it has an amplitude modulated envelop on that. Through simulation i have seen that there should be no such envelop, but actual signal shows that my signal has enevelop which is similar to an amplitude modulated signal, the envelop frequency is 22Khz,

Also i have used differnt card to get this waveform (Netgear and D link) but results are the same,
At higher data rates, only preamble bits has same envelop because preamble is always sent at 1 Mbps,

In general, What ever data is received at 1Mbps, it has an envelop on it and signal looks like amplitude modulated signal. there is no such thing when i observe the tranmitted signal, but only in received signal i see this strange behavior.

Any clues!!
 

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