|
When decimating by a factor of 16 (2 to power 4) only 32 of the
512 fft bins will be used for back transformation.
That is enough, but there is no margin as can be seen in figure 2.
|
Fig. 2.The FFT bandwidth is set to 1 kHz with the gaussian
window and decimation is set to 16 (two to power 4.)
A couple of weak spurs are visible close to the signal at -130 dB.
With real hardware they are well below the noise.
|
|
When decimating by a factor of 32 (2 to power 5) only 16 of the
512 fft bins will be used for back transformation.
The spurs are then at a level of about -90 dB as can be seen in figure 3.
|
Fig. 3.The FFT bandwidth is set to 1 kHz with the gaussian
window and decimation is set to 32 (two to power 5.)
There are spurs that limit the dynamic ranbge to about 90 dB as can be seen
in the S-meter graph.
|
|
Selected FFT1 bandwidth = 1 kHz with the Gaussian window.
The size of the first FFT will be 1024 with the Gaussian window and
the actual bandwidth will be 1.2 kHz.
An FFT1 buffer will span 2.048 ms.
When decimating by a factor of 8 (2 to power 3) the baseband sampling
rate becomes 67.5 kHz but that is not quite enough to avoid spurs as can be
seen in figure 4.
|
Fig. 4.The FFT bandwidth is set to 1 kHz with the gaussian
window and decimation is set to 8 (two to power 3.)
There are spurs that limit the dynamic ranbge to about 90 dB as can be seen
in the S-meter graph.
|
|
By only decimating by a factor of 4 (to 125 kHz) one can get a spur
level of about -115 dB with the Gaussian window at an fft1 bandwidth
of 1 kHz. See figure 5.
|
Fig. 5.The FFT bandwidth is set to 1 kHz with the Gaussian
window and decimation is set to 4 (two to power 2.)
There are spurs that limit the dynamic range to about 115 dB.
|
|
The much nicer spectral response in the main spectrum that can be
obtained with the Gaussian window causes twice as much delay as the
erfc window and further it requires a high baseband sampling
speed for adequate spur suppression.
Such parameters may be fine for QSK CW or fast break-in SSB,
but it will not work with coherent modes or very narrow
baseband filters.
That is not really a limitation because when a narrow baseband filter
is desired one can also set a narrower FFT1 bin bandwidth.
The sine to power 7 window is a little better than the Gaussian window,
but sine squared is not very good.
For very fast response the erfc window is clearly the best.
Selected FFT1 bandwidth = 100 Hz with the erfc window.
The size of the first FFT will be 8192 with the erfc window and
the actual bandwidth will be 81 Hz.
The delay due to the first FFT buffer will thus be 16.4 ms.
Spurs do not become clearly visible unless the decimating factor is
very large.
When decimating by a factor of 512 (2 to power 9) the baseband sampling
rate becomes 976 Hz only and the baseband bandwidth can not be set higher than
about 800 Hz. See figure 6.
The back transformation will then be 16 points only and that results in strong
spurs.
The spurs are all close in frequency however and may be harmless.
The advantage of setting a large decimation is that the low sampling speed
in the baseband will allow very high resoultion in the baseband at a low
CPU load.
|
Fig. 6.The FFT bandwidth is set to 100 Hz with the erfc
window and decimation is set to 512 (two to power 9.)
|
|
With a decimation of 256 and less there are no spurs with the erfc window
at an fft1 bandwidth of 100 Hz or less.
Selected FFT1 bandwidth = 100 Hz with the other windows.
With a decimation of 32 (two to power 5) sine to power 3 and higher is
fine as well as the Gaussian window.
|
| | | | |