ath9k spectral scan:

Atheros 802.11n chipsets include a built-in spectral analysis feature. AR92xx and AR93xx have the ability to report FFT data from the baseband under software controlled conditions, including:

absolute magnitude (|i|+|q|, abs() for I/Q phase of the wireless signal) for each FFT bin (56 for subcarriers in HT20 mode and 128 in HT40 mode)
an index indicating the strongest FFT bin
the maximum signal magnitude for each sample

These information can be used to create an open source spectrum analyzer or interference classifier

Spectral scan parameters:

fft_period: when active and triggered, PHY passes FFT frames to MAC every (fft_period+1)*4uS
period: when active, time period between successive spectral scan entry points (period*256*Tclk). Tclk = 44MHz for HT20 operation, 88MHz for HT40 operation
count: number of scan results requested. There are special meanings in some chip revisions:
- AR92xx: highest bit set (>=128) is interpreted as "never disable"
- AR9300 and newer: 0 is interpreted as "never disable"
short_repeat: controls whether the chip is in spectral scan mode for 4 usec (enabled) or 204 usec (disabled)

Frame format:

HT20

      0       1       2       3       4       5       6       7       8
      +---------------------------------------------------------------+
 0    |       [7:0]: bin -28 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
 1    |       [7:0]: bin -27 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
 2-54 |                                                               |
      +---------------------------------------------------------------+
 55   |       [7:0]: bin 27 magnitude (|i| + |q|) >> max_exp          |
      +---------------------------------------------------------------+
 56   |       [7:0]: all_bins {max_magnite[1:0], bitmap_weight[5:0]}  |
      +---------------------------------------------------------------+
 57   |               [7:0]: all_bins {max_magnite[9:2]}              |
      +---------------------------------------------------------------+
 58   |       [7:0]: all_bins {max_index[5:0], max_magnite[11:10]}    |
      +---------------------------------------------------------------+
 59   |                       [3:0] max_exp                           |
      +---------------------------------------------------------------+

HT40

      0       1       2       3       4       5       6       7       8
      +---------------------------------------------------------------+
0     |       [7:0]: bin -64 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
1     |       [7:0]: bin -63 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
2-125 |                                                               |
      +---------------------------------------------------------------+
127   |       [7:0]: bin 63 magnitude (|i| + |q|) >> max_exp          |
      +---------------------------------------------------------------+
128   |    [7:0]: lower_bins {max_magnite[1:0], bitmap_weight[5:0]}   |
      +---------------------------------------------------------------+
129   |               [7:0]: lower_bins {max_magnite[9:2]}            |
      +---------------------------------------------------------------+
130   |       [7:0]: lower_bins {max_index[5:0], max_magnite[11:10]}  |
      +---------------------------------------------------------------+
131   |    [7:0]: upper_bins {max_magnite[1:0], bitmap_weight[5:0]}   |
      +---------------------------------------------------------------+
132   |               [7:0]: upper_bins {max_magnite[9:2]}            |
      +---------------------------------------------------------------+
133   |       [7:0]: upper_bins {max_index[5:0], max_magnite[11:10]}  |
      +---------------------------------------------------------------+
134   |                       [3:0] max_exp                           |
      +---------------------------------------------------------------+

Riceved power computation:

Assuming the noise floor is equal to -96dbm(*) and the magnitude of each sample in a 20MHz bin equals the RSSI, the received signal strength of each FFT bin on HT20 channel can be computed as follow:

power(i) = nf + RSSI + 10*log(b(i)^2) - bin_sum

where:

RSSI is computed on control chain 0
b(i) is the magnitude in each bin, unscaled by max_exp
bin_sum = 10*log(sum[i=1..56](b(i)^2))

For 40MHz channel, previous formula should be used for 64 bins of control and extension channels, keeping in mind for HT40+ mode the extension channel is above the primary one (lower=ctl, upper=ext) and for HT40- the extension channel is below the primary one (lower=ext, upper=ctl)

(*) nf can differ from -96dbm due to noise and spikes leading to a reduced reported RSSI

Userspace programs:

FFT samples gathered from Atheros NICs could be drawn using userspace programs:

(based on Adrian Chadd's documentation https://wiki.freebsd.org/dev/ath_hal%284%29/SpectralScan)